282 The Alligator and Its Allies 
Figure 12g is through the neurenteric canal (nc), 
a distinct opening through the floor of the spinal 
canal. The section is of course just back of the 
posterior end of the notochord. The entoderm 
(en) along the margin of the neurenteric canal is 
naturally continuous with the wall of the spinal 
cord (sc). The ectoderm (ec) is thicker than ever, 
except in the median plane, where it passes over 
the spinal cord. The mesoblast is more abundant 
than in the preceding figure, and shows on the 
left what appears to be a distinct myoccel (myc), 
though in surface view the mesoblastic somites do 
not extend this far toward the tail. 
STAGE X 
FIGURES 13-13g (PLATES XIX., XX., X XI.) 
This embryo (Fig. 13) is about 5mm. in length, 
and hence is slightly smaller than the preceding 
stage, though somewhat more advanced in develop- 
ment. The medullary canal is still apparently 
unclosed for a short distance at the extreme poste- 
rior end; this appearance is probably due to the 
neurenteric canal (mc) and to the thinness of the 
roof of the medullary canal rather than to any 
lack of fusion of the medullary folds. The optic 
vesicle is more distinct than in the preceding stage; 
a somewhat similar, though smaller, opacity (0) 
marks the position of the ear. There are now 
about twenty pairs of somites, though it is difficult 
