The Development of the Heart in Shad. 



257 



of the peritoneal coelom affects only the hepatic-vein region (Fig. 

 22). It ends so abruptly that the splanchnic peritoneum, passing 

 peripherally from the anterior ends of the hepatic veins to become 

 somatic, forms a partition between sinus-venosus and hepatic-vein 

 regions. This partition of splanchnic peritoneum represents the 

 posterior wall of the sinus venosus, and has been labelled accordingly 

 in Fig. 22. 



In the sinus-venosus region shrinkage of the yolk has brought 

 about marked diminution in the vertical dimension of this part of 



I Ventricle 

 Somatic mesoderm (pericardial) 



23 



Peritoneal 

 coelom / 



R. Hepatic v. 

 (future) 



Splanchnic 



mesoderm 



pericardial) 



L. Hepatic v. 



Pericardial 



coelom 



Somatic mesoderm (pericardial) 

 24 



_ Supraviielline_ 

 Mesoderm^ blood-sinus 

 (peritoneal) 

 25 



Figs. 23, 24, and 25. — Three transverse sections of which the positions are 

 iuarl<ecl on Figs. 22. X 50 diams. 



Yolk, voluntary muscle, and vascular endothelium indicated as in Figs. 

 18 and 19. 



the embryo (compare Figs. 22 and 17). The yolk-process has 

 become thicker and much shorter ; the space between its posterior 

 surface and the yolk (which constituted the posterior communica- 

 tion between the right and left sides of the sinus venosus) is now 

 obliterated. 



The most striking change taking place at this stage occurs, alike, 

 in the sinus-venosus and hepatic-vein regions; it is a process of re- 

 arrangement of the vascular endothelium. The vascular endothelium 

 is rapidly spreading from the splanchnic mesoderm, both pericardial 

 and peritoneal, on to the yolk so as to exclude the latter from the 



