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. 15-8A.B); 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 embryo after closing of neural 

 tube; H, cross section of the same stage; 

 I, longitudinal section of embryo at time 

 of hatching; J, cross section of same 

 stage, ec, ectoderm; en, endoderm; y, 

 yolk cells; b, blastocoel; e, archenteron; 

 m, mesoderm; dl, dorsal lip of blasto- 

 pore; vl, ventral lip of blastopore; yp, 

 yolk plug; n, notochord; nf, neural fold; 

 nc, neural crest (which develops into 

 sensory ganglia); c, coelom; nt, neural 

 tube; bp, blastopore; a, anus; r. rectum; 

 I, liver; k, embryonic kidney tubule; br, 

 brain; sp, spinal cord; t, tail; h, heart; 

 ph, pharynx; mo, mouth; mu, muscle; sg, 

 sensory ganglion; v, vertebra; bv, blood 

 vessel; ms, mesodermal somite. 



