Incestual Reproduction in Animals

Incestual Reproduction in Animals

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I'm quite curious on how animals view incestual pairings. Given there are enough mates around, i have quite observed that most females (subjects are dogs, cats, pigeons and chickens) would go choose another mate that is not a direct family member (parent, sibling, offspring). How does this work? Is it instinct to not favor family members or does it still revolve around who's the fittest mate?

Anecdotally I have never observed a preference in lab mice, but we have never made a detailed study of it. Males do not seem to favor non-incestuous over incestuous breeding. Though inbreeding does tend to produce smaller litter sizes over generations and inbred strains of lab mice do tend to be less hearty. Though lab mice are a bit removed from the situation in the wild.

Incestual Reproduction in Animals - Biology

Most animals are diploid organisms, meaning that their body (somatic) cells are diploid and haploid reproductive (gamete) cells are produced through meiosis. Some exceptions exist: for example, in bees, wasps, and ants, the male is haploid because it develops from unfertilized eggs. Most animals undergo sexual reproduction. This fact distinguishes animals from fungi, protists, and bacteria, where asexual reproduction is common or exclusive. However, a few groups, such as cnidarians, flatworm, and roundworms, undergo asexual reproduction, although nearly all of those animals also have a sexual phase to their life cycle.

Learning Objectives

  • Explain the processes of animal reproduction and embryonic development
  • Compare and contrast the embryonic development of protostomes and deuterostomes
  • Describe the roles that Hox genes play in development

Animal Reproduction and Development

Most animals are diploid organisms (their body, or somatic, cells are diploid) with haploid reproductive ( gamete ) cells produced through meiosis. The majority of animals undergo sexual reproduction. This fact distinguishes animals from fungi, protists, and bacteria where asexual reproduction is common or exclusive. However, a few groups, such as cnidarians, flatworms, and roundworms, undergo asexual reproduction, although nearly all of those animals also have a sexual phase to their life cycle.

Animal Reproduction and the Egg – Guided Learning

I designed this activity for my freshman biology class during the pandemic of 2020. My goal was to provide an overview of how vertebrate groups reproduce. Students in the past would focus on the amniote egg as part of unit on birds and reptiles. I would assign the amniote egg coloring as a way for students to learn the structures of the egg, like the amnion, chorion, allantois, and yolk.

The Eyewitness video series has a great episode on birds and the amniote egg which is now available on Youtube. I even have a worksheet to go with it, though Edpuzzle is also a great app for getting students to watch assigned videos.

This guided learning activity is an interactive exploration of reproduction in different animal groups. It starts with the difference between internal and external fertilization. I use engaging graphics, like gifs to focus student attention on important (or amazing) concepts. For example, swimming salmon show that dark red coloration may be the key to attracting a mate.

Next, students compare three types of reproductive strategies. Some animals, like snakes, lay eggs, whereas some animals retain eggs inside their bodies. Mammals, like humans, nourish the egg inside the uterus. I would like students to understand that in all of these strategies, there are similarities. Even though most mammals do not lay eggs, the retain some of the egg characteristics, like the amnion.

In later slides, students label an image of the amniote egg based on the descriptions. A fill-in slide contains the descriptions for what each membrane does. In another slide, students drag labels to an image.

In the final slide, students must synthesize what they have learned to create a Venn diagram comparing mammal reproduction to bird reproduction. A fun way to make the unit more interactive is to create “reptile” eggs by soaking bird eggs in vinegar. This will remove the calcium from the shell and make it soft and flexible.

In the past, I have also incubated chicken eggs. This can be a major project though, and you’ll need an incubator and fertilized eggs, plus a place to go with the chicks that hatch.


Offspring of biologically related persons are subject to the possible effects of inbreeding, such as congenital birth defects. The chances of such disorders are increased when the biological parents are more closely related. This is because such pairings have a 25% probability of producing homozygous zygotes, resulting in offspring with two recessive alleles, which can produce disorders when these alleles are deleterious. [14] Because most recessive alleles are rare in populations, it is unlikely that two unrelated marriage partners will both be carriers of the same deleterious allele however, because close relatives share a large fraction of their alleles, the probability that any such deleterious allele is inherited from the common ancestor through both parents is increased dramatically. For each homozygous recessive individual formed there is an equal chance of producing a homozygous dominant individual — one completely devoid of the harmful allele. Contrary to common belief, inbreeding does not in itself alter allele frequencies, but rather increases the relative proportion of homozygotes to heterozygotes however, because the increased proportion of deleterious homozygotes exposes the allele to natural selection, in the long run its frequency decreases more rapidly in inbred populations. In the short term, incestuous reproduction is expected to increase the number of spontaneous abortions of zygotes, perinatal deaths, and postnatal offspring with birth defects. [15] The advantages of inbreeding may be the result of a tendency to preserve the structures of alleles interacting at different loci that have been adapted together by a common selective history. [16]

Malformations or harmful traits can stay within a population due to a high homozygosity rate, and this will cause a population to become fixed for certain traits, like having too many bones in an area, like the vertebral column of wolves on Isle Royale or having cranial abnormalities, such as in Northern elephant seals, where their cranial bone length in the lower mandibular tooth row has changed. Having a high homozygosity rate is problematic for a population because it will unmask recessive deleterious alleles generated by mutations, reduce heterozygote advantage, and it is detrimental to the survival of small, endangered animal populations. [17] When deleterious recessive alleles are unmasked due to the increased homozygosity generated by inbreeding, this can cause inbreeding depression. [18]

There may also be other deleterious effects besides those caused by recessive diseases. Thus, similar immune systems may be more vulnerable to infectious diseases (see Major histocompatibility complex and sexual selection). [19]

Inbreeding history of the population should also be considered when discussing the variation in the severity of inbreeding depression between and within species. With persistent inbreeding, there is evidence that shows that inbreeding depression becomes less severe. This is associated with the unmasking and elimination of severely deleterious recessive alleles. However, inbreeding depression is not a temporary phenomenon because this elimination of deleterious recessive alleles will never be complete. Eliminating slightly deleterious mutations through inbreeding under moderate selection is not as effective. Fixation of alleles most likely occurs through Muller's ratchet, when an asexual population's genome accumulates deleterious mutations that are irreversible. [20]

Despite all its disadvantages, inbreeding can also have a variety of advantages, such as reducing the recombination load, [21] and allowing the expression of recessive advantageous phenotypes. It has been proposed that under circumstances when the advantages of inbreeding outweigh the disadvantages, preferential breeding within small groups could be promoted, potentially leading to speciation. [22]

