Franklin P. Mall . 253 
It is seen from what has been said above that it is undoubtedly the 
growth of the tissue of the organ which leads the way. Into this new- 
formed tissue the capillaries grow and they have an inherent power which 
makes them grow into an anastomosing tubular system. The density of 
the capillary plexus is influenced by the tissues into which they grow, 
but their length and arrangement is determined by the circulation 
through them. A vascular proportion is constantly maintained for each 
organ down to the minutest vascular twig. Capillaries through which 
the rate of circulation is below the normal shrink or disappear, and when 
it is above the normal they enlarge into either veins or arteries. A cap- 
illary too long will eventually cut itself off on account of the increased 
resistance to the circulation in its own walls. An increased flow of blood 
rarely causes an artery to empty directly into a vein, because the deter- 
mining factor is nearly always to be found in the capillaries themselves. 
The growth of the capillaries causes some of them to change into arteries 
and veins, and the equilibrium is thus easily maintained. In rare in- 
stances, however, the amount of blood thrown into an organ may be in- 
creased greatly, as is the case when all of the blood from the umbilical 
vein is suddenly forced through the liver. It follows that the circulation 
through a chain of capillaries from the portal to the hepatic vein is much 
above the normal capillary circulation, and, as a result, the ductus 
venosus is formed. 
EARLY DEVELOPMENT OF THE LIVER. 
The early development of the liver has been worked out by His and 
others, and therefore it need not be discussed to any great extent. The 
liver bud, as shown in Fig. 6, is well marked in an embryo at the end of 
the second week (2.1 mm.). It grows rapidly and then encircles the 
left omphalo-mesenteric vein, in the chick and in man, and both the 
right and the left in the dog. At the same time that the liver tissue en- 
circles the vein it also invades it, carrying the endothelial lining ahead 
of the sprouts and thus forms a series of sinuses, or the sinusoids of 
Minot. While this process is taking place the umbilical veins are gaining 
much in importance and a large share of the blood which formerly re- 
turned to the heart through the omphalo-mesenteric now returns through 
the umbilical veins. Figures 7 and 8, from an embryo 4.5 mm. long, 
and Figs. 9 and 10, from an embryo 4.5 mm. long, illustrate this point. 
In the embryo 4.3 mm. long large sprouts of liver tissue have invaded 
the common omphalo-mesenteric veins which have also extended, forming 
a large ring of Minot’s sinusoids encircling the intestine as described by 
