31 Chapter 31: Human and Animal Development

Learning Objectives

By the end of this section, you should be able to:

LO31.1 Describe how the form/structure of egg and sperm cells (generally) support their functions related to fertilization.

LO31.2 Compare different ways to prevent fertilization and polyspermy.

LO31.3 Distinguish between the three different stages of embryonic development (cleavage, gastrulation, and neurulation) and the outcome of each stage.

LO31.4 Construct a model demonstrating how sexual differentiation is not binary (be sure to use examples).

LO31.5 Discuss the relationship between biology and society regarding variation in sexual differentiation and development.


Fertilization, pictured in 31.1a is the process in which gametes (an egg and sperm) fuse to form a zygote. The egg and sperm each contain one set of chromosomes. To ensure that the offspring has only one complete diploid set of chromosomes, only one sperm must fuse with one egg. In mammals, the egg is protected by a layer of extracellular matrix consisting mainly of glycoproteins called the zona pellucida. When a sperm binds to the zona pellucida, a series of biochemical events, called the acrosomal reactions, take place. In placental mammals, the acrosome contains digestive enzymes that initiate the degradation of the glycoprotein matrix protecting the egg and allowing the sperm plasma membrane to fuse with the egg plasma membrane, as illustrated in Figure 31.1b. The fusion of these two membranes creates an opening through which the sperm nucleus is transferred into the ovum. The nuclear membranes of the egg and sperm break down and the two haploid genomes condense to form a diploid genome.

Part A is a micrograph that shows a sperm whose head is touching the surface of an egg. The egg is much larger than the sperm. Part B is an illustration that shows the surface of the egg, which is coated with a zona pellucida. The sperm penetrates the zona pellucida, then the egg plasma membrane and releases its D N A into the egg cytoplasm. At this point, changes in proteins just inside the egg's cell membrane occur, preventing entry of other sperm.
Figure 31.1 (a) Fertilization is the process in which sperm and egg fuse to form a zygote. (b) Acrosomal reactions help the sperm degrade the glycoprotein matrix protecting the egg and allow the sperm to transfer its nucleus. (credit: (b) modification of work by Mariana Ruiz Villareal; scale-bar data from Matt Russell)

To ensure that no more than one sperm fertilizes the egg, once the acrosomal reactions take place at one location of the egg membrane, the egg releases proteins in other locations to prevent other sperm from fusing with the egg. Ernest Everett Just, who was particularly skilled and knowledgeable in handling invertebrate eggs, demonstrated several mechanisms at work at different rates. The fast reaction included what Just called a “wave of negativity,” in which the membrane potential of the egg cell altered quickly. Then, the slow block involved changing the membrane structure itself. If these mechanisms fail, multiple sperm can fuse with the egg, resulting in polyspermy. The resulting embryo is not genetically viable and dies within a few days.

Reading Question #1

What is the name of the reactions that occur once a sperm reaches and binds to the zona pellucida?

a) fertilization reactions

b) fusion reactions

c) acrosomal reactions

d) zona pellucida reactions

Cleavage and Blastula Stage

The development of multi-cellular organisms begins from a single-celled zygote, which undergoes rapid cell division to form the blastula. The rapid, multiple rounds of cell division are termed cleavage. Cleavage is illustrated in (Figure 30.2a). After the cleavage has produced over 100 cells, the embryo is called a blastula. The blastula is usually a spherical layer of cells (the blastoderm) surrounding a fluid-filled or yolk-filled cavity (the blastocoel). Mammals at this stage form a structure called the blastocyst, characterized by an inner cell mass that is distinct from the surrounding blastula, shown in Figure 30.2b. During cleavage, the cells divide without an increase in mass; that is, one large single-celled zygote divides into multiple smaller cells. Each cell within the blastula is called a blastomere.

Part A illustration shows a fertilized egg divided into two, four, eight, sixteen and thirty-two cells. Part B shows a hollow ball of cells. The cells on the surface are called the blastoderm, and the hollow center is called the blastocoel.
Figure 30.2 (a) During cleavage, the zygote rapidly divides into multiple cells without increasing in size. (b) The cells rearrange themselves to form a hollow ball with a fluid-filled or yolk-filled cavity called the blastula.

Cleavage can take place in two ways: holoblastic (total) cleavage or meroblastic (partial) cleavage. The type of cleavage depends on the amount of yolk in the eggs. In placental mammals (including humans) where nourishment is provided by the parent’s body, the eggs have a very small amount of yolk and undergo holoblastic cleavage. Other species, such as birds, with a lot of yolk in the egg to nourish the embryo during development, undergo meroblastic cleavage.