Autosomal recessive disorders occur in individuals who have two copies of an allele for a particular recessive genetic mutation. [23] Except in certain rare circumstances, such as new mutations or uniparental disomy, both parents of an individual with such a disorder will be carriers of the gene. These carriers do not display any signs of the mutation and may be unaware that they carry the mutated gene. Since relatives share a higher proportion of their genes than do unrelated people, it is more likely that related parents will both be carriers of the same recessive allele, and therefore their children are at a higher risk of inheriting an autosomal recessive genetic disorder. The extent to which the risk increases depends on the degree of genetic relationship between the parents the risk is greater when the parents are close relatives and lower for relationships between more distant relatives, such as second cousins, though still greater than for the general population. [24]

Children of parent-child or sibling-sibling unions are at an increased risk compared to cousin-cousin unions. [25] : 3 Inbreeding may result in a greater than expected phenotypic expression of deleterious recessive alleles within a population. [26] As a result, first-generation inbred individuals are more likely to show physical and health defects, [27] [28] including:

  • Reduced fertility both in litter size and sperm viability
  • Increased genetic disorders
  • Fluctuating facial asymmetry
  • Lower birth rate
  • Higher infant mortality and child mortality[29]
  • Smaller adult size
  • Loss of immune system function
  • Increased cardiovascular risks[30]

The isolation of a small population for a period of time can lead to inbreeding within that population, resulting in increased genetic relatedness between breeding individuals. Inbreeding depression can also occur in a large population if individuals tend to mate with their relatives, instead of mating randomly.

Many individuals in the first generation of inbreeding will never live to reproduce. [31] Over time, with isolation, such as a population bottleneck caused by purposeful (assortative) breeding or natural environmental factors, the deleterious inherited traits are culled. [6] [7] [32]

Island species are often very inbred, as their isolation from the larger group on a mainland allows natural selection to work on their population. This type of isolation may result in the formation of race or even speciation, as the inbreeding first removes many deleterious genes, and permits the expression of genes that allow a population to adapt to an ecosystem. As the adaptation becomes more pronounced, the new species or race radiates from its entrance into the new space, or dies out if it cannot adapt and, most importantly, reproduce. [33]

The reduced genetic diversity, for example due to a bottleneck will unavoidably increase inbreeding for the entire population. This may mean that a species may not be able to adapt to changes in environmental conditions. Each individual will have similar immune systems, as immune systems are genetically based. When a species becomes endangered, the population may fall below a minimum whereby the forced interbreeding between the remaining animals will result in extinction.

Natural breedings include inbreeding by necessity, and most animals only migrate when necessary. In many cases, the closest available mate is a mother, sister, grandmother, father, brother, or grandfather. In all cases, the environment presents stresses to remove from the population those individuals who cannot survive because of illness. [ citation needed ]

There was an assumption [ by whom? ] that wild populations do not inbreed this is not what is observed in some cases in the wild. However, in species such as horses, animals in wild or feral conditions often drive off the young of both sexes, thought to be a mechanism by which the species instinctively avoids some of the genetic consequences of inbreeding. [34] In general, many mammal species, including humanity's closest primate relatives, avoid close inbreeding possibly due to the deleterious effects. [25] : 6

Examples Edit

Although there are several examples of inbred populations of wild animals, the negative consequences of this inbreeding are poorly documented. [ citation needed ] In the South American sea lion, there was concern that recent population crashes would reduce genetic diversity. Historical analysis indicated that a population expansion from just two matrilineal lines was responsible for most of the individuals within the population. Even so, the diversity within the lines allowed great variation in the gene pool that may help to protect the South American sea lion from extinction. [35]

In lions, prides are often followed by related males in bachelor groups. When the dominant male is killed or driven off by one of these bachelors, a father may be replaced by his son. There is no mechanism for preventing inbreeding or to ensure outcrossing. In the prides, most lionesses are related to one another. If there is more than one dominant male, the group of alpha males are usually related. Two lines are then being "line bred". Also, in some populations, such as the Crater lions, it is known that a population bottleneck has occurred. Researchers found far greater genetic heterozygosity than expected. [36] In fact, predators are known for low genetic variance, along with most of the top portion of the trophic levels of an ecosystem. [37] Additionally, the alpha males of two neighboring prides can be from the same litter one brother may come to acquire leadership over another's pride, and subsequently mate with his 'nieces' or cousins. However, killing another male's cubs, upon the takeover, allows the new selected gene complement of the incoming alpha male to prevail over the previous male. There are genetic assays being scheduled for lions to determine their genetic diversity. The preliminary studies show results inconsistent with the outcrossing paradigm based on individual environments of the studied groups. [36]

In Central California, sea otters were thought to have been driven to extinction due to over hunting, until a small colony was discovered in the Point Sur region in the 1930s. [38] Since then, the population has grown and spread along the central Californian coast to around 2,000 individuals, a level that has remained stable for over a decade. Population growth is limited by the fact that all Californian sea otters are descended from the isolated colony, resulting in inbreeding. [39]

Cheetahs are another example of inbreeding. Thousands of years ago the cheetah went through a population bottleneck that reduced its population dramatically so the animals that are alive today are all related to one another. A consequence from inbreeding for this species has been high juvenile mortality, low fecundity, and poor breeding success. [40]

In a study on an island population of song sparrows, individuals that were inbred showed significantly lower survival rates than outbred individuals during a severe winter weather related population crash. These studies show that inbreeding depression and ecological factors have an influence on survival. [20]

A measure of inbreeding of an individual A is the probability F(A) that both alleles in one locus are derived from the same allele in an ancestor. These two identical alleles that are both derived from a common ancestor are said to be identical by descent. This probability F(A) is called the "coefficient of inbreeding". [41]

Another useful measure that describes the extent to which two individuals are related (say individuals A and B) is their coancestry coefficient f(A,B), which gives the probability that one randomly selected allele from A and another randomly selected allele from B are identical by descent. [42] This is also denoted as the kinship coefficient between A and B. [43]

A particular case is the self-coancestry of individual A with itself, f(A,A), which is the probability that taking one random allele from A and then, independently and with replacement, another random allele also from A, both are identical by descent. Since they can be identical by descent by sampling the same allele or by sampling both alleles that happen to be identical by descent, we have f(A,A) = 1/2 + F(A)/2. [44]

Both the inbreeding and the coancestry coefficients can be defined for specific individuals or as average population values. They can be computed from genealogies or estimated from the population size and its breeding properties, but all methods assume no selection and are limited to neutral alleles.

