The Development of the Alligator 271 
cavities. The former is no longer enclosed, a dor- 
sal fold in the entoderm being all that remains of 
the cavity that was seen in the more anterior 
figures, while the latter is here reduced to a narrow 
cleft between the somatic and splanchnic meso- 
blast. A thickening of the mesoblast on either 
side of the notochord, especially on the left, repre- 
sents a mesoblastic somite. The medullary canal 
(mc) is more open than in the more anterior sec- 
tions. 
For about one third of the length of the embryo 
posterior to Figure 11f there is a gradual flattening, 
in a dorso-ventral direction, with loss of the am- 
nion, until the condition represented in Figure 11g 
isreached. The most striking feature of this region 
is the great thickness of the ectoderm (ec), which is 
still made up of scattered, irregular cells. In the 
middle line, directly over the medullary canal (here 
a nearly cylindrical tube), is a sort of break in the 
ectoderm, as though there had not been a complete 
fusion of the epidermal layer when the nervous 
layer came together on the closure of the medullary 
groove. This break in the ectoderm may be fol- 
lowed back to the region of the primitive streak, 
and will be mentioned again. As has been noted, 
the medullary canal (mc) is nearly circular in cross- 
section, and is closely underlaid by the notochord 
(nt), which is several times the diameter that it 
was in more anterior sections. The mesoblast 
(mes) is a comparatively thin layer, intermediate 
