264 The Alligator and Its Allies 



the wide, dorsal opening {hip) of the blastopore or 

 neurenteric canal into the medullary groove {mg) is 

 shown. The blastopore or neurenteric canal, then, 

 is still at this stage a passage that leads entirely 

 through the embryo, the medullary canal being in 

 this region unenclosed above. Ventrally it is seen 

 as a narrow opening through the entoderm; it 

 then passes upward and backward, behind the end 

 of the notochord, as a small but very distinct canal, 

 which may be traced through about ten sections. 

 The enclosed portion of the canal lies, as has been 

 stated (Figure 9^, hip), in the center of the mass of 

 cells that is fused with or is a part of the floor of 

 the medullary groove. 



The above-described neurenteric canal is essen- 

 tially like that described by Balfour in the Lacertilia . 

 He does not say, however, and it is not possible 

 to tell from his figures, whether there is a long, 

 gradually diminishing groove posterior to the 

 dorsal opening of the canal, as in the alligator. 

 He says that the medullary folds fuse poster- 

 iorly until the medullary canal is enclosed over 

 the opening of the neurenteric canal; also that 

 "the neurenteric canal persists but a very short 

 time after the complete closure of the medullary 

 canal." 



In Figiire gm, for about thirty sections (one 

 tenth the entire length of the embryo) , behind the 

 section represented in the last figure, there is a very 

 gradual change in the embryo, converting the deep 





