174 OUTLINES OF CHORDATE DEVELOPMENT 



completely separating the somatopleure , or body wall, consisting 

 of the somatic mesoderm and integument, from the splanchno- 

 pleure, or gut wall, consisting of splanchnic mesoderm and 

 enteric wall. 



In the pharyngeal region, the layers of mesoderm remain 

 fused together medially, below the gut; consequently the 

 splanchnoccel is paired in this region, where the heart develops 

 later. The median fusion is a vestige of a ventral mesentery. 

 Along the dorsal side of the enteron, the splanchnic layers of 

 mesoderm push in between the chorda and the enteron, and 

 form the dorsal mesentery by which the gut remains connected 

 with the dorsal wall of the body cavity, and through which 

 later the vessels, nerves, etc., pass to and from the gut. When 

 the yolk is absorbed and the narrowed gut passes to the ventral 

 side of the body cavity, the mesentery forms a thin double 

 fold of membrane. Then as the intestine elongates the mesen- 

 tery is thrown into folds corresponding with those of the gut. 



Through the absence of a ventral mesentery, save in the heart 

 region,- the body cavity is continuous from side to side beneath 

 the gut; dorsally the mesentery interrupts such a communica- 

 tion. Later on, the body cavity becomes incompletely divided 

 transversely into anterior and posterior parts, but this and the 

 formation of the pericardial portion of the body cavity, are 

 more conveniently described in connection with the vascular 

 system. 



V. THE VASCULAR SYSTEM 



1. The Heart 



The first parts of the vascular system to appear are the 

 heart and the large veins connected with its posterior end. 

 We have already said that the cardiac region lies beneath the 

 hinder part of the pharynx, immediately anterior to the liver 

 and posterior to the thyroid body. In this region the somatic 

 and splanchnic layers of the lateral plate are separated by a 

 wide cavity which is the beginning of the pericardial cavity 

 (Fig. 61). This is at first directly continuous posteriorly with 



