Barron's AP Biology by Deborah T. Goldbert

Author:Deborah T. Goldbert
Language: eng
Format: epub
Publisher: Barrons Educational Series
Published: 2017-10-12T16:00:00+00:00

Figure 16.6 Blastocyst

During the next stage of development, the cells of the blastocyst or blastula communicate with each other and begin to differentiate. In many animals, the movement of the cells is so regular that it is possible to label a specific blastomere (individual cells of a blastocyst) and identify the tissue that results as embryonic development proceeds. Labeling these cells produces a fate map.

STUDY TIP

Gastrulation produces three germ layers: ectoderm, mesoderm, and endoderm.

Gastrulation is a process that involves rearrangement of the blastula or blastocyst and begins with the formation of the blastopore, an opening into the blastula. In some animals, the blastopore becomes the mouth; in other animals (deuterostomes), the blastopore becomes the anus. Some of the cells on the surface of the embryo migrate into the blastopore to form a new cavity called the archenteron or primitive gut. As a result of this cell movement, gastrulation forms a three-layered embryo called a gastrula. These three differentiated layers, called embryonic germ layers, are the ectoderm, endoderm, and mesoderm. They will develop into all the parts of the adult animal.

■The ectoderm will become the skin and the nervous system.

■The endoderm will form the viscera including the lungs, liver, and digestive organs, and so on.

■The mesoderm will give rise to the muscle, blood, and bones. Some primitive animals (sponges and cnidarians) develop a noncellular layer, the mesoglea, instead of the mesoderm. See Figure 16.7.

Organogenesis is organ building. It is the process by which cells continue to differentiate, producing organs from the three embryonic germ layers. Three kinds of morphogenetic changes—folds, splits, and dense clustering called condensation—are the first evidence of organ building. Once all the organ systems have been developed, the embryo simply increases in size.

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