DEVELOPMENT OF THE ELASMOBRANCH LIVER 351 



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SO that one looks down on the interior of the ventral half of the 

 gut from above and a little behind. Figure 9 shows the liver as a 

 pair of shallow lateral diverticula, lying mainly behind the point 

 of union of fore gut and blastodermic entoderm or the anterior 

 wall of the yolk-stalk. As the embryo is elevated and farther 

 separated from the blastoderm the anterior wall of the yolk- 

 stalk retreats posteriorly. This brings the anterior ends of the 

 lateral diverticula in contact and they fuse more and more, form- 

 ing the median liver pouch and producing the condition shown in 

 figure 10. 



Here the liver anlage is tZ-shaped with the limbs of the U turned 

 posteriorly and slightly divergent. The fusion of the lateral diver- 

 ticula is continued along with the posterior progression of the 

 anterior wall of the yolk-stalk. At the same time parts of these 

 structures undergo unequal growth. In each diverticulum the 

 middle part above and a little behind the fused median portion 

 begins a rapid dorsal and lateral growth, producing the structures 

 known variously as 'lateral pouches' 'ebauche hepatique,' 

 and 'Seitendivertikel.' The posterior parts of the primitive 

 lateral diverticula, or pars ductus, which extend backward to or 

 beyond the anterior wall of the yolk- stalk share but little in this 

 growth, but remain unchanged in their primitive condition as a 

 pair of shallow lateral diverticula until at a much later period, 

 they are transformed into a part of the ductus choledochus. 



In the meantime the gall bladder arises as an out-pouching of 

 the dorsal part of the anterior wall of the yolk-stalk and being 

 somewhat cut off from that structure by the posterior growth of 

 the fore gut comes to lie between it and the median liver pouch 

 anteriorly and with the lateral median pouches bounding its sides. 

 This stage is represented in figure 11. Figure 12 shows a some- 

 what later stage modified from the preceding by the greater expan- 

 sion of all parts of the hepatic structure and by a still greater 

 elongation of the fore gut. 



This account of the development of the liver is to some extent 

 in accord with that given by Hammar ('93) as opposed to the 

 idea of a single median ventral anlage as advanced by Balfour 

 ('76), Laguesse ('93), Brachet ('96) and Choronschitzky ('00). 



