310 THE EVOLUTION OF MAN. 



closing of the intestinal tube, on the other hand, takes place 

 not only from the right and left, but by a concrescence of the 

 walls on all sides of the intestinal groove towards the navel, 

 as a central point. Moreover, the whole process of the 

 secondary formation of the intestine in the three higher 

 classes of Vertebrates is most closely connected with the 

 formation of the navel, with the "pinching in" of the 

 embryo from the yelk-sac (navel-vesicle). (Cf. Fig. 70, p. 

 283, and Plate V. Figs. 14 and 15.) 



In order to be quite clear about these points, it is neces- 

 sary to bear in mind the relation of the germ-shield to the 

 germ-area and to the intestinal germ- vesicle. Tliis is best 

 accomplished by comparing the five stages which are repre- 

 sented in longitudinal section in Fig. 94. The germ-shield (e), 

 which at first protruded only slightly from the surface of 

 the germ-area, soon begins to raise itself from the latter, and 

 to pinch itself off the intestinal germ-vesicle. During this 

 the germ-shield, seen from the dorsal side, still retains its 

 original simple sole-shape (Figs. 86, 87, p. 298). There is as 

 yet no appearance of any distinction into head, neck, trunk, 

 or limbs. But the germ-shield has grown much thicker, espe- 

 cially in the anterior portion. It now, therefore, protrudes 

 from the surface of the germ-area like a thick, much arched, 

 oval swelling, and begins to separate and free itseli 

 completely from the intestinal germ-vesicle, to which it is 

 attached by its ventral surface. Ihe progress of this separ- 

 ation renders the back continually more curved; in proportion 

 as the embryo grows and becomes larger, the germ- vesicle 

 decreases and becomes smaller, till at last it hangs, in the 

 form of a small bladder, from the abdomen of the embrvo 

 (Fig. 94, 5 cZs). In consequence of the pj'ocesses of growth 



