DEVELOPMENT OF THE ELASMOBRANCH LIVER 381 



valvular intestine; (2) a middle segment formed from the 'pars 

 ductus lateralis' of the primitive lateral hepatic diverticula; 

 (3) a distal and anterior segment which is differentiated from the 

 posterior portion or 'pars ductus mediana' of the secondary me- 

 dian hepatic pouch. Thus the middle and distal portions of the 

 duct are both derived from the primitive lateral diverticula, 

 at a very early stage and are truly hepatic in origin while the 

 proximal or posterior part is formed from the archenteron at a 

 much later stage after the duodenum and valvular intestine. In 

 the fully formed fish the latter segment forms by far the greater 

 part of the duct being represented by practically all the extra 

 hepatic portion. 



The differentiation of the posterior part of the lateral hepatic 

 diverticula into the pars ductus lateralis has already been de- 

 scribed in Part I, as well as the early demarkation of the pars 

 ductus mediana from the median hepatic pouch. At 10 mm. the 

 pars ductus lateralis forms the slightly expanded ventral half 

 of the rather constricted segment of archenteron which connects 

 the median hepatic pouch and the gut above it with the yolk- 

 stalk and mid gut posteriorly (figs. 39 and 40). The pars ductus 

 lateralis is thus continuous anteriorly and below with the gall 

 bladder and median pouch and above with the dorsal part of this 

 stalk which later forms the duodenum. On the dorsal surface 

 of the median pouch (fig. 28) is an oblique ridge which extends 

 from its left anterior to right posterior angle and maps off the 

 area of this structure which later becomes the distal part of the 

 ductus choledochus. At 15 mm. (figs. 41 and 42) both the 

 median hepatic pouch and the pars ductus lateralis behind it are 

 completely cut off from the gut above. The latter now forms a 

 short wide duct about 0.1 mm, in length which extends back- 

 ward a little obliquely from left to right and joins with the floor 

 of the gut a little to the right of the median line. In cross section 

 the duct is equal in diameter to that segment of gut, the duode- 

 num, with which it joins and is triangular in outline with the 

 apex of the triangle directed upward. It thus shows a trace of 

 the pouch-like structure from which it arises but at a little later 

 stage it becomes circular or oval in cross section. The pars duc- 



THE AMERICAN JOURNAL OP ANATOMY, VOI,. 14, NO. 3 



