DEVELOPMENT OF THE ELASMOBRANCH LIVER 383 



with the valvular intestine in the angle between the latter struc- 

 ture and the vitelline duct. The duct lies below and to the right 

 of the median line of the duodenum, but as this structure is con- 

 tinuous only with the left side of the valvular intestine (the right 

 side of which is continuous with the vitelline duct) the ductus 

 choledochus joins the valvular intestine directly in the median 

 line of the gut. 



From this time to until the embryo reaches a length of approxi- 

 mately 25 mm. the duct grows steadily backward and its ostium 

 remains in the median line. It grows past the opening of the 

 vitelline duct passing it to the left but never overtakes the ostium 

 of the pancreatic duct. 



When the embryo reaches the length of about 25 mm., however, 

 the duct becomes involved in the twisting process of the spiral 

 valve and is carried to the left. At 28 mm. its opening is in the 

 middle of the right side of the valvular intestine and at 37 to 48 

 mm. it lies at the junction of the superior and right surfaces. 

 In embryos 60 to 80 mm. long, the duct enters the intestine on its 

 dorsal surface at the edge of the first turn of the spiral valve, and 

 in the new-born fish the opening lies at the junction of the left 

 side with the dorsal surface of the intestine. These changes in 

 position as well as the comparative diameter of the duct at differ- 

 ent stages are shown in table 2. 



Along with the elongation of the ductus choledochus come 

 several modifications of its position besides the posterior shifting 

 just described. The intrahepatic portion is affected by the growth 

 of the internal yolk sac and by the vascular changes already dis- 

 cussed in connection with their effect upon the position of the 

 hepatic and cystic ducts. This part of ductus choledochus is 

 first arched upward in a stiff almost semi-circular curve by the 

 formation of a venous sinus below it and its extreme anterior 

 end is rotated to the right. These changes are shown in figures 

 51, 52 and 54. In later stages and in the new-born fish these 

 changes are less noticeable, being probably compensated by the 

 growth of the hepatic parenchyma, but in the new-born fish as 

 well as the adult the distal end of the duct lies distinctly to right 

 of the median line. As the duodenal flexure is developed, the 



