DEVELOPMENT OF THE ELASMOBRANCH LIVER 363 



tus median a has lost somewhat of its sac-hke form and appears as 

 an irregular dilated chamber which is broader in front than behind 

 and receives the broad short cystic duct from below and the proxi- 

 mal parts of the lateral pouches from the sides. The pars hepatica 

 mediana is somewhat elongated and is still broadly continuous 

 with the pars ductus posteriorly. The formation of tubules 

 from it has progressed considerably and now involves the ventral 

 as well as the anterior surface. The division of this part of the, 

 median pouch into right and left segments by a vertical fissure is 

 well marked. This fissure lies in the same sagittal plane as the left 

 lateral wall of the pars ductus behind it, thus showing a marked 



shift to the left. On the right side the pars hepatica mediana 

 is no longer confluent with the anterior part of the lateral pouch 

 but is separated from it by narrow zone of tubule free surface 

 (fig. 46). On the left side however the pars hepatica mediana 

 extends far laterally and is continuous with the left lateral pouch 

 posteriorly. 



The lateral pouches also show several changes. Their proximal 

 stalks are elongated and the size of their distal expansions is much 

 reduced. The connecting stalk of the left pouch with the pars 

 ductus mediana is shifted so far forward that its posterior margin 

 lies in the same transverse plane as the anterior margin of its 

 fellow of the opposite side. The distal part of the left pouch is 



