DEVELOPMENT OF THE ELASMOBRANCH LIVER 369 



tus and is accentuated by the two short ducts which arise on either 

 side of it. These ducts are the right and left medial hepatic 

 rami and are the derivatives of the right medial and left medial 

 tubule groups respectively. As yet they are very short and large 

 calibered and break up almost immediately into a number of 

 hepatic tubules. Two tubules arising immediately ventral to 

 the left medial ramus probably represent the remains of the ante- 

 rior left tubule group whose fusion with the left medial group has 

 been described. Three additional tubules of large caliber arise 

 from the anterior and ventral surface of the pars hepatica mediana. 

 They may be derived from the right or left medial group but 

 probably have arisen direct from the pouch wall after the main 

 tubule groups were established. 



The left hepatic pouch or duct as we may now term the struc- 

 ture, takes origin entirely from the lateral part of the posterior 

 surface of the pars hepatica mediana, having been entirely sepa- 

 rated from the pars ductus mesialiy. Thus it is already evident 

 that the adult hepatic duct upon this side is made up of two ele- 

 ments, a proximal and transverse part derived from the left part 

 of the pars hepatica mediana and a distal and longitudinal part 

 formed from the left lateral pouch proper. On the opposite 

 side the pouch or duct arises, as in preceding stages, from the 

 lateral surface of the pars ductus or common bile duct. A dis- 

 tinct groove separates the anterior boundary of the right duct 

 from the pars hepatica mediana. Also as in earlier stages the 

 left duct is widely separated from the pars ductus while the right 

 duct lies quite close to its opposite side. The condition of the 

 anterior left tubule group has already been described and on the 

 right side the anterior group is connected with the main hepatic 

 duct by a distinct neck, the anterior right hepatic ramus, which is 

 directed dorsally. On the left side the dorsal posterior cluster or 

 ramus is- no farther developed than before, but the ventral cluste? 

 possesses a short duct which extends posteriorly and bifurcates 

 into upper and lower branches. On the right side the dorsal pos- 

 terior cluster is represented by two ducts, the upper one being 

 particularly prominent and directed posteriorly. The posterior 

 ventral cluster arises from a very short broad diverticulum of the 



