THE VASCULAR SYSTEM. 



521 



form of a half tube (fig. 856), it soon becomes folded in on the dorsal 

 side so as to form for the heart a complete muscular wall. Its two 

 sides, after thus meeting to complete the 

 tube of the heart, remain at first con- 

 tinuous with the splanchnic mesoblast 

 surrounding the throat, and form a pro- 

 visional mesentery — the mesocardium — 

 which attaches the heart to the ventral 

 wall of the throat. The superficial 

 stratum of the wall of the heart differ- 

 entiates itself as the peritoneal covering. 

 The inner epithelioid tube takes its 

 origin at the time when the general 

 cavity of the heart is being formed by 

 the separation of the splanchnic meso- 

 blast from the hypoblast. During this 

 process (fig. 357j a layer of mesoblast 

 remains close to the hypoblast, but con- 

 nected with the main mass of the meso- 

 blast by protoplasmic processes. A 

 second layer next becomes split from 

 the splanchnic mesoblast, connected with 

 the first layer by the above-mentioned 

 protoplasmic processes. These two layers form together the epithe- 

 lioid lining of the heart ; between them is the cavity of the heart, 



Fig. 356. Section through 

 the developing heart of an em- 

 BRYO OF AN Elasmobranch (Pris- 

 tiurus). 



al. alimentary tract ; sp. 

 splanchnic mesoblast ; .so. so- 

 matic mesoblast ; ht. heart. 



Fig. 357. Transverse section through the posterior part of the 



HEAD OF AN EMBRYO ChICK OF THIRTY HOURS. 



hh. hind-brain ; vg. vagus nerve ; ep. epiblast ; ch. notochord ; x. thickening of 

 hypoblast (possibly a" rudiment of the sub-notochordal rod); al. throat; ht. heart; 

 pp. body cavity ; so. somatic mesoblast ; sf. splanchnic mesoblast ; hy. hypoblast. 



