DEVELOPMENT OF THE ELASMOBRANCH LIVER 355 



this union does not occur at the same time along the entire antero- 

 posterior length of the hver but proceeds from the cephalic end 

 backward, thus producing the CZ-shaped figure shown in figures 

 9 and 10. 



It appears that too much importance has been placed upon the 

 position of the liver anlagen in regard to the anterior intestinal 

 portal. Laguesse ('93), Brachet ('96) and Choronschitzky ('00) 

 all emphasize this point. This structure, however, is constantly 

 shifting posteriorly as has been demonstrated by Mayr ('97), and 

 the location of the liver in front of it holds good only for stages 

 which are well advanced. The location of the liver as immedi- 

 ately behind the sinus venosus is perhaps of more value, but in 

 Acanthias at least, the liver is present before the two omphalo- 

 mesenteric veins become confluent to form the endothelial heart, 

 or indeed before they are represented by definite endothelial tubes. 



Laguesse has already called attention to the fact that in Acan- 

 thias the gall bladder appears somewhat later than the remainder 

 of the hepatic apparatus, and seems to be developed from the 

 anterior wall of the yolk-stalk rather than from the posterior 

 part of the median liver pouch. Hammar ('93, '97) appears to 

 hold the same opinion in regard to Torpedo. I believe that my 

 sections and models bear out this conclusion and that this struc- 

 ture can be properly interpreted as an organ arising quite separate 

 from the hepatic anlage at the juncture of the pars ductus of the 

 lateral liver diverticula and the floor of the gut, as represented by 

 the anterior wall of the yolk-stalk portal. The shifting of the 

 sac anteriorly so that its duct comes to lie in front of the openings 

 of the hepatic ducts into the ductus choledochus will be discussed 

 in the following section. 



All specimens after the stage when the median and lateral 

 hepatic pouches are formed show a small but constant rotation 

 of the hepatic anlage to the right. This rotation seems without 

 doubt to be a part of that greater one which produces the spiral 

 valve. Like the latter it is from the left to the right side, i.e., 

 clockwise around an axis corresponding to the longitudinal axis 

 of the gut, and it is coincident with it, appearing when the embryo 

 has from 50 to 60 segments and has reached a length of 6 to 7 mm. 



