254 



H. D. Senior, 



tion both anterior (dorsallj) and posterior (ventrallj) to the yolk 

 process. 



Conditions in the hepatic-vein region become more readily in- 

 telligible after preliminary examination of a section from 0.02 

 mm. posterior to the reconsitrncted part of the embryo. .Here 

 (Fig. 21) the peritoneal mesoderm (in which the ccelom is to 

 a large extent virtual) covers the yolk laterally and dorsally and 

 is generally in contact with it. Contact between the yolk and 

 peritoneum is interrupted on the left side by the interposition of 



Pericardial coelotn 

 18 19 



Figs, IS and 19. — Two transverse sections of which the positions are uiarlved 

 on Fig. 17. X 50 diams. 



Yolli horizontally shaded, voluntary muscle obliquely shaded. The distribu- 

 tion of vascular endothelium, including endocardium, is indicated by large 

 dots. 



the supravitelline blood-sinus (see introduction). In passing tail- 

 ward in the series from the position of Fig. 21, the yolk increases 

 in size and the supravitelline sinus becomes more dorsally placed 

 with regard to the yolk, but is always mainly to the left of the 

 mid-sagittal plane. 



In Fig. 20, from the hepatic-vein region of the reconstniction, 

 contact between the peritoneum and yolk is interrupted on both 

 right and left sides. The interruption on the right of the figure 

 (left of embryo) corresponds to the future left hepatic vein and 



