274 A Study of the Structural Unit of the Liver 
portal vein, but also by the tips of the hepatic artery, as well as those 
of the bile duct. Furthermore, it is probable that the lymphatics arise 
from the same point and that the nerves and connective tissue spread from 
it. From time to time I have found large groups of karyokinetic figures 
there, which, together with the pathological evidence, make this the re- 
generative or growing point of the liver. The portal unit is the true 
structural unit of the liver. 
Fig. 38. Lateral view of a model of a group of lobules of the dog’s liver. 
The outside vessels are terminal portal branches. H, sublobular vein; c, ele- 
vations in the lobule over the tip of the central veins within; n, nodal points; 
p, portal vein. 
GROWTH OF THE HEPATIC LOBULE AND OF THE PorTAL UNIT. 
When I began the study of the development of the lobule of the liver 
twenty years ago, I thought that I had selected the simplest and most 
definite lobule for investigation. It seemed then easy to follow the lobule 
from stage to stage and thereby gain a comprehensive picture of the 
histogenesis of the liver. It has turned out, however, that most of the 
work—the hundreds of injections, experiments and sets of serial sec- 
tions—has been in vain, and but the faintest sketch remains from which 
to picture. The great difficulty is to recognize the same things from step 
to step, and this is obtained with certainty only in very young livers, 
