DEVELOPMENT OF THE ELASMOBRANCH LIVER 359 



II. DEVELOPMENT OF THE HEPATIC DUCTS AND THEIR RAMI 



The elements entering into the formation of the hepatic ducts 

 are the anterior portion of the median hver pouch or pars hepatica 

 mediana, and the right and left lateral hepatic pouches or the 

 pars hepatica lateralis.* These structures are converted into the 

 main hepatic ducts found in the fully developed embrj^o by means 

 of reduction in caliber both relative and actual, by elongation, 

 and by partial separation from the posterior portion of the median 

 liver pouch or pars ductus mediana. That such processes take 

 place has been recognized by Balfour (76), Hammar ('93), 

 Brachet ('96), and other investigators. The details have not been 

 described. The minor ducts are formed in Acanthias in much the 

 same manner as are the major ones, by the differentiation and 

 elongation of the proximal parts or pedicles of certain fairly defi- 

 nitely placed groups of tubules which arise from the surface of 

 the embryonic structures which form the main ducts. This 

 method of formation of the minor ducts probably holds only for 

 selachians in which the omphalo-mesenteric veins are compara- 

 tively small and develop at a late stage. 



An account of the development of these ducts may begin with 

 the description of an ernbryo 15 mm. in length (H.E.C. 227 and 

 No. 26 of the Normal plate series) the general anatomy of which 

 is illustrated in figure 13 of the Normal plates of Squalus acanthias. 



The main divisions of the liver of this specimen and the proximal 

 part of the hepatic tubules arising from them have been recon- 

 structed and figures 41, 42 and 44 are right lateral, left lateral and 

 anterior views of this object. 



The median hepatic pouch is completely separated from the 

 gut above and this separation has extended backward convert- 



* The use of this and the following terms of this paragraph is somewhat of a 

 departure from the classification of the components of the selachian liver pouch 

 into 'pars cystica' and 'pars hepatica' as proposed by Brachet ('96, '97) on the 

 basis of a similar classification employed by Goeppert ('93) in his description of the 

 development of the liver in Teleosts. The term 'pars cystica' as used by Brachet 

 includes the 'pars ductus mediana' and 'pars ductus lateralis' as employed here, 

 as well as the anlage of the gall-bladder and cystic duct. If the conception as 

 presented here, of the gall bladder as an organ with an origin distinct from the 

 liver, is a correct one, then this term 'pars cystica' can hardly be properly applied 



