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 myoccel 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 (a). The medullary folds (mf) 
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 (y) 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 (wl). 
Figure 8f 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 (7/) layers of the ectoderm, but no 
mesoblast is yet to be seen. 
