272 The Alligator and Its Allies 
in thickness between the ectoderm and entoderm. 
It shows laterally a slight separation to form the 
body cavity. 
Figure 11 is about ten sections posterior to 
Figure 11g, and differs from it chiefly in that the 
notochord (né) is continuous with the lower side 
of the medullary canal (mc), though still distinct 
from the underlying entoderm (en). 
Figure 112, four sections farther from the head, 
shows the same greatly thickened ectoderm (ec) 
with the same break (ec’) in the middle line. The 
section is posterior to the notochord and passes 
through the anterior edge of the blastopore or, as 
it may now perhaps better be called, the neuren- 
teric canal. The cells of the medullary wall are 
continuous with those of the entoderm. The 
mesoderm (mes) is still distinct from the other 
germ layers. 
Figure 117 is the next section posterior to the one 
just described and differs from it only in showing 
the actual opening of the neurenteric canal (nc) 
into the medullary canal (mc). The medullary 
canal extends, with gradually diminishing caliber, 
for about fifteen sections posterior to the point 
at which the neurenteric canal empties into it. 
The mesoblast (mes) is so closely attached to the 
lower wall of the neurenteric canal that it seems 
to be actually continuous with it. 
For a considerable distance posterior to the 
end of the medullary canal we find the structure 
