The Development of the Alligator 287 
seen, and also a slight condensation of meso- 
blast, the latter probably being the forerunner of 
cartilage. 
Figure 13¢ passes through the anterior part of 
the heart about seventy-five sections posterior to 
Figure 13). The embryo in this region is narrow 
but deep (dorso-ventrally), the depth being largely 
due to the size of the heart. The ectoderm (ec) 
is considerably thickened on each side of the 
pharynx (ph); this thickened area may be traced 
for some distance both anteriorly and posteriorly 
from this point; its significance could not be deter- 
mined. The spinal cord (sc) and notochord (n#) 
need no special description; the former is smaller 
and the latter larger than in the more anterior 
sections. The two large blood-vessels (ac) near 
the spinal cord and notochord are probably the 
anterior cardinal veins. The aorte are cut by the 
plane of this section just anterior to their point of 
fusion into a single vessel. A few blood corpuscles 
are seen in the right aorta. The enteron (ent), cut 
posterior to the region of the gill clefts, is a large 
elliptical cavity, with its long axis in a transverse 
position. Its entodermal wall is comparatively 
thin and smooth, with the cell nuclei arranged 
chiefly on the outer side, 7. e., away from the 
cavity of the enteron. The body cavity (bc) is 
here still unenclosed, and its walls, the somatic 
stalk, are cut off close to the body of the embryo. 
The heart (ht), the most conspicuous feature of 
