276 A Study of the Structural Unit of the Liver 
likened to a single lobule of the adult. In embryos four weeks old there 
is a distributing and collecting end to each of the lobules present and 
not until the eighth week is the adult form to be recognized in the struc- 
ture of the liver. Furthermore, they state that in their growth the lobules 
split and give rise to numerous new lobules. 
The process of expansion and rearrangement is expressed in the dia- 
gram, Figs. 39 and 40, which may be viewed to represent either a portal 
NIG 39: Fia. 40. 
Fics. 39 and 40. Diagrammatic illustration of two stages of a growing 
hepatic vein. D, main vein; c, its branch; 7, 2, 8, 4, 5, the same vessels in 
both diagrams; a, a, new branches from the stem c; b, a new branch on the 
main stem, d in which the successive stages of the central vein are marked 
éwe, andue”. 
or an hepatic twig. As the vessels represented in Fig. 39 grow, the liver 
tissue increases in quantity, but the liver lobules do not increase in size 
indefinitely, because Thoma’s first law is constantly at work and will soon 
break up the larger lobules into a number of smaller ones. In all cases 
the length of the capillaries remains constant, and when they appear to 
be too long and too numerous it is always found that some of them have 
already turned into small veins and thus mark the beginning of new 
iobules, or of new portal units. A more advanced step is represented in 
Fig. 40. The lobules or units of the two stages are marked with corre- 
sponding numbers. But each of them is splitting at its end and new 
vessels, a, have also arisen from the main trunk. As there were no lobules 
