The Development of the Alligator 279 



than in the preceding figure. Ventral to the 

 enteron is the large heart Qit), 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 

 {sni) 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 {hi), and 

 the latter {so) being carried dorsalward as the 



