The Development of the Alligator 261 
the preceding section, and passes through the point 
of separation of the folds of the entoderm (en). 
From this point the entoderm gradually flattens 
out, leaving the enteron unenclosed. The medul- 
lary canal (mc) and notochord (nt) are about as 
in the preceding section, but the ectoderm (ep) 
is somewhat thinner and more flattened. The 
mesoderm (mes) on the right side exhibits a distinct 
cleavage, the resulting body cavity (bc) being a 
large, triangular space. 
Figure 9g, the twenty-fifth section posterior 
to that represented in Figure 9f, shows a marked 
change in the form of the embryo. While of 
about the same lateral dimensions, the dorso- 
ventral diameter of the embryo in this region is 
less than one half what it was in the head region. 
The epidermal ectoderm (ep) is now nearly hori- 
zontal in position and is not so abruptly separated 
laterally from the thin lateral sheets of ectoblast. 
The medullary groove (mg) is here a very narrow 
vertical slit. At this stage the fusion of the medul- 
lary folds has taken place over the anterior third 
of the embryo. For a short distance, represented 
in about thirty-five sections, the canal is open 
as in the figure under discussion; for the next one 
hundred sections (about one third the length of 
the embryo) in the region of the mesoblastic somites 
the canal is again closed, while throughout the last 
one third of its length the canal is widely open dor- 
sally. The enteron is here entirely open ventrally, 
