Fertilization And Early Development

In animals, fertilization is the union of female and male gametes to form a zygote. Fertilization results in the combination of the haploid sets of chromosomes from two individuals into a single diploid zygote.

Gametes

In most animal species, the sperm cell, shown in Figure 32-11, is specialized for movement—it is very streamlined and small. The head of the sperm contains chromosomes, and the tail of the sperm is composed of a long flagellum.

The egg, also shown in Figure 32-11, is typically large because it has a large store of cytoplasm and yolk. The size of an egg produced by a given species seems to depend on how long the food supply in the yolk must last. For aquatic animals in which the embryo begins to feed itself early, eggs are small, and there is little yolk. In sharp contrast, the embryos of birds must live on the yolk until they hatch. In these eggs, the yolk volume is very large.

Fertilization

At the start of fertilization, the sperm's cell membrane fuses with the egg's cell membrane, and the nucleus of the sperm enters the cytoplasm of the egg. The fusion of the cell membranes of the egg and sperm causes an electrical change in the egg membrane that blocks entry to the egg by other sperm cells. The sperm nucleus merges with the egg nucleus to form the diploid nucleus of the zygote. Once a zygote is formed, replication of DNA begins, and the first cell division soon follows.

objectives

• List the steps of fertilization and development through gastrulation.

• List two body parts formed from each germ layer.

• Identify the three different body cavity structures of animals.

• Name the categories of animals that undergo spiral cleavage and radial cleavage.

• Contrast the two processes of coelom formation.

vocabulary fertilization cleavage blastula gastrulation gastrula archenteron blastopore ectoderm endoderm mesoderm acoelomate pseudocoelom coelom protostome deuterostome schizocoely enterocoely figure 32-11

The small, flagellated sperm is adapted for motility and speed. It must seek out and fertilize the much larger, yolk-filled egg.

figure 32-11

The small, flagellated sperm is adapted for motility and speed. It must seek out and fertilize the much larger, yolk-filled egg.

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