There are several methods to compute this percentage. The two main ways are the path method [45] [41] and the tabular method. [46] [47]

Typical coancestries between relatives are as follows:

  • Father/daughter or mother/son → 25% ( 1 ⁄ 4 )
  • Brother/sister → 25% (
  • 1 ⁄ 4 )
  • Grandfather/granddaughter or grandmother/grandson → 12.5% (
  • 1 ⁄ 8 )
  • Half-brother/half-sister, Double cousins → 12.5% (
  • 1 ⁄ 8 )
  • Uncle/niece or aunt/nephew → 12.5% (
  • 1 ⁄ 8 )
  • Great-grandfather/great-granddaughter or great-grandmother/great-grandson → 6.25% (
  • 1 ⁄ 16 )
  • Half-uncle/niece or half-aunt/nephew → 6.25% (
  • 1 ⁄ 16 )
  • First cousins → 6.25% (
  • 1 ⁄ 16 )

Wild animals Edit

    females regularly mate with their fathers and brothers. [48] : North Carolina State University found that bedbugs, in contrast to most other insects, tolerate incest and are able to genetically withstand the effects of inbreeding quite well. [49] females prefer to mate with their own brothers over unrelated males. [1] : 'It turns out that females in these hermaphrodite insects are not really fertilizing their eggs themselves, but instead are having this done by a parasitic tissue that infects them at birth,' says Laura Ross of Oxford University's Department of Zoology. ‘It seems that this infectious tissue derives from left-over sperm from their father, who has found a sneaky way of having more children by mating with his daughters.' [50]
  • Adactylidium: The single male offspring mite mates with all the daughters when they are still in the mother. The females, now impregnated, cut holes in their mother's body so that they can emerge to find new thrips eggs. The male emerges as well, but does not look for food or new mates, and dies after a few hours. The females die at the age of 4 days, when their own offspring eat them alive from the inside. [51]

Semi-domestic animals Edit

Domestic animals Edit

Breeding in domestic animals is primarily assortative breeding (see selective breeding). Without the sorting of individuals by trait, a breed could not be established, nor could poor genetic material be removed. Homozygosity is the case where similar or identical alleles combine to express a trait that is not otherwise expressed (recessiveness). Inbreeding exposes recessive alleles through increasing homozygosity. [55]

Breeders must avoid breeding from individuals that demonstrate either homozygosity or heterozygosity for disease causing alleles. [56] The goal of preventing the transfer of deleterious alleles may be achieved by reproductive isolation, sterilization, or, in the extreme case, culling. Culling is not strictly necessary if genetics are the only issue in hand. Small animals such as cats and dogs may be sterilized, but in the case of large agricultural animals, such as cattle, culling is usually the only economic option.

The issue of casual breeders who inbreed irresponsibly is discussed in the following quotation on cattle:

Meanwhile, milk production per cow per lactation increased from 17,444 lbs to 25,013 lbs from 1978 to 1998 for the Holstein breed. Mean breeding values for milk of Holstein cows increased by 4,829 lbs during this period. [57] High producing cows are increasingly difficult to breed and are subject to higher health costs than cows of lower genetic merit for production (Cassell, 2001).

Intensive selection for higher yield has increased relationships among animals within breed and increased the rate of casual inbreeding.

Many of the traits that affect profitability in crosses of modern dairy breeds have not been studied in designed experiments. Indeed, all crossbreeding research involving North American breeds and strains is very dated (McAllister, 2001) if it exists at all. [58]

The BBC produced two documentaries on dog inbreeding titled Pedigree Dogs Exposed and Pedigree Dogs Exposed: Three Years On that document the negative health consequences of excessive inbreeding.

Linebreeding Edit

Linebreeding is a form of inbreeding. There is no clear distinction between the two terms, but linebreeding may encompass crosses between individuals and their descendants or two cousins. [54] [59] This method can be used to increase a particular animal's contribution to the population. [54] While linebreeding is less likely to cause problems in the first generation than does inbreeding, over time, linebreeding can reduce the genetic diversity of a population and cause problems related to a too-small gene pool that may include an increased prevalence of genetic disorders and inbreeding depression. [ citation needed ]

Outcrossing Edit

Outcrossing is where two unrelated individuals are crossed to produce progeny. In outcrossing, unless there is verifiable genetic information, one may find that all individuals are distantly related to an ancient progenitor. If the trait carries throughout a population, all individuals can have this trait. This is called the founder effect. In the well established breeds, that are commonly bred, a large gene pool is present. For example, in 2004, over 18,000 Persian cats were registered. [60] A possibility exists for a complete outcross, if no barriers exist between the individuals to breed. However, it is not always the case, and a form of distant linebreeding occurs. Again it is up to the assortative breeder to know what sort of traits, both positive and negative, exist within the diversity of one breeding. This diversity of genetic expression, within even close relatives, increases the variability and diversity of viable stock.

Laboratory animals Edit

Systematic inbreeding and maintenance of inbred strains of laboratory mice and rats is of great importance for biomedical research. The inbreeding guarantees a consistent and uniform animal model for experimental purposes and enables genetic studies in congenic and knock-out animals. In order to achieve a mouse strain that is considered inbred, a minimum of 20 sequential generations of sibling matings must occur. With each successive generation of breeding, homozygosity in the entire genome increases, eliminating heterozygous loci. With 20 generations of sibling matings, homozygosity is occurring at roughly 98.7% of all loci in the genome, allowing for these offspring to serve as animal models for genetic studies. [61] The use of inbred strains is also important for genetic studies in animal models, for example to distinguish genetic from environmental effects. The mice that are inbred typically show considerably lower survival rates.

