13.11: Processes of Animal Reproduction and Development - Biology

13.11: Processes of Animal Reproduction and Development - Biology

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Learning Outcomes

  • Explain the processes of animal reproduction and embryonic development

During sexual reproduction, the haploid gametes of the male and female individuals of a species combine in a process called fertilization. This process produces a diploid fertilized egg called a zygote.

Some animal species—including sea stars and sea anemones, as well as some insects, reptiles, and fish—are capable of asexual reproduction. The most common forms of asexual reproduction for stationary aquatic animals include budding and fragmentation, where part of a parent individual can separate and grow into a new individual. In contrast, a form of asexual reproduction found in certain insects and vertebrates is called parthenogenesis (or “virgin beginning”), where unfertilized eggs can develop into new male offspring. This type of parthenogenesis is called haplodiploidy. These types of asexual reproduction produce genetically identical offspring, which is disadvantageous from the perspective of evolutionary adaptability because of the potential buildup of deleterious mutations. However, for animals that are limited in their capacity to attract mates, asexual reproduction can ensure genetic propagation.

After fertilization, a series of developmental stages occur during which primary germ layers are established and reorganize to form an embryo. During this process, animal tissues begin to specialize and organize into organs and organ systems, determining their future morphology and physiology. Some animals, such as grasshoppers, undergo incomplete metamorphosis, in which the young resemble the adult. Other animals, such as some insects, undergo complete metamorphosis where individuals enter one or more larval stages that may in differ in structure and function from the adult (Figure 1). For the latter, the young and the adult may have different diets, limiting competition for food between them. Regardless of whether a species undergoes complete or incomplete metamorphosis, the series of developmental stages of the embryo remains largely the same for most members of the animal kingdom.

The process of animal development begins with the cleavage, or series of mitotic cell divisions, of the zygote (Figure 2). Three cell divisions transform the single-celled zygote into an eight-celled structure. After further cell division and rearrangement of existing cells, a 6–32-celled hollow structure called a blastula is formed. Next, the blastula undergoes further cell division and cellular rearrangement during a process called gastrulation. This leads to the formation of the next developmental stage, the gastrula, in which the future digestive cavity is formed. Different cell layers (called germ layers) are formed during gastrulation. These germ layers are programmed to develop into certain tissue types, organs, and organ systems during a process called organogenesis.

Watch the following video to see how human embryonic development (after the blastula and gastrula stages of development) reflects evolution:

A link to an interactive elements can be found at the bottom of this page.

Developmental Biology - Gametogenesis, Spermatogenesis, Oogenesis, Fertilization

The process of sexual reproduction ensures the formation of a diploid zygote which could constitute the next generation. A zygote is a single celled structure. By an ontogenetic process the zygote undergoes various develop-mental phases resulting in multicellular embryonic organisation. These phases include cleavage, gastrulation,neurulation, organogenesis and the pe-riod of growth and histological differentiation. Inspite of the fact that or-ganisms vary in their structure, form and mode of life, the processes of embryogenesis, development and differention are remarakably similar in all metazoans. Till later stages of development a fundamental uniform pattern in development can be observed. The ontogenetic stages also reflect the historical development of species or phylogenetic development.

Realising the mode of formation of a young individual of the next gen-eration has always interested human mind. There is a recorded history of human natural curiosity in sexual reproduction from very early period. The ' Susruta samhita', a monumental Indian medical book, written during second or third century A.D., describes the development of a human child in the mother's womb.

The earliest recorded work had been done by Aristotle (384-322 BC). His classical work De Generatione Animalium is concerned with the gen-eration of animals. It describes the reproduction and development of many kinds of animals. In his another work ' De Historia Animalaium', Aristotle provides an account of the development of the hen's egg. He compared reproductive methods of different animals and provided a classification based on that. By observing the development of hen's egg he concluded that the development always proceeds from simple formless beginning to the complex organization of the adult. For this speculative idea he provided the name epigenesis. Through his remarkable observation and speculations Aritotle established ' embryology' as an independent field in science. Thus to-day he is regarded as the founder of the science of embryology.

After the period of early Greek thinkers this discipline once again got the attention of the scientists from the beginning of the 17 th century. Through the contributions made by various workers like Von Baer , E. Haeckel , O. Hertwig, E.B Wilson, Spemann, C.M Child, Maclean and others rapid advancements were being made in the understanding of de velopmental processes in animals. Modern embryology has utilised all tools made available from other branches of science and diversified into branches such as ' Experimental embryology' , ' Chemical embryology', ' Compara-tive embryology' and Descriptive embryology. Such studies have paved the way for meeting the challanges of to-day's world through works on clon-ing techniques, tissue culture, stem cell researches, ' in vitro' fertilisation, organ transplantations, regeneration, tissue grafting and other medical and non-medical fields.

