THE YELK-SAC AND THE PERMANENT INTESTINE. 379 



weeks old, we find a large, round sac, filled with liquid, 

 on the ventral side of the germ. This is the yelk-sac, the 

 so-called navel- vesicle, the origin of which we have already 

 examined (Figs. 132, 133). In proportion as the embryo 

 grows larger, the yelk-sac grows smaller. At a later period 

 it hangs, as a small pear-shaped vesicle, at the end of a long 

 stalk (the yelk-duct), from the abdomen of the embryo 

 (Fig. 139, 5 ds), and is finally detached from the body by the 

 closing of the navel. The wall of this navel- vesicle consists, 

 as we have seen, of an inner layer, the intestinal-glandular 

 layer, and an outer layer, the intestinal-fibrous layer. It is 

 therefore composed of the same constituents as the intestinal 

 wall itself, of which it forms, in fact, a direct continuation. 

 In Birds and Reptiles the yelk-sac is much larger, and con- 

 tains a considerable quantity of albuminous and fatty nutri- 

 tive matter. This penetrates through the yelk-duct into the 

 intestinal cavity and serves as food. In Mammals the yelk- 

 sac plays a much smaller part in the nourishment of the 

 germ, and degenerates at an early period. The relation of 

 the intestine to the yelk sac has very often been entirely 

 mistaken According to the Gastrsea Theory the two form 

 one whole. We may say that the primitive intestine of 

 those Vertebrates which are without a skull afterwards 

 separated in their descendants (in consequence of the 

 accumulation of nutritive yelk) into two parts, a transitory 

 embryonic organ (the yelk-sac), and a permanent intestine 

 (the after-intestine). 



Behind the yelk-sac, a second and much more significant 



appendage forms, at an early period, on the abdomen of the 



vertebrate embryo. This is the allantois, or primitive 



urinary sac, an important embryonic organ, which occurs 



27 