Effects Edit

Inbreeding increases homozygosity, which can increase the chances of the expression of deleterious recessive alleles and therefore has the potential to decrease the fitness of the offspring. With continuous inbreeding, genetic variation is lost and homozygosity is increased, enabling the expression of recessive deleterious alleles in homozygotes. The coefficient of inbreeding, or the degree of inbreeding in an individual, is an estimate of the percent of homozygous alleles in the overall genome. [63] The more biologically related the parents are, the greater the coefficient of inbreeding, since their genomes have many similarities already. This overall homozygosity becomes an issue when there are deleterious recessive alleles in the gene pool of the family. [64] By pairing chromosomes of similar genomes, the chance for these recessive alleles to pair and become homozygous greatly increases, leading to offspring with autosomal recessive disorders. [64]

Inbreeding is especially problematic in small populations where the genetic variation is already limited. [65] By inbreeding, individuals are further decreasing genetic variation by increasing homozygosity in the genomes of their offspring. [66] Thus, the likelihood of deleterious recessive alleles to pair is significantly higher in a small inbreeding population than in a larger inbreeding population. [65]

The fitness consequences of consanguineous mating have been studied since their scientific recognition by Charles Darwin in 1839. [67] [68] Some of the most harmful effects known from such breeding includes its effects on the mortality rate as well as on the general health of the offspring. [69] Since the 1960s, there have been many studies to support such debilitating effects on the human organism. [66] [67] [69] [70] [71] Specifically, inbreeding has been found to decrease fertility as a direct result of increasing homozygosity of deleterious recessive alleles. [71] [72] Fetuses produced by inbreeding also face a greater risk of spontaneous abortions due to inherent complications in development. [73] Among mothers who experience stillbirths and early infant deaths, those that are inbreeding have a significantly higher chance of reaching repeated results with future offspring. [74] Additionally, consanguineous parents possess a high risk of premature birth and producing underweight and undersized infants. [75] Viable inbred offspring are also likely to be inflicted with physical deformities and genetically inherited diseases. [63] Studies have confirmed an increase in several genetic disorders due to inbreeding such as blindness, hearing loss, neonatal diabetes, limb malformations, disorders of sex development, schizophrenia and several others. [63] [76] Moreover, there is an increased risk for congenital heart disease depending on the inbreeding coefficient (See coefficient of inbreeding) of the offspring, with significant risk accompanied by an F =.125 or higher. [27]

Prevalence Edit

The general negative outlook and eschewal of inbreeding that is prevalent in the Western world today has roots from over 2000 years ago. Specifically, written documents such as the Bible illustrate that there have been laws and social customs that have called for the abstention from inbreeding. Along with cultural taboos, parental education and awareness of inbreeding consequences have played large roles in minimizing inbreeding frequencies in areas like Europe. That being so, there are less urbanized and less populated regions across the world that have shown continuity in the practice of inbreeding.

The continuity of inbreeding is often either by choice or unavoidably due to the limitations of the geographical area. When by choice, the rate of consanguinity is highly dependent on religion and culture. [65] In the Western world some Anabaptist groups are highly inbred because they originate from small founder populations and until [ clarification needed ] today [ when? ] marriage outside the groups is not allowed for members. [ citation needed ] Especially the Reidenbach Old Order Mennonites [77] and the Hutterites stem from very small founder populations. The same is true for some Hasidic and Haredi Jewish groups.

Of the practicing regions, Middle Eastern and northern Africa territories show the greatest frequencies of consanguinity. [65] The link between the high frequency and the region is primarily due to the dominance of Islamic populations, who have historically engaged in familyline relations. [68] However, inbreeding culture in Middle East didn't begin with Islam, having roots in ancient Egypt and Mesopotamia.

Among these populations with high levels of inbreeding, researchers have found several disorders prevalent among inbred offspring. In Lebanon, Saudi Arabia, Egypt, and in Israel, the offspring of consanguineous relationships have an increased risk of congenital malformations, congenital heart defects, congenital hydrocephalus and neural tube defects. [65] Furthermore, among inbred children in Palestine and Lebanon, there is a positive association between consanguinity and reported cleft lip/palate cases. [65] Historically, populations of Qatar have engaged in consanguineous relationships of all kinds, leading to high risk of inheriting genetic diseases. As of 2014, around 5% of the Qatari population suffered from hereditary hearing loss most were descendants of a consanguineous relationship. [78]

Hormonal control of oogenesis

Oogenesis is controlled by FSH, LH, estrogen, and progesterone.

  • FSH stimulates development of egg cells that develop in structures called follicles, which are located within the ovaries.
  • LH also promotes development and maturation of eggs and induction of ovulation.
  • Estrogen is the reproductive hormone in females that assists in ovulation and regrowing the lining of the uterus it is also responsible for the secondary sexual characteristics of females such as breast development.
  • Progesterone assists in endometrial re-growth and inhibition of FSH and LH release.

These hormones together regulate the ovarian and menstrual cycles. The ovarian cycle governs the preparation of endocrine tissues and release of eggs, while the menstrual cycle governs the preparation and maintenance of the uterine lining. These cycles occur concurrently and are coordinated over a 22–32 day cycle, with an average length of 28 days:

  • The first half of the ovarian cycle is the follicular phase. Slowly rising levels of FSH and LH cause the growth of follicles on the surface of the ovary. This process prepares the egg for ovulation. As the follicles grow, they begin releasing estrogens. Estrogen levels increase over the course of the follicular phase as the follicles continue to develop. In the menstrual cycle, menstrual flow occurs at the beginning of the follicular phase when estrogen levels are low (when the follicles are only just beginning to develop) rising levels of estrogen then cause the endometrium to proliferate (grow), replacing the blood vessels and glands that deteriorated during the end of the last cycle.
  • Ovulation occurs just prior to the middle of the cycle (approximately day 14), when the high level of estrogen produced by the developing follicles causes FSH and especially LH to rise rapidly, then fall. The spike in LH causes ovulation: the follicle which is most mature ruptures and releases its egg. The follicles that did not rupture degenerate and their eggs are lost. The level of estrogen decreases when the extra follicles degenerate.
  • Following ovulation, the ovarian cycle enters its luteal phase, and the menstrual cycle enters its secretory phase, both of which run from about day 15 to 28. The cells in the follicle undergo physical changes and produce a structure called a corpus luteum, which produces estrogen and progesterone. The progesterone facilitates the regrowth of the uterine lining and inhibits the release of further FSH and LH. The uterus becomes prepared to accept a fertilized egg, should fertilization occur. The inhibition of FSH and LH by progesterone prevents any further eggs and follicles from developing. The level of estrogen produced by the corpus luteum increases to a steady level for the next few days estrogen enhances the effects of progesterone.
  • It takes about seven days for an egg to travel through the Fallopian tube from the ovary to the uterus, and it must be fertilized while in the Fallopian tube:
    • If no fertilized egg is implanted into the uterus, the corpus luteum degenerates and the levels of estrogen and progesterone decrease. The endometrium begins to degenerate as the progesterone levels drop, initiating the next menstrual cycle. The decrease in progesterone also allows the hypothalamus to send GnRH to the anterior pituitary, releasing FSH and LH and starting the cycles again. The figure below visually compares the ovarian and uterine cycles as well as the hormone levels controlling these cycles.
    • If a fertilized egg implants in the endometrial lining of the uterine wall, the embryo produces a hormone called human chorionic gonadotropin (hCG) that maintains the corpus luteum. The ovary continues to produce progesterone at high levels, and the menstrual cycle is arrested for the duration of the pregnancy. Because hCG is unique to pregnancy, it is the hormone detected by pregnancy tests.

