There are four stages in the growth and development of animals. The first is gametogenesis, the formation of sperm and eggs. The second includes embryonic development, which begins with fertilization of the egg and continues to birth. The third stage is the process leading to reproductive maturity (puberty), and the fourth is the aging process to death. In general, the AP exam expects you to know features of only the first two developmental stages, gametogenesis and embryonic development. Gametogenesis was previously described, while a discussion of embryonic development follows.

Some animals resemble mature individuals of their species at birth, while other are physically dissimilar from the adult form. For example, tadpoles and caterpillars are larval forms of their respective adult forms, a frog and a flying insect. The larval form of a sea urchin, a pluteus, is a microscopic, free-floating animal that drifts as plankton until it attaches to a solid surface. Like other larvae, the pluteus undergoes additional development (metamorphosis) that transforms the larva into an adult.

In other animals, embryonic development continues until the birth of an infant that resembles the form of the adult. In mammals, the developmental period is often divided into two stages— embryonic development followed by fetal development. The fetus is an embryo that resembles the infant form.

Many of the events that occur during embryonic development are remarkably similar in all animals. Below, early developmental stages are described as they occur in the sea urchin. These stages are typical of development in most animals. However, some developmental events described occur only in certain groups of animals (for example, deuterostomes, chordates, amniotes, or vertebrates) or are specific to humans. These are clearly noted in the descriptions.

1. Fertilization. Fertilization occurs when the sperm penetrates the plasma membrane of the secondary oocyte. Fertilization is accompanied by the following steps:

• Recognition. Before penetration can occur, the sperm secretes a protein that binds with special receptor molecules that reside on a glycoprotein layer surrounding the plasma membrane of the oocyte. This vitelline layer (or zona pellucida in humans) insures that fertilization occurs only between egg and sperm of the same species.

• Penetration. The plasma membranes of the sperm and oocyte fuse, and the sperm nucleus enters the oocyte.

• Formation of the fertilization membrane. The vitelline layer forms a fertilization membrane which blocks the entrance of additional sperm.

• Completion of meiosis II in the secondary oocyte. In humans, sperm penetration triggers meiosis II in the oocyte, producing an ovum (egg) and polar body. The polar body is discharged through the plasma membrane.

• Fusion of nuclei and replication of DNA. The sperm and ovum nuclei fuse, forming a zygote nucleus consisting of 23 pairs of chromosomes (in humans). Each chromosome replicates so that it consists of two identical chromatids.

2. Cleavage. The zygote now begins a series of cleavage divisions, rapid cell divisions without cell growth. As a result, each of the resulting cells, called blastomeres, contain substantially less cytoplasm than the original zygote. Some characteristics of early cleavages follow:

• Embryo polarity. The egg has an upper, animal pole and a lower, vegetal pole. Cells formed at the vegetal pole contain more yolk, or stored food, because the yolk material, denser than the surrounding cytoplasm, settles to the bottom of the egg.

• Polar and equatorial cleavages. Early cleavages are polar, dividing the egg into segments that stretch from pole to pole (like sections of an orange). Other cleavages are parallel with the equator (perpendicular to the polar cleavages).

• Radial and spiral cleavages. In deuterostomes, early cleavages are radial, forming cells at the animal and vegetal poles that are aligned together, the top cells directly above the bottom cells. In protostomes, cleavages are spiral, forming cells on top that are shifted with respect to those below them.

• Indeterminate and determinate cleavages. A cleavage is indeterminate if it produces blastomeres that, if separated, can individually complete normal development. In contrast, blastomeres produced by a determinate cleavage cannot develop into a complete embryo if separated from other blastomeres. Instead, their developmental program is limited to the production of definite (or determined) cells that contribute to only a part of a complete embryo. Radial cleavages of deuterostomes are usually indeterminate, while spiral cleavages of protostomes are often determinate.

3. Morula. Successive cleavage divisions result in a solid ball of cells called a morula.

4. Blastula. As cell divisions continue, liquid fills the morula and pushes the cells out to form a circular cavity surrounded by a single layer of cells. This hollow sphere of cells is called the blastula, and the cavity is the blastocoel.

5. Gastrula. Formation of the gastrula, or gastrulation, occurs when a group of cells invaginate (move inward) into the blastula, forming a two-layered embryo with an opening from the outside into a center cavity. The following features are associated with the gastrula.

• Three germ layers. A third cell layer forms between the outer and inner layers of the invaginated embryo. These three cell layers, the ectoderm, mesoderm, and endoderm (outside, middle, and inside layers, respectively) are the primary germ layers from which all subsequent tissues develop.

• Archenteron. The center cavity formed by gastrulation is called the archenteron. It is completely surrounded by endoderm cells.

