DEVELOPMENT OF LOBULE OF PIG’S LIVER 319 
ing similar relations to one another. Thus they form undefined 
lobules, which are only different from those which are separated 
from one another by connective tissue septa, in that their vascu- 
lar systems are In more open communication with each other. 
We may unhesitatingly conclude, therefore, that the connec- 
tive tissue septa play no active part in the formation of lobules in 
the pig’s liver. The boundaries of the lobules are determined by 
the vascular system; these in turn become apparent by the paral- 
lel arrangement of the border cells; finally the connective tissue 
fibers invade the reticulum between the parallel layers of cells 
and the septum becomes established. Just what may be the 
stimulus which causes the growth of the connective-tissue fibers, 
I have not determined; it may or it may not be due to certain 
influences of the circulatory system. When this problem is 
solved we may understand why it is that in certain animals the 
septa are complete, in others partially, and in still others entirely 
absent; and further, why in the seal (Mall, ’06) the connective 
tissue septa bound the portal rather than the hepatic lobules. 
THE GROWTH OF THE LIVER 
I have thus far considered the formation of new lobules with- 
out regard to the changes which they produce in the liver as a 
whole. The growth of the liver is accomplished by two means: 
first, by an actual increase in size of the lobules; second, by a 
multiplication of lobules. I have shown in a previous paper the 
approximate rate of growth of the hepatic lobule of the pig. In 
the table below, it will be seen that the size of the larger lobules 
remains constant between the 80-mm. and the 154-mm. stages. 
From the 229-mm. stage on they show a gradual increase. The 
growth of the lobule is shown graphically in figures 16 to 19, all 
of which are camera-lucida drawings of equal magnification. 
It was believed by Illing (05) and Theopold (710) that after 
the connective-tissue septa are laid down the growth of the liver 
takes place entirely by an enlargement of its lobules. Theopold, 
however, states that the septa are not completely formed until 
