DEVELOPMENT OF THE AMERICAN ALLIGATOR — REESE 15 



Figure yd is about a dozen sections posterior to the one just de- 

 scribed and is about one-seventh the length of the embryo from the 

 anterior end. The embryo is much more compressed, in a dorso- 

 ventral direction, and the medullary groove (mg) is correspond- 

 ingly more shallow. Where the ectodorm (ec) curves over to form 

 the medullary folds it becomes much more compact and somewhat 

 thicker. The notochord (tit) is large and distinct, but is still fused 

 with the entoderm (en). The mesoderm (mes) forms a well- 

 defined layer, entirely distinct from both the notochord and the ento- 

 derm. From this region, as we pass caudad, the size of the embryo 

 in cross-section gradually decreases and the medullary groove be- 

 comes more shallow. 



Figure ye is about one-third of the length of the embryo from the 

 posterior end, and is only a few sections from the caudal end of the 

 medullarv groove. The ectoderm (ec) is much thinner than in the 

 preceding figure and the medullary groove (mg) is much more 

 shallow. The notochord (nt) is of about the same diameter as 

 before, but is here quite distinct from the entoderm (en) as well as 

 from the mesoderm (mes). 



Figure yf is seven sections posterior to figure ye. The medullary 

 groove has disappeared and the medullary folds have flattened to 

 form what might be called a medullary plate (at the end of the refer- 

 ence line ec), which continues to the anterior border of the blasto- 

 pore. The notochord (nt) is larger in cross-section than in the 

 more anterior regions ; it is still distinct from the entoderm. 



Figure yg passes through the blastopore and shows essentially the 

 same structure as was described in connection with the correspond- 

 ing section of stage I (fig. 2/). 



Figure yh represents the region of the primitive groove (pg) and 

 primitive streak (ps). The section shows the typical structure for 

 this region— a thick mass of cells is proliferating from the ventral 

 side of the ectoderm (ec) and is spreading laterally to form a dis- 

 tinct mesoderm (mes). The entoderm (en) is entirely distinct from 

 the other layers. 



Stage A' 



Figures 8-8; (Plates VII, VIII, IX) 



On opening the egg this embryo (figs. 8 and 8a) was found to lie 

 on the end of the yolk, near the center of the irregular, lighter area 

 which was mentioned in connection with the description of the egg. 

 The length of the embryo proper is 3 mm. This was the youngest 