• Blastopore. The opening into the archenteron is the blastopore. It becomes the mouth (in protostomes) or the anus (in deuterostomes).

6. Extraembryonic membrane development. In birds, reptiles, and humans, collectively called the amniotes, extraembryonic (outside the embryo proper) membranes develop, as follows:

• Chorion. The chorion is the outer membrane. In birds and reptiles it acts as a membrane for gas exchange. In mammals, the chorion implants into the endometrium. Later, the chorion, together with maternal tissue, forms the placenta. The placenta is a blend of maternal and embryonic tissues across which gases, nutrients, and wastes are exchanged.

• Allantois. The allantois begins as a sac that buds off from the archenteron. Eventually, it encircles the embryo, forming a layer below the chorion. In birds and reptiles, it initially stores waste products (in the form of uric acid). Later in development, it fuses with the chorion, and together they act as a membrane for gas exchange with blood vessels below. In mammals, the allantois transports waste products to the placenta. After subsequent development, it forms the umbilical cord, transporting gases, nutrients, and wastes between the embryo and the placenta.

• Amnion. The amnion, for which this group of vertebrates is named, encloses the amniotic cavity, a fluid-filled cavity that cushions the developing embryo much like the coelom cushions internal organs in coelomate animals.

• Yolk sac. In birds and reptiles, the yolk sac membrane digests the enclosed yolk. Blood vessels transfer the nutrients to the developing embryo. In placental mammals, the yolk sac is empty. Instead, nutrition is obtained through the placenta.

7. Organogenesis. As cells continue to divide after gastrulation, they become different from one another (cell differentiation), taking on characteristics of specific tissues and organs. This development of organs is called organogenesis. The formation of the following organs are characteristic of the chordates:

• Notochord. Cells along the dorsal surface of the mesoderm germ layer form the notochord, a stiff rod that provides support in lower chordates. The vertebrae of higher chordates are formed from nearby cells in the mesoderm.

• Neural tube. In the ectoderm layer directly above the notochord, a layer of cells forms the neural plate. The plate indents, forming the neural groove, then rolls up into a cylinder, the neural tube. The neural tube develops into the central nervous system. Additional cells roll off the top of the developing neural tube and form the neural crest. These cells form various tissues, including teeth, bones, and muscles of the skull, pigment cells in the skin, and other tissues. The stages of embryonic development summarized above, from fertilization to gastrulation, are typical of a sea urchin (an echinoderm). Although these stages are general, important variations that occur among three animals commonly studied in developmental biology follow:

1. Frog (an amphibian)

• Gray crescent. When the sperm penetrates a frog egg, a reorganization of the cytoplasm results in the appearance of a gray, crescent-shaped region, called the gray crescent. Hans Spemann, in a famous experiment, separated the cells formed during early cleavages and showed that each individual cell could develop into a normal frog only if it contained a portion of the gray crescent.

• Gastrulation. During gastrulation, cells migrate over the top edge of the blastopore. The top edge, called the dorsal lip, forms from the same region earlier occupied by the gray crescent. The bottom and sides of the blastopore edge are called the ventral lip and lateral lips, respectively.

• Yolk. The yolk material is much more extensive in the frog than in the sea urchin. Cells from the vegetal pole rich in yolk material form a yolk plug near the dorsal lip.

2. Bird

• Blastodisc. The yolk of the bird egg is very large, and most of it is not involved in cleavages. Instead, the cleavages occur in a blastula that consists of a flattened, diskshaped region that sits on top of the yolk. This kind of blastula is called a blastodisc.

• Primitive streak. When gastrulation begins, invagination occurs along a line (rather than a circle) called the primitive streak. As cells migrate into the primitive streak, the crevice formed becomes an elongated blastopore (rather than a circular blastopore, as found in sea urchins and frogs).

3. Humans and most other mammals

• Blastocyst. The blastula stage, called the blastocyst, consists of two parts—an outer ring of cells, the trophoblast, and an inner mass of cells, the embryonic disc.

• Trophoblast. The outer ring of cells, or trophoblast, serves several functions. First, it accomplishes implantation by embedding into the endometrium of the uterus. Second, it produces human chorionic gonadotropin (HCG) to maintain progesterone production of the corpus luteum (which, in turn, will maintain the endometrium). Later, the trophoblast forms the chorion, the extraembryonic membrane that, together with maternal tissue, forms the placenta.

• Embryonic disc. Within the cavity created by the trophoblast, a bundle of cells called the inner cell mass (ICM), clusters at one pole and flattens into the embryonic disc. The embryonic disc is analogous to the blastodisc of birds and reptiles. A primitive streak develops, gastrulation follows, and development of the embryo and extraembryonic membranes (except the chorion) ensues.