634 



THE HEART. 



356), 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 continuous 

 with the splanchnic mesoblast sur- 

 rounding the throat, and form a pro- 

 visional mesentery the mesocar- 

 dium which attaches the heart to 

 the ventral wall of the throat. The 

 superficial stratum of the wall of 

 the heart differentiates itself as the 

 peritoneal covering. The inner epi- 

 thelioid tube takes its origin at the 

 time when the general cavity of the 

 heart is being formed by the separa- 

 tion of the splanchnicmesoblastfrom 

 the hypoblast. During this process 

 (fig. 357) a layer of mesoblast re- 

 mains close to the hypoblast, but connected with the main mass 



FIG. 356. SECTION THROUGH 

 THE DEVELOPING HEART OF AN 

 EMBRYO OF AN ELASMOBRANCH 

 (Pristiurus). 



al. alimentary tract ; sp. splanch- 

 nic mesoblast ; so. somatic meso- 

 blast ; ht. heart. 



FIG. 357. TRANSVERSE SECTION THROUGH THE POSTERIOR PART OF THE 

 HEAD OF AN EMBRYO CHICK OF THIRTY HOURS. 



hb. hind-brain; vg. vagus nerve; ep. epiblast; ch. notochorcl ; x. thickening of 

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

 //. body cavity; so. somatic mesoblast; sf. splanchnic mesoblast; Ay. hypoblast. 