    The figure below visually compares the ovarian and uterine cycles as well as the hormone levels controlling these cycles.

    Rising and falling hormone levels result in progression of the ovarian and menstrual cycles. Image credit: modification of work from OpenStax Biology and OpenStax Anatomy and Physiology modification of work by Mikael Häggström)

    This video provides a great overview of the human female reproductive system, emphasizing many of the points described above:

    Biology: The Effects on Inbreeding on Animal and Human Populations

    To what extent are biological defects linked to cases of inbreeding and why?


    Inbreeding is a controversial topic that has affecting our nations for centuries. Inbreeding occurs when two individuals from the same species mate, which are more closely related than if you had selected them randomly from the whole population. In simple terms it means mating between individuals that are related to one another. Within animals this can mean mating between brothers and sisters, father and daughter or mother and son, although this degree of inbreeding is rarely seen in humans, it can also be observed and is usually seen as mating between cousins and other family members. Inbreeding is a measure of the probability of identity by descent of two alleles at a given locus in a given individual. Locus refers to the location of a gene on the chromosome and an allele is a genetic variant, if there is variation in genetic type at a locus, then you have at least two alleles. Identity by descent is straightforward, it simply means that the two alleles are the same because they derive from the same common ancestor. Loss of heterozygosity (and therefore a gain in homozygosity), whereby loci are more likely to carry identical alleles. As heterozygosity is the biological source of hybrid vigour, this vigour is lost in what is called inbreeding depression and A decrease in genetic variation for traits. As a practice that has been occurring for many generations the question, what affects does inbreeding have on animal and human populations?, has begun to arise in the scientific community. Through the study of genetics the answer to this question is hoped to be discovered.

    This investigation examines the genetic traits associated with inbreeding and the biological defects that can result from it. In order to understand this study there must be an understanding of Genetics. Genetics is the branch in biology which studies the origin, transmission and expression of genetic information, and the variation it causes amongst species. Inbreeding is most commonly used as a technique for the retention of desirable characteristics or the elimination of undesirable ones. This technique has been most commonly used with animals in order to produce offspring with certain desired traits. However, over the years it has been discovered that the descendants of inbreed couples are being born with more genetic defects and are not living as long as their ancestors. In order to discover the cause of these defects, further exploration of genetic genealogy is imperative.


    In order to successfully investigate this topic tedious amounts of research on other studies and experiments needed to be done since I personally could not do an experiment. The method chosen was the research three different types of studies revolving around inbreeding within both human and animal populations. The first study chosen is about a certain breed of cow that has a certain genetic disorder linked to it, the second study is based around the effect of inbreeding on humans using a royal family. The family genetic history and family relations is used to prove why incest caused the downfall of the dynasty and how it ultimately led to its extinction. The final study used is also about animals but it focuses on how inbreeding is used to make “pure bred animals” and how the prolonged use of this technique can have detrimental affect on these animals health.

    Research Question:

    To what extent are biological defects linked to cases of inbreeding and why?


    In recent years many studies have been conducted in efforts to connect certain diseases to inbreeding. In the study Svara et al. Acta Veterinaria Scandinavica conducted in 2016, it was founded that inbreeding is directly related to certain biological defects in Cika cattle. Svara studied two cows from the same farm that both had stillbirths and traced their genetic lineage using a pedigree of their familial relationships that went back five generations.

    Familial relationship of the two PHA affected calves. Males are represented by squares, females circles. Full shading designates affected.

    Using the pedigree Svara discovered that both the cows previously had multiple normal offspring with different cows but when they were breeded with the same 2-year-old Cika cow their offsprings both had the same genetic defect, since the trait was recessive it meant that both the mothers also were carriers for the disease, meaning that they most likely had common ancestors since it was such a small farm. Based on the information in the pedigree it can be determined that the two affected cows were a result of inbred mother-son and aunt-nephew relations. Furthermore the two dams were found to be paternal half-sisters and had the same maternal grandmother. As a result of the mothers having the same lineage both of their offspring had the same genetic deficiencies, gross lesions were similar in both affected calves. Their bodies were severely deformed due to severe diffuse subcutaneous oedema with multiple cysts of various sizes, filled with serohemorrhagic fluid. This is recessive gene for Cika cows so when Svara discovered that two cows from the same farm had the same disease she used a pedigree to put inbreeding at default.

    As well as in animals, inbreeding can also have an affect on the genetic makeup of humans. A great example of this is the extinction of a European royal family. In 2009 Gonzalo Alvarez did a study, from the genetic point of view, as to whether or not inbreeding was the major cause responsible for the extinction of the Spanish Habsburg dynasty. Alvarez used an extended pedigree up to 16 generations in depth, involving more than 3,000 individuals. He found that the inbreeding coefficient of the Spanish Habsburg kings increased strongly along generations from 0.025 for king Philip I, the founder of the dynasty, to 0.254 for Charles II and several members of the dynasty had inbreeding coefficients higher than 0.20. The inbreeding coefficient uses an equation to measure the degree of consanguinity between two individuals, it is the probability that an individual receives at a given locus two genes identical by descent due to the common ancestry between his parents.The last king of the Spanish Habsburg dynasty was Charles II and as stated earlier he had the highest inbreeding coefficient in his family. Charles II presented important physical and mental disabilities, he suffered from a number of different diseases during his lifetime, the main ones being pituitary hormone deficiency and distal renal tubular acidosis. Alvarez speculated that the health problems suffered by Charles II could have been caused by the action of detrimental recessive genes given his high inbreeding coefficient with 25.4% of his autosomal genome expected to be homozygous. Since Charles II suffered from two rare genetic disorders that are both determined by recessive alleles but carried at two unlinked loci it is assumed that his disorders are a result of inbreeding. Alvarez came to the conclusion that the probability of an individual suffering from two very rare recessive traits is low but inbreeding can increases those chances and cause an association of two recessive traits even for unlinked loci.

