334 THE EVOLUTION OF MAN. 



I 



this globular formation is that at one point in the germ i 

 disc, immediately below the notochord, and, therefore, below I 

 the axis of the developing body, a furrow-like depression I 

 arises. This is the primitive groove (Fig. 100, B). It gradually | 

 becomes deeper and broader, assumes the form of a canal, ! 

 and completely separates from the germ-vesicle, of which j 

 it originally formed a part (Fig. 100, D — H). At first ! 

 the whole intestinal germ- vesicle is, in a certain sense, the  

 intestinal cavity. We may, therefore, compare the entire ' 

 intestinal germ vesicle of the Mammal, the wall of which, I 

 closed on all sides, is formed by the intestinal layer, with the 

 primitive intestine of a Gastrula, the primitive mouth of i 

 which has closed. This primitive intestine separates into | 

 two parts, the permanent after-intestine (d), and the tran- | 

 sient navel-vesicle (nh). \ 



This is also true of the formation of the intestine in ^ 

 Birds and Reptiles. For in these, the large yelk-sac, filled ; 

 with nutritive yelk, represents the smaller mammalian 

 navel-vesicle, filled with clear liquid. In Birds and Reptiles ; 

 again, the later, permanent intestine also separates itself \ 

 from the yelk-sac by the intestinal groove changing into a  

 canal, into the intestinal tube. This tube is formed from 

 the intestinal-furrow in the same way as the medullary 

 tube originates from the dorsal furrow. The groove grows \ 

 deeper and deeper ; its edges grow downwards towards each ; 

 other, and coalesce at the point at which they meet. But j 

 the difference between the structure of the intestinal tube ;! 

 and that of the medullary tube consists, as we have shown I 

 in the fact that the medullary tube is closed equally along 

 its whole length in a suture, while the intestinal tube 

 grows together more concentrically, not only from the two 



