During cleavage, the zygote divides repeatedly without undergoing cell growth, producing a many-celled hollow blastula.
The series of cell divisions that occurs immediately following fertilization is termed cleavage. Figure 32-12, steps O, ©, and © show that as cleavage progresses, the number of cells increases, from 2 to 4, then to 8, and so on. During cleavage, mitotic divisions rapidly increase the number of cells, but the cells do not grow in size. Thus, cleavage yields smaller and smaller individual cells. Cleavage increases the surface area-to-volume ratio of each cell, which enhances gas exchange and other environmental interactions.
In most species, cleavage produces a raspberry-shaped mass of 16 to 64 cells, as shown in step Q. As the number of dividing cells further increases, the mass becomes a hollow ball of cells called a blastula, shown in step G. The central cavity of a blastula is called the blastocoel (BLAS-toe-SEEL).
At the start of the next stage of development, shown in Figure 32-13, an area of the blastula begins to collapse inward. As shown in steps O and ©, reorganization of the cells of the hollow blastula begins with the inward movement of cells at one end of the blastula. This process, called gastrulation, transforms the blastula into a multilayered embryo, called the gastrula, shown in step ©. Gastrulation is marked by changes in the shape of cells and the way the cells interact with each other.
As the inward folding continues, the now cup-shaped embryo enlarges, and a deep cavity, called the archenteron, or primitive gut, develops. The open end of the archenteron is called the blastopore. Forming the outer layer of the gastrula is the outer germ layer, the ectoderm, shown in blue in step ©. The inner germ layer, the endoderm, is shown in yellow. In most phyla, the gas-trula does not remain a two-layer structure. As development progresses, a third layer, the mesoderm, forms between the endoderm and the ectoderm.
Each of the germ layers formed during gastrulation develops into certain organs in a process called organogenesis. The endo-derm forms the lining of the urinary system, the reproductive system, and most of the digestive tract; it also forms the pancreas, liver, lungs, and gills. The ectoderm forms the outer layer of skin, hair, nails, and the nervous system. The mesoderm forms many body parts, including the skeleton, muscles, the inner layer of skin, the circulatory system, and the lining of the body cavity.
Echinoderms, such as the sea urchin, undergo the gastrulation process shown here. The blastula reorganizes and forms the cup-shaped gastrula. Other phyla have somewhat different patterns of gastrulation.
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