The Development of the Alligator 241 
ary fold taking place in the head-fold. This grows 
posteriorly along the median dorsal line, forming 
a V-shaped process with the apex pointing back- 
ward toward the blastopore. There is quite a 
deep groove between the arms ofthe V. The head- 
fold on the ventral side, as seen in Figure 4a, made 
from the same embryo as Figure 4, grows most 
rapidly on the mid-line, and also becomes thicker at 
that place. The medullary folds now begin to 
form on either side of the medullary groove, end- 
ing posteriorly on either side of the blastopore and 
anteriorly on either side of the point of the V- 
shaped process in the middle of the head-fold. 
This is seen in Figure 5, which is a dorsal view of an 
embryo from an egg three days after it was taken 
out of an alligator. A ventral view of the same 
embryo (Fig. 5a) represents the thickened process 
on the mid-line at its greatest development. For 
some reason the notochord did not show in this 
embryo, possibly owing to particles of the yolk 
material adhering about the mid-line. 
‘“‘In an embryo a day or two older, the V-shaped 
fold of the head-fold is seen to have broken through 
at the apex, and each of the arms thus separated 
from one another unites with the medullary fold 
of its respective side. This can be seen in Figure 
6, which is a dorsal view of part of an embryo a day 
or two older than the one represented by Figures 
5 and 5a. 
“This is so unexpected a method of formation 
16 