    Another main issue related to inbreeding is its manipulated use within animals in order to create purebreds. Although pure bred dogs are coveted for their beauty and desired for certain traits they often suffer for many problems caused by inbreeding. The main issues with continuously making purebred offspring is that in order to make a purebred you need to animals from the exact same gene pool. As a result this severely limits the gene pool variety and then on top of that most breeders will animals from the same family causing and even higher inbreeding coefficient. Most frequently this method of breeding is used within dogs and recent studies have put it at blame for many disease found in dogs like, higher risks of cancer and tumors eye and heart disease joint and bone disorders skin, immune system and neurological diseases and even epilepsy. “One study found that ten thousand pugs have the same genetic diversity as 50 individuals”(barnes). This means that most inbred animals will be more likely to receive the negative recessive genes from their lineage causing them to have severe health problems.

    Incest Not So Taboo in Nature

    This recent story went wide: British fraternal twins who were adopted separately at birth later married without realizing they were brother and sister. Why does this make us so instantly and overtly squeamish?

    Lord David Alton of Liverpool — a member of British parliament — discussed the couple's case during a government session on in vitro fertilization as he pushed for identity rights of children conceived by the technique. On his Web site, Alton noted that a similar adopted brother-sister marriage was recently avoided through detailed identity records.

    Incest is considered taboo in nearly every human culture around the world, researchers have found. Yet as the 21st century waxes, questions about the behavior remain unanswered.

    Where does our aversion to incest come from — genetics or society — and what's so bad about it, anyway?

    Incestuous ancestry

    Scientists think Earth's earliest life emerged about 3.8 billion years ago and slowly evolved into the diversity of organisms seen today. Until roughly 1.2 billion years ago, however, sex didn't exist.

    Nathaniel Wheelwright, an evolutionary biologist at Bowdoin College in Maine, said asexual reproduction was the first type of reproduction to evolve. In its most basic form, called parthenogenesis, it involves one-celled organisms such as bacteria dividing in two. But more complex creatures do it, too.

    "Asexual reproduction is [like] the ultimate in incest because you're breeding with yourself," Wheelwright told LiveScience. "You can still see species asexually reproducing, or cloning themselves, in situations where there is no advantage to [sex]," he said, "and you can see species that commit incest where there is no penalty to inbreeding."

    Aside from microbes, most of which reproduce asexually, Wheelwright said mountaintops, small islands and other isolated habitats are places where today's incestuous reproducers are most commonly found. "If your relatives are the only game in town you don't have much of a choice," he said.

    But Wheelwright explained that sexual reproduction — the current reproductive norm among plants and animals — gives creatures a leg-up in life. "Sex results in . diverse offspring and maintains a diversity of genes," he said.

    It's like nature's way of avoiding putting all its eggs in one basket: Where one copy of a gene may spell doom for one organism, a different version spread through sex in another creature may help it survive.

    "People who domesticated plants and animals were likely the first to figure this out," Wheelwright said. "When they inbred, they got lower birth weights, increased embryo death and decreased fertility."

    Still, genetic diversity is at times less important than other advantages, such as better guarding of offspring in some African fish that inbreed. On the whole, however, the risk of incest in plants and animals generally outweighs any of its benefits.

    Bad combination

    The problem with incest is that it can keep so-called "bad" genes in the gene pool and compound their effects, said Debra Lieberman, an evolutionary psychologist at the University of Hawaii.

    "Close genetic relatives run the risk of having offspring that have a reduced chance of surviving," Lieberman said.

    To understand the dangers of incest in humans, she explained, one needs to know that DNA — the blueprint of life — is divvied up into two sets of 23 chromosomes for a total of 46 in the average human being. One set of 23 comes from the father while the other comes from the mother.

    While Lieberman cautioned it's never plain when it comes to genetics, she offered a simplified example to illustrate the risks associated with incest.

    "Let's say you get a bad gene, which scientists call deleterious, from your mom. But your dad's copy of the same gene functions normally," Lieberman said. "The good version acts like a backup, effectively preventing disease the bad gene might have caused."

    But having a kid with your sibling, she explained, drastically increases the chances of getting two copies of the deleterious gene as compared to reproducing with someone outside of your family.

    "Each of you would have a copy of that bad gene, so there's a good chance your kid won't have a normal copy to work with," she said. Multiply that by any other deleterious genes sprinkled among an estimated 50,000 active genes in humans, she explained, and there are bound to be some life-shortening problems.

    Naturally unselected

    Because so-called higher organisms such as humans are susceptible to life-shortening genetic combinations, Lieberman thinks nature has weeded out incestuous behavior over time through natural selection. Humans and other animals, she said, likely evolved ways to detect and avoid mating with their close relatives.

    "We don't have DNA goggles to detect our relatives, but I think we've evolved psychological systems that help us do so," Lieberman said, including face recognition and even scent. But Lieberman thinks the strongest cue humans have is growing up with a sibling under the same roof.

    "People refer to this as the Westermarck Effect, which essentially says children who co-reside are much less likely to breed with each other when they reach adulthood," she said.

    Even unrelated children who grow up together exhibit avoidance toward inbreeding, she said.

    "The Kibbutz communities in Israel are a good example," she said. Only weeks after birth, mothers give their kids to a "children's society" staffed by trained caregivers. Lieberman said people raised in the same community are much less likely to marry each other than someone from a neighboring area.

    Another example Lieberman noted are 1800s records of arranged Taiwanese "minor" marriages, where parents would arrange a marriage for their daughter by handing her over to the future groom's household shortly after birth.

    "Compared to 'major' marriage arrangements, where a couple meets just before the wedding, minor couples had fewer kids," she told LiveScience. "Minor couples frequently refused to consummate their marriage, so the fathers would stand outside their door until they did."

    Lieberman thinks minor couples had such trouble because they grew up with one another, "activating the genetic cues that screamed, 'Avoid mating with this person,'" she said. "Those cues probably didn't get activated with the brother-sister couple who married. They didn't grow up together."

    Incestuous mysteries

    Although no genes for incest avoidance cues have been pinpointed yet, Lieberman thinks they will eventually be tracked down.

    "It would be wonderful to isolate those genes," she said. "I think we will some day, but we need to know if there are other cues used to avoid mating with a relative."

    But how does Lieberman explain incestuous behavior both in captive and wild animals, such as juvenile male chimps who attempt sex with their mothers?

