The Development of the Alligator 263 
embryo, therefore, through the posterior one fourth 
of its length. The chief difference between this 
and the preceding section is in the medullary canal, 
which is here open and is in the form of a wide 
groove with an irregular, rounded bottom and 
vertical sides. The size of the section is consider- 
ably greater than in the preceding, the increase 
being especially noticeable in the notochord (nt), 
which is cut near its posterior end. ‘There is little 
or no sign of mesoblastic cleavage. 
Figure 9j is about twenty sections posterior 
to Figure 97. The medullary groove (mg) is con- 
siderably larger than in the more anterior regions, 
and its folds are somewhat inclined toward each 
other, though still wide apart. The notochord 
and entoderm are fused to form a large, compact 
mass of tissue close under the ventral wall of the 
medullary groove. On the ventral side of this 
mass of cells a groove (blp) marks the anterior and 
ventral opening of the blastopore shown in the next 
figure. The mesoblast shows no sign of cleavage. 
Figure 9k shows the medullary groove (mg) 
in about the same position as in the preceding 
section. The blastopore (b/p) is here seen as a 
small cavity in the center of the large mass of cells 
that was noted in the last figure. The entoderm 
(en) is continuous from side to side, but is not so 
sharply differentiated from the other germ layers 
as is represented in the figure. 
Figure 9/ is four sections back of the preceding; 
