360 RICHARD E. SCAMMON 



ing the pars ductus lateralis into a short tube of large caliber. 

 This is the middle part of the ductus choledochus. It joins with 

 the floor of the duodenum a little to the right, thus preser\dng the 

 same relation observed in younger embryos. The obliquely 

 placed swelling upon the dorsal surface of the median pouch 

 which represents the course of the distal portion of the ductus 

 choledochus is present but is not so marked as in the embryo 10 

 mm. in length described in the preceding section (p. 348) . 



The pars hepatica mediana or anterior part of the median pouch 

 is broadly continuous with the pars ductus medialis behind and 

 with the lateral pouches posteriorly and laterally. Its anterior 

 surface (fig. 44) is rendered extremely irregular by the formation 

 of a number of hepatic tubules. The origin of these structures 

 from ridges in the pars hepatica was noted in connection with the 

 description of an embryo 10 mm. in length, in the preceding 

 section of this paper. At the present stage the tubules arising 

 from the pars hepatica mediana are little more than short conical 

 evaginations of the pouch wall and only one shows any evidence 

 of the complex branching which all soon undergo. The tubules 

 of the pars hepatica mediana are divided into two groups, a right 

 and a left, by a deep vertical furrow which lies somewhat to the 

 left of the median plane and extends from the dorsal to the ventral 

 surface of the pouch. . The right subdivision thus formed is more 

 extensive than the left, but a smaller number of tubules arise 

 from it. The groups of tubules established by this subdivision 

 will be termed in this paper the right medial group and the left me- 

 dial group respectively. The lower surface of the pars mediana 

 remains smooth at this stage and rests upon a mass of mesenchyma 

 which extends from the anterior surface of the gall bladder to the 

 anterior mesothelial wall of the liver. 



to all of these structures and to use it for structures which are later wholly incor- 

 porated in tlie ductus choledochus and not in the vessica fellae or its duct at all, 

 seems inadvisable. The use of the expression 'mediana' in connection with 'pars 

 ductus' is not intended to convey the meaning that this portion of the ductus 

 choledochus is a direct derivative of the median part of the gut jirimarily, but that 

 it is formed from the median pouch produced by the fusion of the anterior parts of 

 the original lateral diverticula while the more posterior part of the ductus chole- 

 dochus is/ormed from the hinder parts of the lateral diverticula without the inter- 

 vention of a median pouch stage. 



