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 descril)ed 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, etc., 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 w'hole 

 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 



