The Development of the Alligator 249 



is of about the same diameter as before, but is here 

 quite distinct from the entoderm {ei2) as well as 

 from the mesoderm (mes) . 



Figure 7/ 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 

 reference line ec), which continues to the anterior 

 border of the blastopore. The notochord (;//) 

 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 de- 

 scribed in connection with the corresponding 

 section of Stage I (Fig. 2/). • 



Figure yh represents the region of the primitive 

 groove (pg) and primitive streak (ps). The sec- 

 tion 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 distinct mesoderm (^nes) . The entoderm 

 (en) is entirely distinct from the other layers. 



STAGE V 

 Figures 8-8j (Plates XII., XIII., XIV.) 



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 



