The Development of the Alligator 237 
in this embryo was probably not so far developed 
as it was in the embryo shown in Figures 3 and 3a. 
Not having seen the embryo, however, before it was 
sectioned, the writer cannot be certain of this 
point. The ectoderm and entoderm are here of 
nearly the same thickness. 
Figure 3e is a short distance posterior to the 
preceding. It shows a marked thickening of the 
ectoderm in the medial region (ec), which is con- 
tinuous posteriorly with the anterior ends of the 
medullary folds that are just beginning to differ- 
entiate (Figs. 3f-h). 
Figure 3g passes through the anterior end of the 
medullary plate or folds (mf), whichever they may 
be called. The ectoderm of the folds is thickened 
and is considerably elevated above the rest of the 
blastoderm. There is scarcely any sign, in this 
region, of a medullary groove. The entoderm (em) 
is considerably thickened in the medial region, this 
thickening being continuous posteriorly, as in the 
preceding stage, with the mesoderm. 
In Figure 3h, cut in a plane at some distance 
posterior to the preceding, the medullary groove 
(mg) is well marked; its bordering folds grad- 
ually thin out laterally to the thickness of the 
ordinary ectoderm. The medial thickening of 
the entoderm is very marked, but it has not in 
this region separated into a distinct mesoblastic 
layer. 
Immediately under the medullary groove is a 