  • The process by which mature individuals produce offspring is called reproduction.
  • Reproduction is a characteristic of all living organisms and prevents extinction of a species.
  • There are two types of reproduction: sexual and asexual reproduction.
  • Sexual reproduction involves the fusion of male and female gametes to form a zygote.
  • Asexual reproduction does not involve gametes.
  • Cell division starts with division of nucleus.
  • In the nucleus are a number of thread-like structures called chromosomes, which occur in pairs known as homologous chromosomes.
  • Each chromosome contains-genes that determine the characteristics of an organism.
  • The cells in each organism contains a specific number of chromosomes.

There are two types of cell division:

  • This takes place in all body cells of an organism to bring about increase in number of cells, resulting in growth and repair.
  • The number of chromosomes in daughter cells remain the same as that in the mother cell.
  • This type of cell division takes place in reproductive organs (gonads) to produce gametes.
  • The number of chromosomes in the gamete is half that in the mother cell.


  • Mitosis is divided into four main stages.
  • Prophase, Metaphase, Anaphase and Telophase.
  • These stages of cell division occur in a smooth and continuous pattern.


  • The term interphase is used to describe the state of the nucleus when the cell is just about to divide.
  • During this time the following take place:
    • Replication of genetic material so that daughter cells will have the same number of chromosomes as the parent cell.
    • Division of cell organelles such as mitochondria, ribosomes and centrioles.
    • Energy for cell division is synthesised and stored in form of Adenosine Triphosphate (ATP) to drive the cell through the entire process.
    • Chromosomes are seen as long, thin, coiled thread-like structures.
    • Nuclear membrane and nucleolus are intact.


    • The chromosomes shorten and thicken.
    • Each chromosome is seen to consist of a pair of chromatids joined at a point called centromere.
    • Centrioles (in animal cells) separate and move to opposite poles of the cell.
    • The centre of the nucleus is referred to as the equator.
    • Spindle fibres begin to form, and connect the centriole pairs to the opposite poles.
    • The nucleolus and nuclear membrane disintegrate and disappear.


    • Spindle fibres lengthen.
    • In animal cells they attach to the centrioles at both poles.
    • Each chromosome moves to the equatorial plane and is attached to the spindle fibres by the centromeres.
    • Chromatids begin to separate at the centromere.


    • Chromatids separate and migrate to the opposite poles due to the shortening of spindle fibres.
    • Chromatids becomes a chromosome.
    • In animal cell, the cell membrane starts to constrict.


    • The cell divides into two.
    • In animal cells it occurs through cleavage of cell membrane.
    • In plants cells, it is due to deposition of cellulose along the equator of the cell.(Cell plate formation).
    • A nuclear membrane forms around each set of chromosome.
    • Chromosomes later become less distinct.

    Significance of Mitosis

    • It brings about the growth of an organism:
    • It brings about asexual reproduction.
    • Ensures that the chromosome number is retained.
    • Ensures that the chromosomal constitution of the offspring is the same as the parents.


    • Meiosis involves two divisions of the parental cell resulting into four daughter cells.
    • The mother cell has the diploid number of chromosomes.
    • The four cells (gametes) have half the number of chromosomes (haploid) that the mother cell had.
    • In the first meiotic division there is a reduction in the chromosome number because homologous chromosomes and not chromatids separate.
    • Each division has four stages Prophase, Metaphase, Anaphase and Telophase.


    • As in mitosis the cell prepares for division.
    • This involves replication of chromosomes, organelles and build up of energy to be used during the meiotic division.

    First Meiotic division

    Prophase I

    • Homologous chromosomes lie side by side in the process of synapsis forming pairs called bivalents.
    • Chromosomes shorten and thicken hence become more visible.
    • Chromosomes may become coiled around each other and the chromatids may remain in contact at points called chiasmata (singular chiasma).
    • Chromatids cross-over at the chiasmata exchanging chromatid portions.
    • Important genetic changes usually result.

    Metaphase I

    • Spindle fibres are fully formed and attached to the centromeres.
    • The bivalents move to the equator of the spindles.

    Anaphase I

    • Homologous chromosomes separate and migrate to opposite poles.
    • This is brought about by shortening of spindle fibres hence pulling the chromosomes.
    • The number of chromosomes at each pole is half the number in the mother cell.

