322 THE DEVELOPMENT OF THE CHICK 



tribution of the cephalic duct to the left, and the caudal duct to 

 the right lobe. 



The liver is primarily an unpaired median organ. Its division 

 into right and left lobes is therefore secondary and has no funda- 

 mental embryological significance. The factors that determine 

 its definitive external form are the following: (a) the relative 

 power of growth of its various parts; (6) limitation of its exten- 

 sion to the septum transversum and its connections; (c) the limi- 

 tations of space in the ccelome. 



Bearing these principles in mind, the growth of the liver 

 may be described as follows: three primary divisions succeed- 

 ing one another in a cranio-caudal direction, may be distinguished 

 at an early stage, viz., an antero-dorsal division, abutting on the 

 postero-dorsal part of the sinus venosus, formed by the anterior 

 end of the cephalic hepatic diverticulum; an intermediate division, 

 surrounding the meatus venosus in which both cephalic and 

 caudal hepatic diverticula are concerned; and a postero- ventral 

 division, beneath the posterior end of the meatus venosus and the 

 right omphalomesenteric vein, formed exclusively by the caudal 

 diverticulum. 



The growth of the liver causes expansion of the median mass 

 of the septum transversum in all directions, excepting anteriorly, 

 and the substance of the liver extends more or less into all the 

 connections of the latter, viz., the lateral mesocardia, the lateral 

 closing plates associated with the umbilical veins, the primary 

 ventral ligament, the mesentery of the vena cava, the gastro- 

 hepatic ligament, and that part of the hepatic portal vein formed 

 by the right omphalomesenteric vein. 



At the stage of 96 hours the anterior division spreads 

 out in the lateral mesocardia behind the Cuvierian ducts nearly 

 to the lateral body-wall on each side. The intermediate division, 

 on the other hand, lies largely on the right side of the middle 

 line, owing to the displacement of the stomach to the left and the 

 meatus venosus to the right. A small lobe is, however, pushing 

 itself to the left beneath the gastro-hepatic ligament. The pos- 

 terior division lies entirely on the right ventral side of the hinder 

 end of the meatus venosus and right omphalomesenteric vein, 

 as far back as the dorsal anastomosis. There are, of course, 

 no sharp lines of demarcation between the divisions, so that in 

 general it may be said that the liver substance tends more and 



