334 raE EVOLUTION OF MAW. 



this globular formation is that at one point in the germ 

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

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

 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 

 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, 

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

 primitive intestine of a Gastrula, the primitive mouth of 

 which has closed. This primitive intestine separates into 

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

 sient navel-vesicle (nb). 



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 

 the difference between the structure of the intestinal tube 

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

 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 



