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at the points marked a before, the new ones must have been differentiated 
from adjoining lobules. The process by which this has taken place may 
be expressed by the twig 6. At a much earlier stage all this tissue was 
arranged in.a single lobule around the vessel d. Then it spread over the 
branch ec, and finally as the vessel d elongated a new lobule, b, arose. Its 
central vein was marked successively by the vessels, e, e’ and e”. 
It may be considered that the Figs. 39 and 40 represent the process of 
growth of the liver in one dimension of space, which becomes much more 
complex when viewed in two dimensions, as is shown in Figs. 41, 42 and 
43. The single branch, a, Fig. 41, becomes the main branch, a, Fig. 43. 
Hire: 424i. 1G. - 42. Fig. 43. 
Fics. 41, 42 and 43. Diagrams of three successive stages of the portal and 
hepatic veins in a growing liver. A, hepatic side; d, portal side; b, and c. 
successive stage of the hepatic vein; e and f, successive stages of the portal 
vein. 
The successive orders of new branches, 6 and c, explain themselves. 
Finally the lobule ¢* is adjoined by two different kinds of lobules, ¢° 
younger than itself, and b older. The portal vein has spread into this 
region, corresponding with the hepatic, and we see the branches reaching 
out in all directions, keeping equidistant from one another. This diagram 
is practically correct for most regions of the liver where there are a suc- 
cession of portal and hepatic veins alternating, so that in the region, L, 
there is another vessel like a, and in the region F another like d. But 
when these vessels, a and d, are the only ones that run out to the edge of 
the liver, the vessel e must cross a into the field R, and the vessel b must 
