264 The Alligator and Its Allies 



the wide, dorsal opening (blp) 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&, blp), 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 Figure 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 



