The Development of the Alligator 253 



cially the second, third, and fourth, is made up of 

 a mass of mesoblast whose cells are compactly 

 arranged peripherally, but are scattered in the 

 center, where a small myocoel may be seen. 



A series of transverse sections of the embryo 

 shown in Figures 8 and 8a is represented in Figures 

 8d-j. 



Figure 8d is through the anterior end of the 

 embryo ; the posterior edge of the amnion is cut 

 only on one side (c/). The medullary folds {711 f) 

 are shown as two distinct masses of tissue, sepa- 

 rated by a considerable space from each other, 

 both dorsally and ventrally; they are underlaid 

 by the ectoderm of the head-fold, through which 

 the section passes. A mass of yolk (3') is shown 

 at one side of the section. 



Figure 8e represents a section a short distance 

 posterior to the one just described, and passes 

 through the short region where the dorsal edges of 

 the medullary folds have fused with each other. 

 The ventral side of the medullary groove (mg) is, 

 as in the preceding section, still unclosed. An 

 epidermal layer of ectoblast (ep) is now distinct 

 from the nervous layer (w/) . 



Figure 8/ is through a region still farther toward 

 the posterior end. Here the medullary groove 

 is again open above, and is still open below. A 

 well-marked space is seen between the epidermal 

 (ep) and nervous (;?/) layers of the ectoderm, but no 

 mesoblast is yet to be seen. 