    "These systems aren't foolproof," she said. "Sometimes the [female chimp mother] lets her male offspring mount her if they're frightened and want to calm down. But most of the time, females squawk and reject the attempts."

    David Spain, an emeritus University of Washington anthropologist who has followed incest research since 1968, said incest "defeats the whole point of sex" — mixing up the gene pool — and is ultimately why the behavior is astonishingly rare among first relatives.

    "Cousin marriages don't have as much in the way of deleterious effects, so we see those partnerships more often," Spain said. "Evolution weeds out the things that don't work."

    Better birth certificates?

    Spain thinks the now-unmarried twins, whose identities and anullment details have been concealed, would be fascinating to interview.

    "This is definitely a one-in-a-million type thing. The psychoanalyst side of me definitely wants to know what was going through their minds after they discovered they were brother and sister," Spain said, noting that such an analysis might offer important scientific clues about incest.

    Other than that, he said, the couple's story simply excites human aversion to incest. "Just look to popular culture to understand why," he said. "It's sort of like a 'Star Wars' story that ends up with Luke Skywalker and Princess Leah marrying each other."

    Yet Dan Boucher, a spokesperson for Lord Alton, said the couple's tale might repeat itself as more people choose to conceive their children through sperm donors.

    "A donor can be used to conceive up to 10 children," Boucher said, and according to Alton's Web site up to 25 children have been conceived from a single donor. "That greatly increases the chances of something like this happening again."

    Offering two birth certificates to IVF children, he said, could help: One "long" version would indicate the genetic father as well as the mother, while a "short" version without such details could be used to maintain the person's privacy.

    Lesson Plan: Animal Patterns of Reproduction

    Image shows nine animals species that exhibit sexual dimorphism or unique reproductive strategies: elephant seals, seahorses, mallad ducks, Indian peafowl, Mormon cricket, honey bee, magnificent riflebird, clownfish, and slipper limpet.

    In this lesson, students learn briefly about the reproductive strategies nine different animal species. Student use a tally to track what strategies are most common - for example, male competition, female mate choice. More unique patterns like sex-changing clownfish and touch-mediated sex development are included within the nine species.

    When I taught this lesson, students learned about each species through short video recordings of teachers in our school describing the species. For more general use, I changed the lesson as posted on this website to have students do online research instead of watching the videos.

    Asexual and Sexual Reproduction in Animals | Zoology

    A new animal comes into existence by the transformation of some part of a pre-existing animal this is reproduction.

    The animals reproduce their kind by two fundamental methods:

    Asexual reproduction involves only one parent and no special reproductive structures. Sexual reproduc­tion usually involves two parents and the union of two germ cells, or of two cells of some kind, or of two nuclei derived from different cells.

    1. Asexual Reproduction:

    It occurs only in simpler and lower forms of animals. Verte­brates never reproduce asexually. The more important types of asexual reproduction are: binary fission, multiple fission, and budding.

    In this an individual divides into two equal halves after which each part grows to the original form. When occurring in a protozoon, the division of the cell body is always preceded by the division of the nucleus.

    Fission may occur in a transverse plane, as in Amoeba and Paramoecium, or it may be longi­tudinal, as in Euglena. The most important feature of fission is that the parent disappears as an individual, and the two new indivi­duals take its place.

    This is found only in protozoa where the nucleus fragments repeatedly and then the cytoplasm divides, so that a part of it surrounds each of the nuclear fragment. Each bit of nucleated fragment is virtually an asexually produced spore hence multiple fission is also known as sporulation. Multiple fission is usually preceded by encystment as in Amoeba. In Plas­modium, however, there is no encystment.

    When an animal divides into two unequal parts, it is said to reproduce by budding. The larger portion is regarded as the parent and the smaller one as the offspring. A bud usually arises as a small protuberance from the body of the parent it grows larger and develops similar parts. It may remain attached to the parent or it may separate and live independently. Budding is the usual method of reproduction in Hydra.

    2. Sexual Reproduction:

    Higher animals usually reproduce sexually each new organism originates from the union of two germ cells. These are the male and female gametes, and are known respectively as spermatozoa and ova.

    The gametes are produced in gonads, the ova in ovaries and the spermatozoa in testes. There is a great interest in the ultimate origin of the gametes, because they serve to transmit hereditary characters from the parent to the offspring.

    August Weismann (1834-1914) held that the gametes or germplasm are totally kept apart from the influence of other body cells or Somatoplasm, and are transmitted continuously from generation to generation. It has been definitely established that at least there is a continuity of the chromosomes by which the hereditary characters are transmitted from parents to offspring.

    The germ cells are derived from the germinal epithelium of the gonads. The primordial germ cells are known as spermatogonia in the male, and as oogonia in the female. At the approach of sexual maturity these cells multiply rapidly by mitosis and the nucleus of each cell contains a dual set of chromosomes, which is the diploid number and is represented as 2n.

    The process by which the spermatogonia become spermatozoa and the oogo­nia become ova is known as gametogenesis. It involves two suc­cessive divisions—one of which is a reduction division or meiosis. As the gametes of the two sexes differ in form, size, and behaviour, the gametogenesis, therefore, occur differently in the two but the meiotic changes associated with the formation of the gametes are identical.

    In male, the spermatogonia are converted into full-grown cells known as the primary spermatocytes, each of which has 2n chromosomes. Every primary spermato­cyte divides twice in quick succession. The first division is meio­tic, and produces two secondary spermatocytes, each having in chromosomes in its nucleus.

    During the following mitotic division each secondary spermato­cyte divides into two spermatids. Thus a primary spermatocyte yields four spermatids, each containing n chromosomes, which represents the haploid number.

    It may be pointed out here that from each pair homologous chromosomes in a primary spermatocyte, any one spermatid receives only one. The spermatids undergo a process of metamorphosis and are converted into the spermatozoa. The whole process is known as spermatogenesis.

    The spermatozoon is small and motile. It is usually thread-like in appearance and consists of a head, a mid­dle piece and a tail. The oval head represents the nucleus and the middle-piece encloses the central-body. The flagella-like wavy tail is composed of cytoplasm.

    In females, the oogenesis is slightly different. The oogonia are converted into primary oocytes with 2n chro­mosomes. Each primary oocyte undergoes two successive matu­ration divisions of which the first one is meiotic.

    In the first divi­sion it is converted into two unequal cells—a large secondary oocyte, and a small first polar body with very little cytoplasm. Later division results in the formation of a second polar body and the mature ovum.