    Telophase I

    Second Meiotic Division

    • Usually the two daughter cells go into a short resting stage (interphase)
    • but sometimes the chromosomes remain condensed and the daughter cells go straight into metaphase of second meiotic division.
    • The second meiotic division takes place just like mitosis.

    Prophase II

    Metaphase II

    • Spindle forms and are attached to the chromatids at the centromeres.
    • Chromatids move to the equator.

    Anaphase II

    • Sister chromatids separate from each other
    • Then move to opposite poles, pulled by the shortening of the spindle fibres.

    Telophase II

    • The spindle apparatus disappears.
    • The nucleolus reappears and nuclear membrane is formed around each set of chromatids.
    • The chromatids become chromosomes.
    • Cytoplasm divides and four daughter cells are formed.
    • Each has a haploid number of chromosomes.

    Significance of Meiosis

    • Meiosis brings about formation of gametes that contain half the number of chromosomes as the parent cells.
    • It helps to restore the diploid chromosomal constitution in a species at fertilisation.
    • It brings about new gene combinations that lead to genetic variation in the offsprings.


    Budding is a form of asexual reproduction that results from the outgrowth of a part of the body leading to a separation of the “bud” from the original organism and the formation of two individuals, one smaller than the other. Budding occurs commonly in some invertebrate animals such as hydras and corals. In hydras, a bud forms that develops into an adult and breaks away from the main body (Figure 13.3).

    Figure 13.3 (a) Hydra reproduce asexually through budding: a bud forms on the tubular body of an adult hydra, develops a mouth and tentacles, and then detaches from its parent. The new hydra is fully developed and will find its own location for attachment. (b) Some coral, such as the Lophelia pertusa shown here, can reproduce through budding. (credit b: modification of work by Ed Bowlby, NOAA/Olympic Coast NMS NOAA/OAR/Office of Ocean Exploration) Part a: This shows a hydra, which has a stalk-like body with tentacles growing out the top. A smaller hydra is budding from the side of the stalk. Part b: This photo shows branching white coral polyps.

    Complex Tissue Structure

    Animals, besides Parazoa (sponges), are characterized by specialized tissues such as muscle, nerve, connective, and epithelial tissues.

    Learning Objectives

    List the various specialized tissue types found in animals and describe their functions

    Key Takeaways

    Key Points

    • Animal cells don’t have cell walls their cells may be embedded in an extracellular matrix and have unique structures for intercellular communication.
    • Animals have nerve and muscle tissues, which provide coordination and movement these are not present in plants and fungi.
    • Complex animal bodies demand connective tissues made up of organic and inorganic materials that provide support and structure.
    • Animals are also characterized by epithelial tissues, like the epidermis, which function in secretion and protection.
    • The animal kingdom is divided into Parazoa (sponges), which do not contain true specialized tissues, and Eumetazoa (all other animals), which do contain true specialized tissues.

    Key Terms

    • Parazoa: a taxonomic subkingdom within the kingdom Animalia the sponges
    • Eumetazoa: a taxonomic subkingdom, within kingdom Animalia all animals except the sponges
    • epithelial tissue: one of the four basic types of animal tissue, which line the cavities and surfaces of structures throughout the body, and also form many glands

    Complex Tissue Structure

    As multicellular organisms, animals differ from plants and fungi because their cells don’t have cell walls their cells may be embedded in an extracellular matrix (such as bone, skin, or connective tissue) and their cells have unique structures for intercellular communication (such as gap junctions). In addition, animals possess unique tissues, absent in fungi and plants, which allow coordination (nerve tissue) and motility (muscle tissue). Animals are also characterized by specialized connective tissues that provide structural support for cells and organs. This connective tissue constitutes the extracellular surroundings of cells and is made up of organic and inorganic materials. In vertebrates, bone tissue is a type of connective tissue that supports the entire body structure. The complex bodies and activities of vertebrates demand such supportive tissues. Epithelial tissues cover, line, protect, and secrete these tissues include the epidermis of the integument: the lining of the digestive tract and trachea. They also make up the ducts of the liver and glands of advanced animals.

    The animal kingdom is divided into Parazoa (sponges) and Eumetazoa (all other animals). As very simple animals, the organisms in group Parazoa (“beside animal”) do not contain true specialized tissues. Although they do possess specialized cells that perform different functions, those cells are not organized into tissues. These organisms are considered animals since they lack the ability to make their own food. Animals with true tissues are in the group Eumetazoa (“true animals”). When we think of animals, we usually think of Eumetazoans, since most animals fall into this category.

