260 H. D. Senior. 



The endocardium originally lined the ventral surface of the 

 splanchnic mesoderm anterior to the junction of the head with the 

 first body somite. Of the splanchnic mesoderm lined ventrally by 

 endocardium the (larger) part anterior to the site of discharge of 

 the jugular veins became pericardial and the (smaller) part between 

 the site of discharge of the jugulars and the first body somite became 

 peritoneal. The disturbance of the original relations between the 

 structures in the posterior head region which occurred between the 

 stage of 7.2 mm. and that of 166 hours has been so extensive that 

 it is very difficult to determine the exact amount of splanchnic peri- 

 toneum which was originally situated anterior to the line of junction 

 of the first body somite with the head ; nevertheless the account given 

 below of the eventual distribution of the endocardium is probably 

 not very far from the truth. The endocardium lining the ventral 

 surface of the pericardial splanchnic mesoderm was partially, in- 

 eluded in the conus, ventricle and atrium to line these cavities; the 

 remainder formed the lining of the anterior wall of the sinus venosus. 

 The anterior wall of the sinus venosus assists in the formation of 

 the anterior ends of the right and left hepatic veins (see Fig. 24). 

 The vascular endothelial cells, migrating from the anterior wall of 

 the sinus venosus on to the yolk to furnish the ventral lining of the 

 hepatic veins, are, thus, derived from endocardium. 



The splanchnic peritoneum forming the posterior wall of the sinus 

 venosus is so close to the site of discharge of the jugular veins that 

 the vascular endothelial cells lining the posterior wall of the sinus 

 venosus are undoubtedly (like those lining the remainder of the heart) 

 endocardial in origin. Since the ends of the hepatic veins im- 

 mediately adjoining the sinus venosus are lined ventrally by endo- 

 cardial cells migrating from the anterior wall of the sinus venosus, 

 there is a great probability that the cells migrating from the splanch- 

 nic peritoneum to provide their dorsal lining (see Fig. 24), are also 

 endocardial in origin. 



The main stem of the hepatic vein is composed of vascular endo- 

 thelium which originally lined the roof of the supravitelline sinus, 

 and it is not at all unlikely that the adjoining roots of the right 

 and left hepatic veins (eventually contained within the liver) are 

 of similar origin. 



