HISTOGENESIS OF THE LIVER 269 



portions of the terminal bile ducts, the development of which 

 will be described in a later part of this paper. Such embryos of 

 Torpedo as I have examined agree with Acanthias in lack of 

 any definite nests or reserves of undifferentiated cells about 

 the hepatic-portal veins. In well advanced Mustelus embryos, 

 however, the hepatic cells which surround the larger branches 

 of the hepatic-portal veins do remain somewhat smaller than those 

 of the remainder of the liver and are not so completely charged 

 with fat. 



In summary it may be said that the chief differences between 

 the two types of selachian liver, that represented by Acanthias 

 and that by the several other forms mentioned, lies almost en- 

 tirely in the earlier stages. These differences seem to be depend- 

 ent on the difference in the size and arrangement of the omphalo- 

 mesenteric veins at the time of the formation of the hepatic 

 tubules. 



DEVELOPMENT OF THE MINOR R.\MI OF THE HEPATIC DUCTS 



The formation of the minor rami of the hepatic ducts is closely 

 associated with the history of the hepatic cylinders. In a pre- 

 ceding publication (Scammon '13) it was stated that all of the 

 major and some of the minor rami of the hepatic ducts arise from 

 the constricted bases of certain fairly definitely placed clusters 

 of hepatic tubules. In following the history of these tubule 

 clusters it was found that they become separated from the 

 hepatic pouch from which they arise by a broad and rather in- 

 definite peduncle which is at first hardly more than an extension 

 of the pouch wall. Later this peduncle becomes constricted and 

 elongated, forming a small branch from the pouch which by this 

 time is transformed into a segment of the hepatic duct. This 

 development begins before tubule anastomosis gets fairly under 

 way and continues at the time when anastomoses are taking 

 place. No new major rami arise as outpouchings after anastomo- 

 sis is established and aside from the actual lengthening of already 

 established rami, which is not great, all further growth of these 

 structures takes place by the transformation of pre-existent 

 hepatic tubules into ducts. 



