380 RICHARD E. SCAMMON 



In this way the yolk sac presses upon the right side and lower 

 surface of the gall bladder. The first effect of this pressure is to 

 force the posterior part of the gall bladder upward so that this 

 structure retraces in part the path of downward extension which 

 it followed in earlier stages (figs. 22, 23, 24, 25 and 26 G). At 

 the same time the gall bladder is elongated and somewhat flat- 

 tened vertically and its anterior end is pushed upward and for- 

 ward. At first this only causes an abrupt curve in the cystic 

 duct (fig. 54), but as this process is continued the gall bladder is 

 finally forced anteriorly and dorsally beyond the distal end of the 

 ductus choledochus and the cystic duct proceeds backwards over 

 its dorsal surface to join that structure. In this way the ostium 

 of the cystic duct which was originally in the floor of the ductus 

 choledochus is rotated to its anterior surface and the two ducts 

 together form one continuous tube sometimes called the ductus 

 cystocholedochus, the juncture of the two elements of which is 

 at the ostia of the hepatic ducts. This is also illustrated in figures 



22, 23, 24, 25 and 26 G. 



« 



IV. DEVELOPMENT OF THE DUCTUS CHOLEDOCHUS 



The development of the ductus choledochus is better known 

 than that of any other part of the biliary apparatus in elasmo- 

 branchs. For our information in regard to the earlier stages 

 we mainly have to thank Hammar ('93), Brachet ('96), Mayr 

 ('97) and Choronschitzky ('00) while the later stages have been 

 most successfully studied in connection with the development of 

 the spiral valve by means of the reconstruction method by 

 Riickert ('96, '97). 



The development of this structure in Acanthias has been to 

 some extent described incidentally in connection with the account 

 of the early stages of the liver and of the hepatic and cystic ducts 

 and gall bladder in this paper so that only a short summary need 

 be given here. 



In Acanthias, the ductus choledochus when fully formed is a 

 complex consisting of three embryonic elements: (1) a proximal 

 or posterior segment derived from the floor of the duodenum and 



