266 The Alligator and Its Allies 



STAGE VII 



FIGURES 10 AND ioa (PLATES XV., XVI.) 



Although of practically the same size as the 

 preceding, this stage has advanced sufficiently in 

 development to warrant a description. 



The medullary folds are apparently still slightly 

 open for the greater part of their length, though 

 they are evidently fused together in the head 

 region, except at the extreme end. Transverse sec- 

 tions, however, of Figure 12, in which the medul- 

 lary folds, from the dorsal aspect, seemed open 

 (mg) as in Figure 10, have shown that these folds 

 are fused throughout their length. 



The first cerebral vesicle (v') is clearly indicated as 

 an enlargement of the anterior end of the nervous 

 system, and a slight enlargement (v w ) posterior to the 

 first probably represents the second cerebral vesicle. 



There are now eight pairs of somites (s) . 



The head-fold (h) now shows in both dorsal and 

 ventral views, appearing in the former, when 

 viewed by transmitted light, as a lighter, circular 

 area on either side of the body, just posterior to the 

 hinder edge of the amnion. 



The head-fold of the amnion (a) has extended 

 about twice as far backward as it did in the preced- 

 ing stage. 



Owing to the opacity caused by the medullary 

 folds being in contact along the middle line, the 

 notochord is no longer visible in surface views. 



