. Principles of modern biology. Biology. 278 - Multicellular Animals, Especially Man But in higher animals, as cleavage con- tinues, the cells in the vegetal region appear to be crowded inward, so that they encroach upon the blastocoel (Fig. 15-2). This in- pocketing, or invagination, continues until a tubular pouch, the archenteron, is formed (Fig. 15-2). The archenteron is the embry- onic forerunner of the digestive tract. The cells forming the wall of the archenteron are specified as the endoderm; and the cells re- maining at the surface of the embryo are called the ectoderm. Such a two-lay
. Principles of modern biology. Biology. 278 - Multicellular Animals, Especially Man But in higher animals, as cleavage con- tinues, the cells in the vegetal region appear to be crowded inward, so that they encroach upon the blastocoel (Fig. 15-2). This in- pocketing, or invagination, continues until a tubular pouch, the archenteron, is formed (Fig. 15-2). The archenteron is the embry- onic forerunner of the digestive tract. The cells forming the wall of the archenteron are specified as the endoderm; and the cells re- maining at the surface of the embryo are called the ectoderm. Such a two-layered em- bryo is a gastrula; and the orifice leading into the archenteron is the blastopore. In heavily yolked eggs gastrulation is modi- fied considerably (Fig. ); but it is always possible to recognize a gastrula. This two-layered embryo possesses a saccular archenteron, which communicates with the exterior through a blastopore. The Mesoderm and the Coelom. Very primitive animals, such as Hydra and other Coelenterata (p. 630), do not progress fur- ther than the gastrula stage. The adult body of such animals is two-layered, consisting en- tirely of ectoderm and endoderm. In all higher animals, however, a third layer of cells, the mesoderm, is formed between the ectoderm and endoderm. These three em- bryonic layers—the ectoderm, endoderm, and mesoderm—are celled the primary germ layers of the embryo. The ectoderm is des- tined to give rise to all epithelial layers on the external surface of the bod)', and also to the sensory and nervous tissues of the ani- mal. The endoderm provides a glandular. Fig. 15-8. Amphibian embryos, show- ing development of typical vertebrate structures. A, median longitudinal sec- tion of early gastrula; B, similar section of late gastrula; C, cross section of same stage; D, dorsal view of slightly later stage, showing neural folds; E, cross sec- tion of same stage; F, similar section of later stage; G, median longitudinal sec- tion of embr
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