The Development of the Alligator 259 
rounds the embryo as an irregular membrane of 
some thickness in which no arrangement into 
layers can be seen. The epidermal ectoderm (ep) 
is composed of the usual loosely arranged cells, 
so that it is clearly distinguishable from the com- 
pactly arranged cells of the nervous layer (7), from 
which it is separated by only a line. 
In Figure 9b, which shows a section a short dis- 
tance posterior to the preceding, the medullary 
canal (mc) is somewhat deeper and is still open 
ventrally. There is a distinct space between 
the nervous (ml) and epidermal (ep) layers of the 
ectoderm, in which space a few mesoblast cells 
(mes) may be seen. The section is cut just poste- 
rior to the edge of the amnion, so that there is now 
neither amnion nor yolk above the embryo. 
Figure 9c is about ten sections posterior to Figure 
9b. The section passes through the anterior wall 
of the bent-under part of the medullary canal 
(mc’), so that the actual canal is shown only on the 
dorsal side (mc), where it is completely closed and 
begins to assume the shape of the typical embry- 
onic spinal cord. The space between the super- 
ficial (ep) and nervous (/) layers of the ectoderm is 
quite extensive and is largely filled by a fairly 
compact mass of mesoderm (mes). 
Figure 9d, although only five sections posterior 
to the preceding, shows a marked change in struc- 
ture. The medullary canal (mc) is here of the 
typical outline for embryos of this age. A large, 
