DEVELOPMENT OF THE ELASMOBRANCH LIVER 387 



sinus below and to the left of the anterior end of the ductus 

 choledochus. 



9. The difference in size of the right and left omphalo-mesen- 

 teric veins may be due to the longitudinal rotation of the gut 

 mentioned in section 5 of this summary. By this rotation the 

 space between the left hepatic pouch and the median pouch and 

 gut is somewhat increased while the corresponding space on the 

 opposite side is decreased. 



10. A third shifting of the duct system and the gall bladder 

 forward and upward occurs at a much later stage. It is probably 

 brought about through the great increase in size of the internal 

 yolk sac. 



11. The minor hepatic ducts arise as the elongated pedicles of 

 definitely placed groups of hepatic tubules. These tubule groups 

 are differentiated at the time when the liver loses its dorsal con- 

 nection with the fore gut. 



12. The variations in position of the minor hepatic ducts in 

 the adult depend upon the degree of shifting of these tubule 

 groups at the time when the main hepatic pouches are differentiated 

 into hepatic ducts and when the rotations mentioned in sections 

 5 and 8 take place. 



13. The gall bladder appears much later than do the primary 

 lateral hepatic diverticula. It arises posterior to the hepatic 

 anlage as a distinct evagination of the gut at the juncture of the 

 floor of the fore gut and anterior wall of the yolk sac, and its inti- 

 mate connection with the liver duct system is acquired secondarily. 



14. The gall bladder loses its dorsal and posterior connection 

 with the gut and is shifted forward and downward until its great- 

 est axis is in the transverse plane of the body. In this way its 

 connection with the ductus choledochus comes to lie in front of 

 that of the lateral hepatic ducts. 



15. Subsequently the gall bladder is again shifted upward and 

 forward so that the cystic duct passes backward and downward 

 to join the ductus choledochus. This is brought about by the 

 agency mentioned in section 10. 



16. The ductus choledochus is formed from three distinct 

 elements : the anterior part from the pars ductus mediana, a deriva- 