In mammals, the blastula forms the blastocyst in the next stage of development. Here the cells in the blastula arrange themselves in two layers: the inner cell mass, and an outer layer called the trophoblast. The inner cell mass is also known as the embryoblast and this mass of cells will go on to form the embryo. At this stage of development, illustrated in Figure 30.3 the inner cell mass consists of embryonic stem cells that will differentiate into the different cell types needed by the organism. The trophoblast will contribute to the placenta and nourish the embryo.

Illustration shows a hollow ball of cells with an inner cell mass clustered to one side. The exterior is called the trophoblast.
Figure 30.3 The rearrangement of the cells in the mammalian blastula to two layers—the inner cell mass and the trophoblast—results in the formation of the blastocyst.


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


The typical blastula is a ball of cells. The next stage in embryonic development is the formation of the body plan. The cells in the blastula rearrange themselves spatially to form three layers of cells. This process is called gastrulation. During gastrulation, the blastula folds upon itself to form the three layers of cells. Each of these layers is called a germ layer and each germ layer differentiates into different organ systems.

The three germ layers, shown in Figure 30.4, are the endoderm, the ectoderm, and the mesoderm. The ectoderm gives rise to the nervous system and the epidermis. The mesoderm gives rise to the muscle cells and connective tissue in the body. The endoderm gives rise to columnar cells found in the digestive system and many internal organs.

Illustration shows cells associated with the internal endoderm, the middle mesoderm, and the external ectoderm. Lung, thyroid and digestive tissues are associated with the endoderm. Muscle, kidney and blood cells are associated with the mesoderm. Skin, neurons, and pigment cells are associated with the ectoderm.
Figure 30.4 The three germ layers give rise to different cell types in the animal body. (credit: modification of work by NIH, NCBI)


Are Designer Babies in Our Future?

Illustration shows a tree with words such as genetics, statistics, medicine, economics, and genealogy associated with the roots. The word eugenics is emblazoned across the upper trunk. To the side of the tree is the text that reads, Eugenics is the self-direction of human evolution.
Figure 30.5 This logo from the Second International Eugenics Conference in New York City in September of 1921 shows how eugenics attempted to merge several fields of study with the goal of producing a genetically superior human race.

If you could prevent your child from getting a devastating genetic disease, would you do it? Would you select the sex of your child or select for their attractiveness, strength, or intelligence? How far would you go to maximize the possibility of resistance to disease? The genetic engineering of a human child, the production of “designer babies” with desirable phenotypic characteristics, was once a topic restricted to science fiction. This is the case no longer: science fiction is now overlapping into science fact. Many phenotypic choices for offspring are already available, with many more likely to be possible in the not too distant future. Which traits should be selected and how they should be selected are topics of much debate within the worldwide medical community. The ethical and moral line is not always clear or agreed upon, and some fear that modern reproductive technologies could lead to a new form of eugenics.

Eugenics is the use of information and technology from a variety of sources to improve the genetic makeup of the human race. The goal of creating genetically superior humans was quite prevalent (although controversial) in several countries during the early 20th century, but fell into disrepute when Nazi Germany developed an extensive eugenics program in the 1930s and 40s. The Nazis forcibly sterilized hundreds of thousands of the so-called “unfit” and killed tens of thousands of people with disabilities who resided in institutions, both as part of a systematic program to develop a genetically superior race of Germans known as Aryans. Ever since, eugenic ideas have not been as publicly expressed, but there are still those who promote them.

Efforts have been made in the past to control traits in human children using donated sperm from people with desired traits. In fact, eugenicist Robert Klark Graham established a sperm bank in 1980 that included samples exclusively from donors with high IQs. The “genius” sperm bank failed to capture the public’s imagination and the operation closed in 1999.

In more recent times, the procedure known as prenatal genetic diagnosis (PGD) has been developed. PGD involves the screening of human embryos as part of the process of in vitro fertilization, during which embryos are conceived and grown outside the mother’s body for some period of time before they are implanted. The term PGD usually refers to both the diagnosis, selection, and the implantation of the selected embryos.

In the least controversial use of PGD, embryos are tested for the presence of alleles which cause genetic diseases such as sickle cell disease, muscular dystrophy, and hemophilia, in which a single disease-causing allele or pair of alleles has been identified. By excluding embryos containing these alleles from implantation into the mother, the disease is prevented, and the unused embryos are either donated to science or discarded. There are relatively few in the worldwide medical community that question the ethics of this type of procedure, which allows individuals scared to have children because of the alleles they carry to do so successfully. The major limitation to this procedure is its expense. Not usually covered by medical insurance and thus out of reach financially for most people, only a very small percentage of all live births use such complicated methodologies. Yet, even in cases like these where the ethical issues may seem to be clear-cut, not everyone agrees with the morality of these types of procedures. For example, to those who take the position that human life begins at conception, the discarding of unused embryos, a necessary result of PGD, is unacceptable under any circumstances.

