Wtu

(c) COELOMATE

figure 32-14

(c) COELOMATE

figure 32-14

In three-layered acoelomates (a), the endodermic gut is surrounded by a solid layer of mesoderm. In pseudocoelomates (b), the endodermic gut is suspended in a fluid-filled cavity that is surrounded by mesoderm. In coelomates (c), the endodermic gut is surrounded by and suspended by mesoderm, which also surrounds the coelom, or body cavity.

(a) SCHIZOCOELY

Blastocoel Archenteron

(primitive gut) Anus

Coelom

(a) SCHIZOCOELY

Coelom

Mouth

Coelom

Mouth

(b) ENTEROCOELY

Archenteron

Blastocoel (primitive gut)

Coelom Mouth

(b) ENTEROCOELY

Archenteron

Blastocoel (primitive gut)

Coelom Mouth

Anus

Endoderm Mesoderm I Ectoderm figure 32-15

Anus

Endoderm Mesoderm I Ectoderm figure 32-15

In protostomes, the coelom arises in a process called schizocoely (a), and the blastopore becomes the mouth. In deuterostomes, the coelom arises by enterocoely (b), and the blastopore becomes the anus.

Protostomes and deuterostomes also differ in how early the cells of the embryo specialize. If the cells of some protostome embryos are separated at the four-cell stage of development, each cell will develop into only one-fourth of a complete embryo, and the developing organism will die. Thus, the path of each cell is fixed early in the development of the protostome in a pattern called determinate cleavage.

In contrast, if the cells of most four-celled deuterostome embryos are separated, each cell will embark on its own path to become a separate organism. This type of development is called indeterminate cleavage. Indeterminate cleavage is responsible for the development of identical twins in humans.

Coelom Formation

The way in which the coelom forms in many protostomes differs from the way it forms in many deuterostomes. Figure 32-15a shows coelom formation in protostomes. Cells located at the junction of the endoderm and ectoderm (at the rim of the cup-shaped embryo) move toward the interior of the gastrula. Rapid division of these cells (shown in pink) in the blastocoel forms the mesoderm. The mesoderm then spreads and splits to form the coelom. This process of coelom formation is called schizocoely (SKIZ-oh-SEEL-ee), or "split body cavity."

Figure 32-15b shows coelom formation in deuterostomes. The mesoderm forms when the cells lining the dorsal, or top, part of the archenteron begin dividing rapidly. These rapidly dividing cells (shown in pink) form pouches that become mesoderm. The coelom develops within the mesodermal pouches. This process of coelom formation is called enterocoely (EN-tuhr-oh-SEEL-ee), which means "gut body cavity." During both enterocoely and schizocoely, mesodermal cells spread out to completely line the coelom, and the blastocoel disappears. Thus, in both protostomes and deuterostomes, mesoderm lines the interior of the outer body wall and surrounds the gut.

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