382 RICHARD E. SCAMMON 



tus mediana is still sac-like and like the duct behind it is inclined 

 a little from left to right. 



In 15.5 to 18 mm. embryos the part of the duct derived from the 

 pars lateralis is nearly twice as long as in the above specimen 

 (figs. 45 and 46) and extends directly backward joining the pos- 

 terior end of the duodenum in the median line. Its diameter is 

 about 0.1 mm., being distinctly less than that of the earlier stage 

 and about one»-half that of the gut which it joins. The pars duc- 

 tus mediana is rapidly taking on a duct-like form although this 

 feature is somewhat obscured by the large ducts which arise from 

 it. 



D-chol. 



D.pOTH 



29 



Fig. 28 Dorsal view of a reconstruction of an Acanthias embryo 10 mm. long 

 (S.C. 20). X 36. Lateral and anterior views of this model are shown in figures 

 38 and 39. A, anterior part; B, posterior part of ridge marking the formation of 

 the anterior part of ductus cholcdochus from the pars ductus medialis of the 

 median liver pouch. F.g., fore gut; L.hep.p., lateral hepatic pouches which are 

 cut away dorsally; Sp.v., spiral valve; Y.s., yolk stalk. 



Fig. 29 Reconstruction of the mid gut region of an Acanthias embryo 20.5 

 mm. long (S.C. 5). X 50. Duo., duodenum; D.chol., ductus cholcdochus; 

 D.panc, pancreatic duct; D.vit., vitelline duct; V.int., valvular intestine. 



In the stages which now follow, as is shown in figures 51, 52 

 and 54, the duct rapidly increases in length due to the elongation 

 of its extra-hepatic portion. This growth seems to be derived 

 from the floor of the digestive tube for there extends in the floor 

 of the gut posterior to the ostium of the duct a deep groove which 

 is lined with epithelium of the same nature as that of the duct 

 itself. At 15.5 mm., the duct joins the gut near the posterior end 

 of the duodenum. At 20.5 mm., as is shown in figure 29, this 

 opening lies at the point where the duodenum becomes continuous 



