The Development of the Heart in Shad. 



255 



is a direct continuation forward of the main supra-vitelline sinus. 

 The interruption on the left side of the figure (right of embryo) 

 corresponds to the future (smaller) right hepatic vein, which is 

 not at present in direct communication with the supravitelline 

 sinus. The future right hepatic vein communicates anteriorly with 

 the future sinus and posteriorly ends blindly. 



The dorsal mesentery in this stage, as in the adult, is absent in 

 the region anterior to the stomach. 



Fig. 22 is a left lateral view of a reconstruction of the posterior 



Peritoneal 

 coelom 



Supravitelline 

 blood-sinus 



Mesoderm (peritoneal) 



Figs. 20 aud 21.— Two transverse sections of which the positions are marked 

 on Fig. 17. X 50 diams. 



Yollv and voluntary muscle shaded and vascular endothelium dotted as in 

 the preceding figure. 



gill region of a shad of 166 hours (9.46 mm. in length). At this 

 stage the peritoneal coelom in the hepatic-vein region is so large that 

 it can be opened by merely removing the body-wall; the anterior 

 pole of the yolk is shown in situ. The plane of separation between 

 sinus-venosus and hepatic-vein regions is indicated, as before, by 

 the point of discharge of the left cardinal vein. 



It will be convenient to consider tJie hepatic-vein region first. In 

 this region the peritoneal coelom has undergone great expansion by 

 reason of the extensive separation of splanchnic from somatic meso- 



