244 The Alligator and Its Allies 
preceding figure. This thickened appearance of 
the entoderm is due to the fact that the section 
passes through the anterior limit of a tall fold 
of that layer, which underlies the similar fold 
of the ectoderm that has already been described. 
This secondary fold of the entoderm is seen on 
the left side of the section. It may be traced 
through several sections, but soon flattens out 
posteriorly. 
Figure 6d is a short distance posterior to tne 
preceding. The secondary folds are here much 
less pronouncedly arched and the deep groove 
between them is reduced to a line (J). The ento- 
derm (en) is no longer markedly arched and is 
closely adherent, along the median plane, to the 
ectoderm, where there is seen the thickening 
(th) that has been mentioned by Clarke (see above). 
Springing from the entoderm on each side of this 
thickening is a small mass of mesoderm (mes). 
The section immediately posterior to the one 
just described is represented in Figure 6e. The 
line (1) which separated the two secondary folds 
in the preceding section is no longer present, so 
that the ectoderm (ec) is continuous from side to 
side, with only a shallow depression (mg), which 
may be considered as the extreme anterior end 
of the medullary groove. The median thickening 
(th) is cut near its posterior limit and still shows a 
close fusion of the germ layers. There is no line 
of demarcation between the gradually flattening 