    Sponges: Sponges, such as those in the Caribbean Sea, are classified as Parazoans because they are very simple animals that do not contain true specialized tissues.

    The different types of tissues in true animals are responsible for carrying out specific functions for the organism. This differentiation and specialization of tissues is part of what allows for such incredible animal diversity. For example, the evolution of nerve tissues and muscle tissues has resulted in animals’ unique ability to rapidly sense and respond to changes in their environment. This allows animals to survive in environments where they must compete with other species to meet their nutritional demands.

    Reproductive systems of invertebrates

    Although asexual reproduction occurs in many invertebrate species, most reproduce sexually. The basic unit of sexual reproduction is a gamete (sperm or egg), produced by specialized tissues or organs called gonads. Sexual reproduction does not necessarily imply copulation or even a union of gametes. As might be expected of such a large and diverse group as the invertebrates, many variations have evolved to ensure survival of species. In many lower invertebrates, gonads are temporary organs in higher forms, however, they are permanent. Some invertebrates have coexistent female and male gonads in others the same gonad produces both sperm and eggs. Animals in which both sperm and eggs are produced by the same individual (hermaphroditism) are termed monoecious. In dioecious species, the sexes are separate. Generally, the male gonads ripen first in hermaphroditic animals (protandry) this tends to ensure cross-fertilization. Self-fertilization is normal, however, in many species, and some species undergo sex reversal.

    Animal Reproduction Science

    Animal Reproduction Science publishes results from studies relating to reproduction and fertility in animals. This includes both fundamental research and applied studies, including management practices that increase our understanding of the biology and manipulation of reproduction. Manuscripts should.

    Animal Reproduction Science publishes results from studies relating to reproduction and fertility in animals. This includes both fundamental research and applied studies, including management practices that increase our understanding of the biology and manipulation of reproduction. Manuscripts should go into depth in the mechanisms involved in the research reported, rather than give a mere description of findings. Results and conclusions should contribute to improving the management of an animal species or population, with regard to its fertility or reproductive efficiency. Results and conclusions should contribute to improving the management of an animal species or population, with regard to its fertility or reproductive efficiency. The focus is on animals that are useful to humans including food- and fibre-producing companion/recreational captive and endangered species including zoo animals, but excluding laboratory animals unless the results of the study provide new information that impacts the basic understanding of the biology or manipulation of reproduction.

    The journal's scope includes the study of reproductive physiology and endocrinology, reproductive cycles, natural and artificial control of reproduction, preservation and use of gametes and embryos, pregnancy and parturition, infertility and sterility, diagnostic and therapeutic techniques.
    The Editorial Board of Animal Reproduction Science has decided not to publish papers in which there is an exclusive examination of the in vitro development of oocytes and embryos however, there will be consideration of papers that include in vitro studies where the source of the oocytes and/or development of the embryos beyond the blastocyst stage is part of the experimental design.

    Submission is encouraged of manuscripts that are focused on reproduction in aquatic animals. Manuscripts focused on reproduction in insects, however, do not fit the scope of the Journal and will be rejected without peer review.

    Authors with any concerns are encouraged to contact the Editor-in-Chief to enquire about the suitability of the content of their paper for submission. There are no page charges for manuscripts published in Animal Reproduction Science and publication of papers only takes place after rigorous peer review.

    Concept in Action

    Visit the Virtual Human Embryo project at the Endowment for Human Development site to click through an interactive of the stages of embryo development, including micrographs and rotating 3-D images.

    The cells in the blastula then rearrange themselves spatially to form three layers of cells. This process is called gastrulation. During gastrulation, the blastula folds in on itself and cells migrate to form the three layers of cells (Figure 13.10) in a structure, the gastrula, with a hollow space that will become the digestive tract. Each of the layers of cells is called a germ layer and will differentiate into different organ systems.

    Figure 13.10 Gastrulation is the process wherein the cells in the blastula rearrange themselves to form the germ layers. (credit: modification of work by Abigail Pyne)

    The three germ layers are the endoderm, the ectoderm, and the mesoderm. Cells in each germ layer differentiate into tissues and embryonic organs. The ectoderm gives rise to the nervous system and the epidermis, among other tissues. The mesoderm gives rise to the muscle cells and connective tissue in the body. The endoderm gives rise to the gut and many internal organs.

