262 The Alligator and Its Allies 



the entoderm being almost flat and horizontal. 

 The notochord {nt) is distinctly outlined and is 

 somewhat flattened in a dorso-ventral direction. 

 The body cavity {he) is well marked, but is sepa- 

 rated by a considerable mass of uncleft mesoblast 

 from the notochord and the walls of the medullary 

 groove. 



A space of about one hundred sections, or one 

 third the length of the embryo, intervenes between 

 Figures 9g and 9^*. This is the region of the meso- 

 blastic somites, and in this region, as has been 

 above stated, the medullary canal is completely 

 enclosed. It is evident then that the entire ante- 

 rior two thirds of the medullary canal is enclosed 

 except for the short region represented in Figure 8g. 

 Whether or not this short open region between the 

 two longer enclosed regions is a normal condition 

 the material at hand does not show. 



Figure 9/? represents a typical section in the 

 region of the mesoblastic somites just described. 

 It shows the enclosed medullary canal {mc), the 

 body cavity (he) on the right, and a mesoblastic 

 somite with its small cavity {myc) on the left. 

 The entire section is smaller than the sections 

 anterior or posterior to this region, and seems to 

 be compressed in a dorso-ventral direction, this 

 compression being especially marked in the case 

 of the notochord. 



Figure 9^' is through a region nearly one hundred 

 sections posterior to the preceding, and cuts the 



