The Development of the Alligator 279 
than in the preceding figure. Ventral to the 
enteron is the large heart (ht), projecting below 
the body cavity, which is no longer enclosed. The 
mesodermic wall (mes’) of the heart is still com- 
paratively thin and is separated by a considerable 
space from the membranous endocardium (en). 
The extent and shape of the heart are shown in the 
surface view of this stage. On the right side of the 
section the body cavity extends to a point nearly 
opposite the middle of the spinal cord, considerably 
dorsal to the notochord, while on the left side the 
dorsal limit of the body cavity is scarcely level 
with the lower side of the notochord. Between 
the dorsal end of the body cavity and the side of the 
spinal cord, on the left, is a dense mass of mesoblast 
(s), one of the mesoblastic somites. A few sections 
either anterior or posterior to the one under dis- 
cussion will show the condition of the two sides 
reversed—that is, the body cavity will extend to 
the greater distance on the left and will be inter- 
rupted by a mesoblastic somite on the right. It 
is evident, then, that the upper angle of the body 
cavity is extended dorsally as a series of narrow 
pouches between the somites. The mesoblast 
that lines the body cavity, the splanchnopleure 
(sm) and somatopleure (so), is somewhat denser 
than the general mass of mesoblast, so that these 
layers are quite distinct, the former (sm) extend- 
ing around the enteron (ent) and heart (ht), and 
the latter (so) being carried dorsalward as the 