    14.8 Mammalian reproductive organs and gametes

    1. Cat ovary (Figure 14.12)
      • Identify primary, secondary and Graffian follicles
    2. Graafian follicle (Figure 14.14).
    3. Cat ovary Corpus Luteum
      • ocate corpus luteum
    4. Uterus (Figure 14.13)
      • Identfy: endometrium, myometrium, perimetrium
    5. Monkey testis (Figure 14.15)
      • Locate: semimiferous tubules, spermatozoa
    6. Human testis (Figure 14.16)
      • Locate: semimiferous tubules, spermatozoa
    7. Human sperm smear (Figure 14.17)
      • Identify: sperm, sperm head, sperm neck and sperm tail

    Figure 14.14: Graafian follicle in cat ovary.

    Figure 14.15: Monkey testis.


    43.1 Reproduction Methods

    Reproduction may be asexual when one individual produces genetically identical offspring, or sexual when the genetic material from two individuals is combined to produce genetically diverse offspring. Asexual reproduction occurs through fission, budding, and fragmentation. Sexual reproduction may mean the joining of sperm and eggs within animals’ bodies or it may mean the release of sperm and eggs into the environment. An individual may be one sex, or both it may start out as one sex and switch during its life, or it may stay male or female.

    Sexual reproduction starts with the combination of a sperm and an egg in a process called fertilization. This can occur either outside the bodies or inside the female. Both methods have advantages and disadvantages. Once fertilized, the eggs can develop inside the female or outside. If the egg develops outside the body, it usually has a protective covering over it. Animal anatomy evolved various ways to fertilize, hold, or expel the egg. The method of fertilization varies among animals. Some species release the egg and sperm into the environment, some species retain the egg and receive the sperm into the female body and then expel the developing embryo covered with shell, while still other species retain the developing offspring through the gestation period.

    43.3 Human Reproductive Anatomy and Gametogenesis

    As animals became more complex, specific organs and organ systems developed to support specific functions for the organism. The reproductive structures that evolved in land animals allow males and females to mate, fertilize internally, and support the growth and development of offspring. Processes developed to produce reproductive cells that had exactly half the number of chromosomes of each parent so that new combinations would have the appropriate amount of genetic material. Gametogenesis, the production of sperm (spermatogenesis) and eggs (oogenesis), takes place through the process of meiosis.

    43.4 Hormonal Control of Human Reproduction

    The male and female reproductive cycles are controlled by hormones released from the hypothalamus and anterior pituitary as well as hormones from reproductive tissues and organs. The hypothalamus monitors the need for the FSH and LH hormones made and released from the anterior pituitary. FSH and LH affect reproductive structures to cause the formation of sperm and the preparation of eggs for release and possible fertilization. In the male, FSH and LH stimulate Sertoli cells and interstitial cells of Leydig in the testes to facilitate sperm production. The Leydig cells produce testosterone, which also is responsible for the secondary sexual characteristics of males. In females, FSH and LH cause estrogen and progesterone to be produced. They regulate the female reproductive system which is divided into the ovarian cycle and the menstrual cycle. Menopause occurs when the ovaries lose their sensitivity to FSH and LH and the female reproductive cycles slow to a stop.

    43.5 Human Pregnancy and Birth

    Human pregnancy begins with fertilization of an egg and proceeds through the three trimesters of gestation. The labor process has three stages (contractions, delivery of the fetus, expulsion of the placenta), each propelled by hormones. The first trimester lays down the basic structures of the body, including the limb buds, heart, eyes, and the liver. The second trimester continues the development of all of the organs and systems. The third trimester exhibits the greatest growth of the fetus and culminates in labor and delivery. Prevention of a pregnancy can be accomplished through a variety of methods including barriers, hormones, or other means. Assisted reproductive technologies may help individuals who have infertility problems.

    43.6 Fertilization and Early Embryonic Development

    The early stages of embryonic development begin with fertilization. The process of fertilization is tightly controlled to ensure that only one sperm fuses with one egg. After fertilization, the zygote undergoes cleavage to form the blastula. The blastula, which in some species is a hollow ball of cells, undergoes a process called gastrulation, in which the three germ layers form. The ectoderm gives rise to the nervous system and the epidermal skin cells, the mesoderm gives rise to the muscle cells and connective tissue in the body, and the endoderm gives rise to columnar cells and internal organs.

    43.7 Organogenesis and Vertebrate Formation

    Organogenesis is the formation of organs from the germ layers. Each germ layer gives rise to specific tissue types. The first stage is the formation of the neural system in the ectoderm. The mesoderm gives rise to somites and the notochord. Formation of vertebrate axis is another important developmental stage.

    Watch the video: Animal Reproduction u0026 Development Notes (February 2023).