The Development of the Alligator 247 
STAGE IV 
Ficures 7a-7h (PLATES XI., XII.) 
No surface view of this stage was seen by the 
writer, and hence is not figured. The figures were 
drawn from one of the series of sections obtained 
through the courtesy of Prof. S$. F. Clarke. This 
series was marked ‘‘3 Urwirbeln,’”’ so that the 
embryo was apparently slightly younger than the 
youngest stage obtained by myself and represented 
in Figures 8 and 8a. 
Figure 7a represents a section that passed 
through the head-fold of the amnion (a) just in 
front of the head-fold of the embryo; the amniotic 
cavity here appears as a large empty space. 
Figure 7b is several sections posterior to the 
preceding; it passes through the head-fold of the 
embryo, but is just back of the head-fold of 
the amnion. The deep depression of the ectoderm 
(ec) and entoderm (e7) caused by the head-fold is 
plainly seen. In this depression lie the ends of 
the medullary folds, distinct from each other both 
dorsally and ventrally. Each medullary fold is 
made up of two parts—a medial, more dense nerv- 
ous layer (nl), and a distal, less dense epidermal 
layer (ep). The section corresponding to this one 
will be more fully described in connection with 
the following stage. 
Figure 7c is some distance posterior to the pre- 
ceding, though the exact distance could not be 
