DEVELOPMENT OF LOBULE OF PIG’S LIVER a 
down from the surface, bisecting the angle formed by the new 
branches of the central vein. This septum instead of following 
the central vein, either turned off to one side or was met by 
another septum from the side, thus leaving the old central vein 
completely surrounded by liver cells. That such central veins 
do not long remain within the lobules is evident, since none of 
the larger sublobular veins are found passing through single 
lobules. The manner in which such a vein leaves its lobule, 
however, I have not fully determined. It may be that they 
later become interlobular in position by the ingrowth of addi- 
tional septa. It seems more probable, however, that they grad- 
ually become shifted to one side, since in several other similar 
lobules, I have found them eccentrically placed. In these 
lobules new central veins were found arising at right angles 
from the eccentrically placed ones, seemingly marking the 
path along which the old veins must have shifted. 
From the above description, it is evident that lobule forma- 
tion takes place by the binary fission of an elongated lobule. 
This is preceded by either a bifurcation of the central vein or 
by the sprouting out of a new vein from it. In either case, the 
two veins extend into the halves of the old lobule. By the time 
the cleavage is complete, each half of the old lobule represents 
a fully formed new lobule. The self-explanatory figures 11 
to 15 show diagrammatically the manner in which a single lob- 
ule may give rise to a larger group. 
In figure 28 is shown a wax reconstruction of a group of lob- 
ules, all of which are supplied by a single sublobular vein. This 
vein and its accompanying branches are shown in figure 27. 
From the above description, it follows that all the lobules of 
this group must have arisen from a single lobule, and, con- 
versely, all the lobules which have arisen from this original 
lobule belong to this group. Each branch of the hepatic tree 
goes to an individual lobule and represents an independent 
cleavage of the hepatic parenchyma. With the exception 
of a few of the terminal branches, it is impossible to tell which 
have arisen through a dichotomous bifurcation of terminal cen- 
tral veins and which as new outgrowths from existing stalks. 
