322 THE EVOLUTION OF MAN. 



gives rise to the various parts of the differentiated intestinal 

 system, and the transient yelk-sac, which serves only as a 

 storehouse of materials for the building of the embryo. 

 The yelk-sac attains its greatest development in Primitive 

 Fishes (Selachii), Bony Fishes (Teleostei), Reptiles, and 

 Birds. In Mammals, and especially in Placental Animals, 

 it is atrophied. The peculiar intestinal development of the 

 Cyclostomi, Ganoids, and Amphibia must be regarded as 

 an intermediate form, between the palingenetic intestinal 

 development of the Skull-less animals, and the kenoge- 

 netic intestinal development of the Amnion Animals (Am- 

 niota). 



We have already seen in what a peculiar way the 

 development of the intestine takes place ontogenetically in 

 the human embryo and in that of other Mammals. Imme- 

 diately from the gastrula of these originates a globular 

 intestinal germ-vesicle (gastrocystis), filled with fluid (Figs. 

 72, 73, vol. i. p. 289). In the wall of this is formed the 

 lyre-shaped germ-shield, on the lower side of which, along 

 the middle line, appears a shallow groove, the first rudi- 

 ment of the future, secondary intestinal tube. 



This intestinal groove grows constantly deeper, and its 

 edges curve toward each other, to grow together at last and 

 form a tube (Fig. 100, vol. i. p. 333). The wall of this 

 secondary intestinal tube consists of two membranes of the 

 inner, intestinal -glandular layer, and of the outer, intestinal- 

 fibrous layer. The tube is completely closed at the ends, 

 having only an opening in the centre of the lower wall, 

 by which it is connected with the intestinal germ-vesicle 

 (Plate V. Fig. 14). The latter, in the course of development, 

 becomes continually smaller, as the intestinal canal continues 



