The Development of the Heart in Shad. 



259 



passing from the liver to the sinus venosus ; this function is now 

 shared bj the right branch. At eleven days the yolk has entirely 

 disappeared; the final disappearance of the (latterly torpedo-shaped) 

 yolk leaves the anterior pole of the liver still some distance posterior 

 to the pericardio-peritoneal septum. The liver now grows forward 

 (somewhat slowly) into the space formerly occupied by the yolk and, 

 therefore, receives its peritoneal covering from the splanchnic meso- 

 derm formerly enveloping the yolk and supravitelline sinus. The 

 advancing liver receives into itself the entire main hepatic stem 

 and the adjacent parts of the right and left branches. Finally, the 

 anterior pole of the liver reaches the posterior surface of the peri- 



Pericardial coelom' 



Somatic mesoderm (pericardial) 

 26 



Splanchnic mesoderm 

 (pericardial) 



Splanchnic mesoderm 



Ectoderm 

 Common Hepatic ' 



(peritoneal) 



Mesoderm (peritoneal) 



27 



Figs. 26 and 27. — Transverse sections of sbad, stage of 8 days. X 50 diams. 

 The transverse diameter of tlie sections is diminished from a tendency of 

 the notochord to collapse. Yolk, voluntary muscle, and vascular endothelium 

 as in Figs. 18 and 19. 



cardio-peritoneal septum and protruding from its anterior end are the 

 right and left hepatic veins of the adult which transmit blood into the 

 sinus venosus. 



The vascular endothelium lining the heart and the hepatic veins 

 has been derived from two sources : Firstly, from the vascular endo- 

 thelium (of which the origin has been described in the section deal- 

 ing with the formation of the heart anlage) which has been referred 

 to as endocardium. Secondly, from the vascular endothelium 

 originally lining the roof of the supravitelline sinus, of which the 

 origin has not been studied. 



