ALIMENTARY TRACT AND ITS APPENDAGES 323 
more to the right side of the body from its fairly symmetrical 
anterior end backwards. 
The lines of development of the liver are thus marked out. 
On the sixth day the anterior division is larger on the left than 
on the right side, owing no doubt to the incorporation of the sinus 
venosus into the right auricle, thus leaving more room for the 
liver on the left side. Passing backwards in a series of sections 
to the region of the center of the meatus venosus, we find the liver 
larger on the right than on the left side, being centered around 
the meatus, but a small lobe extends over to the left side ventral 
to the stomach. The posterior division, again, is confined to 
the right side and ends in a free right lobe projecting caudally to 
the region of the umbilicus. The division of the liver into right 
and left lobes thus takes place on each side of its primary median 
ligaments, dorsal or gastrohepatic, and primary ventral; expan- 
sion being inhibited in the median line by the stomach above and 
heart below, it takes place on both sides, but particularly on the 
right side where there is more space. 
The reader is referred to Chapter XI for description of the 
origin of the ligaments of the liver and the relations of the liver 
to the pericardium and other structures; also to Chapter XII for 
description of its blood-vessels. 
The histogenesis of the liver should be finally referred to. 
This organ is remarkable in possessing no mesenchyme in the 
embryonic stages (Minot, 1900); but from the start the hepatic 
cylinders are directly clothed with the endothelium of the blood- 
vessels, so that only the thickness of the endothelial wall separates 
the hepatic cells from the blood in the sinusoids. The hepatic 
cylinders have been described as arising in the form of solid buds 
from the primary diverticula; the buds first formed branch 
repeatedly, forming solid buds of the second, third, ete., orders, 
and wherever buds come in contact they unite, forming thus a 
network of solid cylinders of hepatic cells. The solid stage does 
not, however, last very long, for on the fifth day it can be seen 
that many of them have developed a small central lumen by dis- 
placement of the cells. Thus there gradually arises a network 
of thick-walled tubes instead of solid cylinders, and the whole 
system opens into the primary diverticula from which it arose. 
The Pancreas. The pancreas arises as three distinct entodermal 
diverticula, the origin of which has been already described, and 
