254 The Alligator and Its Allies 
Figure 8g passes through the middle part of the 
hhead-fold, and shows that the medullary folds in 
this region are fused below, but are widely sepa- 
rated above, where their margins are markedly bent 
away from the mid-line. Between the epidermal 
and nervous layers of the ectoderm a considerable 
mass of mesoderm cells (mes) is seen. The curious 
appearance of the preceding four figures, as well 
as the first three figures of the next stage, was at 
first quite puzzling, until a model of the embryo 
was made from a series of sections. It was then 
plain, as would have been the case before, except 
for the unusual depth dorso-ventrally of the head of 
the embryo, why the medullary canal should at the 
extreme anterior end be open both dorsally and 
ventrally, while a few sections caudad it is open 
only ventrally, and still farther toward the tail it is 
again open both above and below. These condi- 
tions are produccd by the bending under of the 
anterior region of the medullary folds, probably by 
the formation of the head-fold. It is apparently 
a process distinct from the ordinary cranial flexure, 
which occurs later. The fusion of the medullary 
folds to form a canal begins, as has been already 
mentioned, near the anterior end, whence it extends 
both forward and backward. Hence, if the ante- 
rior ends of the medullary folds be bent downward 
and backward, their unfused dorsal edges will come 
to face ventrally instead of dorsally and sections 
through the anterior part of this bent-under region 
