Franklin P. Mall - 255 
end of the third week, it is composed of a single lobule with the vein en- 
tering it on one side and the collecting vein leaving it on the opposite 
side. The turning point between the first and second stages of develop- 
ment is shown in Fig. 11. In the embryo represented by this figure the 
omphalo-mesenteric veins are broken completely into capillaries in the 
liver, and one umbilical vein has been transferred from the ductus 
Cuvieri to the lower part of the liver. The single liver lobule here is 
perfect; it is composed of a complete capillary network without an anas- 
iS 
ULV. 
HIG. 7% 
Fic. 7. Section through a human embryo 4.3 mm. long (No. 148). X 25. 
T,, T;,, T;, third, fourth and fifth thoracic myotomes; i, intestine; 7, liver; 
v, ventricle; ba, bulb of the aorta; am, amnion; wv, umbilical vein. 
Fic. 8. Semidiagrammatic reconstruction of the veins of the liver of a 
human embryo 4.3 mm. long (No. 148). JL, liver; wv, umbilical vein; vom, 
omphalo-mesenteric vein; i, intestine. 
tomosing vein through it. A rough estimation of the vascular proportion 
shows that the area of the capillaries is fully 100 times that of the enter- 
ing veins. In the next two embryos, Figs. 12, 13 and 14, all of the blood 
from the left umbilical vein passes through the liver—the right vein 
having been obliterated. Within the liver it is seen that the right om- 
phalo-mesenteric vein is open, while the main branches of the hepatic and 
portal veins have made their appearance. With the growth of the liver 
the capillary bed has increased which is naturally followed by a more 
rapid circulation in the distributing and collecting capillaries, and con- 
