The Development of the Alligator 243 
The pushing under of the head causes a forward 
projection of the secondary folds, so that the fold 
to the right appears as a rounded mass of cells with 
a small cavity near its center. On the left the 
plane of the section passes through the posterior 
limit of the head-fold, and shows the cells of the 
secondary fold continuous with the dorsal side of 
the ectoderm (ec). As pointed out above by Clarke, 
the secondary folds are here some distance apart, 
and gradually approach each other as we proceed 
toward the posterior. The entoderm (ez) is here 
flat and takes no part in the secondary folds. 
In Figure 6), a short distance back of the one 
just described, the secondary folds (sf) are much 
larger and are closer together. On the right the 
section passes through the extreme limit of the 
head-fold, so that the secondary fold of that side 
is still a closed circle, with a few scattered cells 
enclosed. On the left the section is posterior to 
the head-fold; on this side the secondary fold is 
seen as a high arch of ectoderm, with a thick mass 
of entoderm beneath it. 
Figure 6c represents a section which passes 
back of the head-fold on both sides. The second- 
ary folds (sf) are seen as a pair of ectodermal arches 
continuous with each other in the middle line of the 
embryo. The ectoderm of the folds is much 
thickened and gradually becomes thinner distally. 
On the right the entoderm shows the same thicken- 
ing (en) that was shown on the left side of the 
