Veins in Pig Embryos. 465 
Fig. 1. Embryo 8m.m. No. 2302. Sagittal section. 1004. Showing one 
of the subcardinal-portal anastomoses. The veins are ruled horizontally, the 
arteries in white, the tissues dotted. At the left is seen one of the small aortic 
mesonepbric branches. 
It has been stated above that the vena cava inferior is formed 
through an inosculation between one of the posterior hepatic veins 
and the right subeardinal vein. This needs no proof, having been 
shown clearly by Lewis. An idea of the topography of the viscera 
at this time will be had by reference to Plates I and II, which 
represent a 7.5 mm. embryo, and where, it will be seen, the liver is 
very small, and the subeardinals are fully formed. In spite of the 
undoubtedly distinct character of these veins, no channels are yet 
to be made out entering them from the mesentery. This would 
seem to contradict the theory of Lewis concerning their origin. 
_ They have not, however, reached their full extent in an anterior 
direction, and for that reason approach nowhere close to the liver. 
Notwithstanding this fact, there is open communication between 
the subeardinal system and the hepatic system. A channel is seen 
