252 The Alligator and Its Allies 



in the region of the head, where the section passes 

 through one of the medullary folds (nif) at its 

 thickest part; and also explains the fact that the 

 ectoblast is thinner in the middle region (ec), 

 where the section passes through the medullary 

 groove, than it is farther toward the blastopore 

 where the section cuts the edge of the medullary 

 folds. The outlines of the middle and extreme 

 posterior regions of the ectoblast are much more 

 irregular and ragged than is shown in the figure. 

 The plane of the section passes through the noto- 

 chord (nt) in the posterior region, but not in the 

 anterior end of the embryo, where a layer of 

 mesoblast (mes) is seen. The great size of the 

 blastopore (blp) is well shown, as is the beginning 

 of the foregut ( fg) . Comparison of this figure with 

 the more anterior transverse sections and with the 

 dorsal surface view of this stage will make the 

 rather unusual conditions comprehensible. 



Figure 8c is cut to one side of the median plane, 

 distal to the medullary folds. Being outside of 

 the medullary folds, the ectoderm (ec) is thinner 

 and less dense than in Figure 8b ; anteriorly it is 

 pushed down and back as the head-fold, and pos- 

 teriorly it becomes thin where it forms the dorsal 

 boundary of the primitive streak (ps) . 



The foregut (fg), as would be expected, is not so 

 deep as in the median section (8b). The most 

 striking feature of the section is the presence of 

 five mesoblastic somites (s). Each somite, espe- 