A murkier ethical situation is found in the selection of a child’s sex, which is easily performed by PGD. Currently, countries such as Great Britain have banned the selection of a child’s sex for reasons other than preventing sex-linked diseases. Other countries allow the procedure for “family balancing”, based on the desire of some parents to have at least one child of each sex. Still others, including the United States, have taken a scattershot approach to regulating these practices, essentially leaving it to the individual practicing physician to decide which practices are acceptable and which are not.

Even murkier are rare instances of people with disabilities who select embryos via PGD to ensure that they share their disability. These parents usually cite many positive aspects of their disabilities and associated culture as reasons for their choice, which they see as their moral right. To others, to purposely cause a disability in a child violates the basic medical principle of Primum non nocere, “first, do no harm.” This procedure, although not illegal in most countries, demonstrates the complexity of ethical issues associated with choosing genetic traits in offspring.

Where could this process lead? Will this technology become more affordable and how should it be used? With the ability of technology to progress rapidly and unpredictably, a lack of definitive guidelines for the use of reproductive technologies before they arise might make it difficult for legislators to keep pace once they are in fact realized, assuming the process needs any government regulation at all. Other bioethicists argue that we should only deal with technologies that exist now, and not in some uncertain future. They argue that these types of procedures will always be expensive and rare, so the fears of eugenics and “master” races are unfounded and overstated. The debate continues.

Reading Question #2

What is the name of the process that results in the formation of three germ layers?

a) cleavage

b) blastula

c) gastrulation

d) fertilization

Reading Question #3

Which of the three germ layers will eventually give rise to the heart?

a) ectoderm

b) mesoderm

c) endoderm



Organs form from the germ layers through the process of differentiation. During differentiation, the embryonic stem cells express specific sets of genes which will determine their ultimate cell type. For example, some cells in the ectoderm will express the genes specific to skin cells. As a result, these cells will differentiate into epidermal cells. The process of differentiation is regulated by cellular signaling cascades.

Scientists study organogenesis extensively in the lab in fruit flies (Drosophila) and the nematode Caenorhabditis elegansDrosophila have segments along their bodies, and the patterning associated with the segment formation has allowed scientists to study which genes play important roles in organogenesis along the length of the embryo at different time points. The nematode C.elegans has roughly 1000 somatic cells and scientists have studied the fate of each of these cells during their development in the nematode life cycle. There is little variation in patterns of cell lineage between individuals, unlike in mammals where cell development from the embryo is dependent on cellular cues.

In vertebrates, one of the primary steps during organogenesis is the formation of the neural system. The ectoderm forms epithelial cells and tissues, and neuronal tissues. During the formation of the neural system, special signaling molecules called growth factors signal some cells at the edge of the ectoderm to become epidermis cells. The remaining cells in the center form the neural plate. If the signaling by growth factors were disrupted, then the entire ectoderm would differentiate into neural tissue.

The neural plate undergoes a series of cell movements where it rolls up and forms a tube called the neural tube, as illustrated in Figure 30.6. In further development, the neural tube will give rise to the brain and the spinal cord.

Illustration shows a flat sheet. The middle of the sheet is the neural plate, and the epidermis is at either end. The neural plate border separates the neural plate from the epidermis. During convergence the plate folds, bringing the neural folds together. The neural folds fuse, forming the neural plate into a neural tube. The epidermis separates and folds around the outside.
Figure 30.6 The central region of the ectoderm forms the neural tube, which gives rise to the brain and the spinal cord.

The mesoderm that lies on either side of the vertebrate neural tube will develop into the various connective tissues of the animal body. A spatial pattern of gene expression reorganizes the mesoderm into groups of cells called somites with spaces between them. The somites illustrated in Figure 30.7 will further develop into the cells that form the vertebrae and ribs, the dermis of the dorsal skin, the skeletal muscles of the back, and the skeletal muscles of the body wall and limbs. The mesoderm also forms a structure called the notochord, which is rod-shaped and forms the central axis of the animal body.

Embryo resembles a segmented earthworm with a bulging head.
Figure 30.7 In this five-week old human embryo, somites are segments along the length of the body. (credit: modification of work by Ed Uthman)

Reading Question #4

Which of the following will eventually give rise to the brain and spinal cord?

a) mesoderm

b) neural tube

c) neural plate

d) endoderm

Reading Question #5

The process of differentiating cells to form organs is called

a) cleavage

b) gastrulation

c) organogenesis

d) fertilization


Adapted from:

Adapted from Clark, M.A., Douglas, M., and Choi, J. (2018). Biology 2e. OpenStax. Retrieved from https://openstax.org/books/biology-2e/pages/43-introduction


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Introductory Biology 2 Copyright © 2023 by Lisa Limeri and Joshua Reid is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.

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