258 The Alligator and Its Allies 
Figures 8 and 10. The amnion and head-fold are 
nearly the same as in Figure 8; the medullary folds 
are intermediate in development, the anterior end 
not showing so marked an enlargement as shown in 
Figure 10, v’. There are six or seven faintly dis- 
tinguisnable somites. 
Figure 9a represents a section through the 
anterior part of the head-fold; it shows one unusual 
condition: the head lies entirely beneath the surface 
of the yolk. This condition is quite confusing when 
the section is studied for the first time. The push- 
ing of the head under the yolk is shown at its com- 
mencement in Figure 11. The process continues 
until nearly the entire anterior half of the embryo 
is covered; but when the embryo attains a consider- 
able size it is seen to lie entirely above the yolk, as 
in the chick. According to Voeltzkow’s figures 
(78) this same condition is found in the crocodile, 
and Balfour also mentions it in connection with 
the lizard. The fusion of the medullary folds has 
made considerable progress, so that the entire an- 
terior end of the canal is enclosed, except in the 
region where the folds are bent down and back, as 
in the preceding stage; here the folds are still 
distinct from each other, leaving the medullary 
canal open on the ventral side, as shown in Figures 
9 and 9). In the section under discussion the 
ectoderm (ec) is a very thin membrane on top of 
a considerable mass of yolk, while no entoderm can 
be distinguished. The amnion (a) completely sur- 
