252 The Alligator and Its Allies 
in the region of the head, where the section passes 
through one of the medullary folds (mf) at its 
thickest part; and also explains the fact that the 
ectoblast is thinner in the middle region (ec), 
where the section passes through the medullary 
groove, than it is farther toward the blastopore 
where the section cuts the edge of the medullary 
folds. The outlines of the middle and extreme 
posterior regions of the ectoblast are much more 
irregular and ragged than is shown in the figure. 
The plane of the section passes through the noto- 
chord (nf) in the posterior region, but not in the 
anterior end of the embryo, where a layer of 
mesoblast (mes) is seen. The great size of the 
blastopore (b/p) is well shown, as is the beginning 
of the foregut (fg). Comparison of this figure with 
the more anterior transverse sections and with the 
dorsal surface view of this stage will make the 
rather unusual conditions comprehensible. 
Figure 8c is cut to one side of the median plane, 
distal to the medullary folds. Being outside of 
the medullary folds, the ectoderm (ec) is thinner 
and less dense than in Figure 8); anteriorly it is 
pushed down and back as the head-fold, and pos- 
teriorly it becomes thin where it forms the dorsal 
boundary of the primitive streak (ps). 
The foregut (fg), as would be expected, is not so 
deep as in the median section (8b). The most 
striking feature of the section is the presence of 
five mesoblastic somites (s). Each somite, espe- 
