296 RICHARD E. SCAMMON 



14. The early increase in size of the cyhnders is due to a muiti- 

 pUcation of cells. The later increase is due to a growth in the 

 size of the individual cells. 



15 The hepatic nuclei which are originally large and elongately 

 oval, become somewhat reduced in size and spherical in shape 

 at the time when the tubules are formed from the tubule ridges. 

 Thereafter they undergo a slow shrinkage in size ap to the time of 

 birth. In later stages they may again become oval or gibbus in 

 outline, due to the pressure of the intracellular fat. The nuclei 

 are at first basal in position and later become either central or 

 peripheral. 



16. Hepatic nuclei first have the common embryonic type of 

 structure which they retain longer than do the nuclei of most 

 tissue. The embryonic type of structure is lost first in the 

 nuclei of the gall bladder and major hepatic ducts, next in the 

 minor hepatic ducts, and finally in the hepatic cylinders. 



17. Fat appears in the cylinders at an early stage and eventually 

 fills almost the entire cell as in the typical fat cell. 



18. Selachians in which the omphalo-mesenteric veins are 

 early developed to large proportions have slender hepatic tubules 

 which indent the walls of the vessels and anastomose at a very 

 early stage. 



B. Develojmient of the minor ducts 



1. Two forms of bile duct formation exist in the selachian liver, 

 that of evagination of the liver pouch, and that of transformation 

 of pre-existing cylinders into bile ducts. The first and most 

 primitive is more active in the selachians than in higher verte- 

 brates and forms the major ducts and the proximal parts of the 

 minor ones. The distal parts of the minor ducts are formed by 

 cylinder transformation. 



2. Ducts are only difTerentiated from cylinders which lie in 

 contact with branches of the omphalo-mesenteric veins. The 

 side of the cylinder lying towards the vein is always the first 

 to differentiate and in the smaller ducts is the only part to be 

 differentiated from the true hepatic cells. 



