282 The Alligator and Its Allies 



*fc) 



Figure 12 g is through the neurenteric canal (nc), 

 a distinct opening through the floor of the spinal 

 canal. The section is of course just back of the 

 posterior end of the notochord. The entoderm 

 (en) along the margin of the neurenteric canal is 

 naturally continuous with the wall of the spinal 

 cord (sc) . The ectoderm {ec) is thicker than ever, 

 except in the median plane, where it passes over 

 the spinal cord. The mesoblast is more abundant 

 than in the preceding figure, and shows on the 

 left what appears to be a distinct myocoel {myc), 

 though in surface view the mesoblastic somites do 

 not extend this far toward the tail. 



STAGE X 



Figures 13-13^ (Plates XIX., XX., XXI.) 



This embryo (Fig. 13) is about 5mm. in length, 

 and hence is slightly smaller than the preceding 

 stage, though somewhat more advanced in develop- 

 ment. The medullary canal is still apparently 

 unclosed for a short distance at the extreme poste- 

 rior end; this appearance is probably due to the 

 neurenteric canal {no) and to the thinness of the 

 roof of the medullary canal rather than to any 

 lack of fusion of the medullary folds. The optic 

 vesicle is more distinct than in the preceding stage ; 

 a somewhat similar, though smaller, opacity {0) 

 marks the position of the ear. There are now 

 about twenty pairs of somites, though it is difficult 



