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 FlG . 356 . SECTION THROUGH 

 heart is being formed by the separa- THE DEVELOPING HEART OF AN 



,. r ,, , , . i i r EMBRYO OF AN ELASMOBRANCH 



tion of the splanchnic mesoblast from (Pristiurus). 



the hypoblast. During this process /. alimentary tract ; j/.splanch- 



/r -\ i r 11 n ic mesoblast; so. somatic meso- 



(fig- 357) a layer of mesoblast re- blast ; ht. heart. 



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



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. notochord ; x. thickening of 

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

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



