DEVELOPMENT OF LOBULE OF PIG’S LIVER 309 
numerous, it is always found that some of them have already turned 
into small veins and thus mark the beginning of new lobules or of new 
portal units. 
While in general the idea expressed in the above statement 
may be correct, nevertheless it should be emphasized that the 
sinusoids of the liver are not all of equal length. Mall, him- 
self, shows that within a single lobule they are of various lengths, 
that. there are short paths and longer ones between the portal 
and hepatic veins. Yet he believes that all the capillaries of 
the lobule and of the liver as a whole are equally favored by 
the circulation. In a former paper (’18 a) I have shown that 
in the adult pig the lobules vary greatly in diameter; the lengths 
and numbers of their capillaries, consequently, must also vary. 
Again, in my study of the development of the lobules, I find 
that the lobules do not reach a certain constant size before 
they begin to divide into new ones; a large lobule and a smaller 
one placed side by side may be in a similar stage of division. 
In his recent studies on the growth of the blood-vessels in 
the tail of the frog larva, Clark (’18) has shown that as certain 
areas of tissue increase in size, the surrounding capillaries send 
into the area endothelial sprouts which develop into additional 
capillaries. When these new capillaries become functional, that 
is, when a flow of blood through them is inaugurated, there 
results an increased flow through and an increase in the size of 
certain ones of them which are favorably located. According to 
Clark, it is the increased amount of blood flowing through these 
vessels which causes them to enlarge. Conversely, Clark has 
shown that when the flow through capillaries is diminished, the 
capillaries become smaller, when the flow stops altogether, they 
close, break in two, and are gradually retracted into the vessels 
with which they were originally connected. 
The application of these observations to the growing liver 
may be made as follows: As the hepatic cells increase in number, 
new capillaries are formed, bringing about an increased flow 
of blood in, and a subsequent increase in size of, certain capil- 
laries leading to and from the new ones. These enlarged cap- 
illaries become new branches of the portal and hepatic veins. 
