258 A Study of the Structural Unit of the Liver 
already taken place in the specimens shown in Figs. 17 and 20; in a later 
stage, Fig. 25, the omphalo-mesenteric still remains open after the ductus 
venous is formed. 
During all this time the vascular proportion remains normal, that is, 
the area of the capillaries is about 50 times that of the main portal trunk. 
The distributing branches are on one side of the lobule, and the collecting 
branches on the other. With an increase of the number of lobules, how- 
ever, they are no longer set parallel, but at various angles with one an- 
other. Were they continued parallel they would have to spread as a 
Ties 18 Fic. 14. 
Fic. 13. Section through the liver of a human embryo 5 mm. long (No. 
80). X25. C sixth cervical myotome; a, aorta; cv, cardinal vein; s, 
stomach; w, umbilical vein; l/pc, lesser peritoneal cavity. 
Fig. 14. Semidiagrammatic reconstruction of the veins of the liver of a 
e 
human embryo 5 mm. long (No. 80). JL, liver; vu, umbilical vein; 7. vom, 
right omphalo-mesenteric vein; rhs, ramus hepatic sinistra; rw, recessus um- 
bilicalis; ra, ramus angularis; m, mesenteric vein; i, intestine. 
sheet with a thickness of a millimeter, the maximum normal length of a 
capillary. In an embryo at the end of the fifth week, Fig. 25, two new 
lobules have made their appearance and the two primary lobules have 
begun to divide. The hepatic and portal veins are telescoping; they are 
beginning to dovetail with each other. The new branches of the portal 
have gone into the field of the hepatic and the new hepatic veins have 
entered into the portal field. By this process, and by this process only, 
can a spherical vascular organ be built up maintaining a normal vascular 
