The Development of the Alligator 249 
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 7f is seven sections posterior to Figure 
7e. 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 (nf?) 
is larger in cross-section than in the more anterior 
regions; it is still distinct from the entoderm. 
Figure 7g passes through the blastopore and 
shows essentially the same structure as was de- 
scribed in connection with the corresponding 
section of Stage I (Fig. 2f). 
Figure 7/ 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 (mes). The entoderm 
(en) is entirely distinct from the other layers. 
STAGE V 
Ficures 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 
