The Development of the Alligator 255 
will show the medullary canal open both above 
and below, as in Figure 8d, while sections farther 
caudad pass through the short region where the 
folds are fused, and we have the appearance repre- 
sented in Figure 8e. In Figure 8f is shown a section 
passing posterior to the short, fused region of the 
folds, and we again have the medullary canal open 
both above and below. Figure 8g represents a 
section through the tip of the bent-under region of 
the medullary folds, which are here fused below 
and open above. 
Figure 8h passes through the posterior part of 
the head-fold, between the limits of the fold of the 
ectoderm and that of the entoderm. The medul- 
lary groove (mg) is here very wide and compar- 
atively shallow; its walls are continued laterally as 
the gradually thinning ectoderm (ec). The enteron 
(ent) is completely enclosed, and forms a large, some- 
what compressed, thick-walled cavity. Between 
the dorsal wall of the enteron and the lower side of 
the medullary canal lies the notochord (nt), a small, 
cylindrical rod of closely packed cells derived, in this 
region at least,from the entoderm. Inthe posterior 
region of the embryo it is not possible to determine 
with certainty the origin of the notochord, owing to 
the close fusion of all three germ layers. Between 
the wall of the enteron and the lower side of the ecto- 
derm is a considerable mass of mesoderm (mes), 
which here consists of more scattered and angular 
cells than in the preceding section. 
