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shifted ; they are present in those portions of the liver which had to make 
way for encroaching organs. Not only must large masses of the liver 
disappear entirely, but also smaller areas throughout the liver, especially 
along the trunks of the main vessels, as the liver is growing from its cen- 
ter towards its periphery. Hand in hand with this change, the plexus 
of bile ducts which surrounds the main trunks of the portal vein shifts 
towards the periphery, leaving only a single vessel in its place, which, of 
necessity, becomes very variable, as has been pointed out by Rex. : 
In a liver which has been well formed, as in the rabbit’s liver shortly 
after birth, the cells radiate from the terminal bile ducts towards 
the nodal points and the central veins, as indicated by the lines in Fig. 1. 
The point of juncture between the bile ducts and liver cells is not sharp 
and the younger the liver the more difficult it is to determine it. In fact, 
in young embryos it is extremely difficult to follow the bile ducts into the 
structural units, t. e., to the lobules. Injections show that the younger 
the specimen the more extensive is the plexus of bile ducts around the 
terminal veins, which indicates that an intermediate tissue, neither true 
lobule tissue nor true bile ducts encircle the terminal portal veins in 
growing livers, as is shown beautifully in a pig two months after birth. 
When the liver is finished, this tissue is reduced to a minimum. When 
the liver begins to regenerate, it becomes conspicuous again as “ newly- 
formed bile ducts.” With a plexus of bile ducts encircling a portal vein 
of the sixth order throughout its whole extent we have the most intimate 
connection between the bile ducts and the center of the portal unit, from 
which additions can be added to the unit. As the cells are added they 
seem to pile up in the nodal points, for the distance between the terminal 
portal and hepatic veins does not increase but remains constant. Hand 
in hand with the growth of the nodal points the capillaries follow, and on 
account of their increased number the resistance to the circulation 
through them is diminished, and, according to Thoma’s first law, veins 
from both sides are extended into them; these alternate, as the obserya- 
tions above described have demonstrated. . é 
To obtain an additional key by which we may unravel the growth and 
architecture of the liver units numerous tests have been made in the 
Anatomical Laboratory of the Johns Hopkins University, by Hendrick- 
son, Sudler, Johnson, Sabin, Hill, and myself, with more or less satis- 
factory results. At present we are able to follow in a connected way the 
formation of the bile ducts and capillaries in embryos from 5 em. long 
upward. Possibly at some later date they may be followed back to their 
earliest appearance. Furthermore, it is probable that some simple meth- 
ods will soon be found, by which the history of the blood-vessels can be 
