258 H. D. Senior. 



vascular system (see Figs. 23 and 24). This process has already- 

 progressed so far that the sinus venosus has a complete lining of 

 vascular endothelium, which is true also of the adjacent part of the 

 left hepatic vein. The yolk is still uncovered in the region of 

 Fig. 25. 



The sinus venosus, since the yolk has been consigned to a posi- 

 tion definitely external to it, may now be looked upon as a complete 

 structure in which the achievement of the adult condition is merely 

 a matter of detail. The layers of splanchnic mesoderm, pericardial 

 and peritoneal, which have been called the anterior and posterior 

 walls of the sinus venosus are, to some extent, in mutual contact 

 just behind and above the apex of the ventricle (see Fig. 22). 

 Where these two layers are in contact the terms anterior and pos- 

 terior wall of sinus venosus are not strictly applicable, for they 

 together form the anlage of the pericardio-peritoneal septum. During 

 the process of disappearance of the yolk, the pericardio-peritoneal 

 septum undergoes further increase, and later gowth produces the 

 extensive structure of the adult. 



A few words may be added with regard to the completion of the 

 hepatic vein. The conditions indicated in Figs. 24 and 25 are tend- 

 ing in a direction which ends in the complete formation of the hepatic 

 vein, as shown in Figs. 26 and 27. The latter figures come from 

 sections of an embryo eight days old, and correspond in position 

 to Figs. 24 and 25. In order to understand the condition of the 

 hepatic vein at eight days it is necessary to appreciate the follow- 

 ing facts: The yolk is not only smaller at its equator but the dis- 

 tance between the poles is also much diminished. The liver, which 

 has grown forward so as to keep pace with the posterior pole of 

 the yolk, is now very much nearer the heart than before. Redis- 

 tribution of vascular endothelium, excluding the yolk from the vas- 

 cular system (see Figs. 26 and 27), has now occurred throughout 

 the entire extent of the yolk {i. e., from heart to liver). 



The hepatic vein is, at this time, rather a long vessel and consists 

 of a main stem bifurcating anteriorly into short right and left 

 branches ; both stem and branches replace the original supravitelline 

 blood sinus. The left branch formerly transmitted all the blood 



