262 A Study of the Structural Unit of the Liver 
main trunks of the portal veins; each interlobular vein at this stage is 
to form a main trunk in the adult. At this time we have terminal vessels 
to follow from stage to stage, which is impossible to do in adult speci- 
mens. 
The process of sprouting and interlacing continues at a rapid pace 
from now on, and for the present I shall give illustrations from the livers 
of two embryos of the eighth week. The first (No. 22), Fig. 26, is from 
a wax plate reconstruction carried as far as I could conveniently, and 
Fig. 27 is from a photograph. The second (No. 6) Fig. 28, is from a 
graphie reconstruction which could be carried out pretty well, and Figs. 
— 
Fic. 21. Lateral view of a model of the liver in position of a human em- 
bryo 11 mm. long (No. 109). X 8%. Li, liver; 1, lung; r, first rib; ph, 
phrenic nerve; s, stomach; wf, Wolffian body; pp, pleuro-peritoneal mem- 
brane. 
29-31 are three views of a wax model of the exterior of the liver. These 
illustrations together show the form of the liver and the main vessels with 
their lobular branches. There are about 700 branches of the third order 
in the adult liver, and rough estimations made from Figs. 27, 29-31 give. 
about this number. The lobules in these specimens are about 0.4 mm. in 
diameter, considerably smaller than in the adult. In general branches 
of the hepatic and portal veins of the same order are as far apart as 
possible with a tendency to run at right angles to each other. The 
branches of the first order or main trunks have been present from the 
time of the earliest differentiation of the liver, while those of the second 
