270 RICHARD E. SCAMMON 



This illustrates well the two methods of hepatic duct formation. 

 The first and more primitive type is that suggested by Minot 

 ('93, p. 763) in which the duct is the result of a direct outpouch- 

 ing of the wall of the hepatic diverticulum. In the second and 

 specialized type the duct is produced by the transformation of 

 portions of the network of hepatic cylinders. Selachians clearly 

 stand near the bottom of the scale in this phase of development. 

 Here the ductus choledochus, the hepatic ducts and their major 

 rami arise as outpouchings and only the minor rami and the most 

 distal portions of the major ones are of trabecular origin. In 

 ganoids, amphibians, reptiles and birds the ductus choledochus 

 and the proximal part, at least, of the hepatic ducts are formed by 

 outpouching and the remainder of the duct system from tra- 

 beculae. In the mammals apparently the ductus choledochus 

 alone is the result of an outpouching, but further investigation 

 may change this conception. 



The differentiation of hepatic cylinders into bile ducts is very 

 closely associated with their relations to the blood vessels. Bile 

 ducts are only formed from cylinders which are in contact with 

 the main trunks of the hepatic-portal veins or their larger and 

 more definite branches. Still more striking is the fact that the 

 side of the cylinder towards the vessels precedes in a very marked 

 degree the differentiation of the opposite side and in fact in the 

 smallest ducts the cells of the opposite side of the cylinder may 

 never be transformed into duct epithelium at all but complete 

 the ordinary development of true hepatic cells. Such a terminal 

 duct from the liver of an embryo 95 mm. in length is shown in 

 figure 37. On the other hand, of the many cylinders which 

 closely surround the vascular trunk only a very small percentage 

 is transformed into ducts. The development of the minor 

 ducts is extremel}^ small in proportion to the amount of hepatic 

 parenchyma, smaller, I think, than in any other group of verte- 

 brates. There is absolutely no indication of any system of 

 intercalated ducts. 



In the differentiation of a hepatic cylinder into a bile duct the 

 former first approaches more nearly a perfect circle in cross 