    The polar bodies are useless and never function as gametes. Thus each primary oocyte yields but one mature ovum, containing n chromosomes yet the nuclear chan­ges that produce the ovum and the polar bodies are equivalent to those by which four spermatozoa are derived from a primary spermatocyte.

    The mature ovum is a spherical or oval, non-motile cell. It usually contains a variable quantity of yolk to nourish the future embryo. The hen’s ovum is about 30 mm. in diameter, whereas a toad’s ovum is only 2 mm.

    The human ovum contains very little yolk hence it is as small as 0.15 mm. in diameter. When the quantity of yolk present in the ovum is very large, it is usually located at one pole of the cell, the vegetative pole such an ovum is said to be telolecithal.

    If, on the other hand, the quantity of yolk is very small, the ovum is said to be alecithal or microlecithal. The ovum is surrounded by a thin vitelline membrane which is derived from the oocyte. There is a prominent nucleus embedded in the centre of the cytoplasm. A central body is absent.

    The union of a mature spermatozoon with a mature ovum is known as fertilization, and the resulting cell is a zygote. In fertilization, the fusion takes place primarily between the two nuclei, each contributing n chromosomes. This restores the normal diploid number (2n) of the species, in the nucleus of the zygote.

    Meeting of the ovum and spermatozoon always takes place in a fluid medium in order to permit the active move­ments of the motile male gamete. The ovum remains passive during the whole period. There is evidence to believe that the outer layer of the ovum secretes a chemical substance called fertilizing which attracts spermatozoa by positive chemo taxis.

    On reaching the ovum, the head of the spermatozoon penetrates through the vitelline membrane along with the middle-piece the tail, however, is left behind. A central body arises from the middle piece of the sperm and soon forms the spindle. The male and female pro-nuclei now break up into chromosomes which take their position in the equator of the spindle.

    During this process the homologous chromosomes lie side by side in pairs. In every such homologous pair, one chromosome is contributed by each pro-nucleus. The fertilized ovum or zygote now divides into cells, containing 2n chromosomes. The products of division are known as the blastomeres which ultimately develop into a young of the same species.

    Special Types of Sexual Reproduction:

    In some protozoa, sexual reproduction involves union bet­ween two similar cells which are known as isogametes. The pro­cess is known as conjugation and is found in Monocytes. In other protozoa, such as Plasmodium, the gametes are dissimilar and hence they are anisogametes. The smaller and motile microgamete fertilises the larger and passive macrogamete.

    In Paramoectum, two individuals conjugate temporarily and make an exchange of nuclear material. After this they separate and reproduce by binary fission. Parthenogenesis s Sexual reproduction usually involves two parents and the fusion of two germ cells.

    An ovum, however, may develop into a young individual without being fertilized by a sperm. This peculiar phenomenon is known as partheno­genesis (parthenos =*virgin genesis—origin).

    Parthenogenesis occurs normally in ants, bees, wasps and aphids. The female of these animals lay both fertilized and un­fertilized eggs. She can control fertilization by releasing or retain­ing the sperms which are stored in her spermatheca. In bees the males are derived from the unfertilized eggs, and the females from the fertilized eggs. In certain insects, having no males, re­production is entirely parthenogenetic.

    Parthenogenesis can be induced artificially in the egg of many animals which ordinarily require fertilization. Thus shaking the eggs vigorously, heating them, or pricking them with a needle, all have started development in certain eggs.

    Loeb, by pricking thousands of frog-eggs with pointed needles obtained over two hundred tadpoles, and reared nearly one hundred frogs. Recently a rabbit-egg has also been developed parthenogenetically to produce a young rabbit.

    Although parthenogenesis involves only one parent, yet by origin the cells giving rise to the new individuals are ova. It is for this reason that parthenogenesis is regarded as a kind of sexual reproduction.

    Sexual reproduction is usually carried on by adult animals. But this is not always the case. There are cer­tain species who have remarkable power of reproducing sexually when they are in the larval state.

    This peculiar sexual reproduction by a larval animal is known paedogenesis. The axolotl larva of the tiger salamander attains sexual maturity and breeds under cer­tain conditions. Larva of certain flies produce ova which may de­velop by parthenogenesis. This is an instance of parthenogenetic paedogenesis.

    Development in Animals:

    As a result of sexual reproduction a new individual starts life as a single cell. It is the fertilized or activated eggs, or zygote. The zygote by repeated mitotic division produces many cells which in due time differentiate into the tissues and organs of the developing embryo.

    This process is known as embryogeny and the branch of zoology which deals with the development of indivi­duals is known as embryology. The following is a generalized account of the early development of an embryo.

    Soon after an egg is fertilised, the single-celled zygote begins to divide by mitosis. The first division results in two cells, the two divide into four, and so on. The process is known as cleavage and it results in the segmentation of the egg into a large number of smaller cells, each of which is a blastomere.

    The solid cell mass, thus produced, is known as the morula. If the ovum is alecithal or microlecithal, the cleavage is total or holoblastic. If, on the other hand, the ovum is telolecithal, then the cleavage is. meroblastic and the blastomeres are unequal.

    As cleavage continues, the blastomeres become arranged in the form of a hollow ball, or blastula. The cavity of the blastula is designated as the blastocoele, or segmentation cavity.

    The blastula now undergoes gastrulation. This is a com­plicated process by which the hollow ball of cells is converted into a double walled cup, the gastrula. The blastocoele is gra­dually obliterated by pushing in of one side of the ball into the other, that is by invagination. The cavity of the cup constitutes the primitive gut or archenteron which communicates with the exterior by the blastopore.

    When complete, the gastrula con­sists of:

    (1) An outer layer of cells, the ectoderm,

    (2) An inner layer of cells, the endoderm, and between these two layers deve­lops

    (3) A third cell-layer, the mesoderm.

    These are the three primary germ layers which by modification and orientation produce the various tissues and organs. The ectoderm forms the outer covering of the body, the nervous system and the sense- organs.

    The endoderm is transformed into the lining membrane of the digestive canal and the other structures associated with it. The mesoderm produces the supporting tissues, vascular tissues, muscles, and epithelial lining of the body cavity.

    The preceding description of the formation of a new indivi­dual is more or less generalized. An outline of the development of a chick-embryo will now furnish a good example to illustrate the principles of embryology.

    Watch the video: Γάτες εν δράσει (February 2023).