PRONEPHRIC DUCT IN ELASMOBRANCHS 347 



terior through the cloaca. On this hypothesis the nephridia of verte- 

 brates always open by their original epiblastic pores, primitively directly 

 to the exterior, secondarily into a canal separated from the epiblast. 



Hertwig ('92, pp. 358-9) states the case very clearly and brings 

 the discussion down to the date of his own publications. He 

 speaks of the view entertained by almost all investigators that 

 ''the duct arises from the mesoderm and grows by proliferation 

 of its own cells as far as the hind gut (proctodaeum) and thus was 

 constricted off from neither the outer nor the middle germ-layers 

 nor yet derived from them cell-material for its increase." He 

 claims, however, that this interpretation has become untenable, 

 quoting various authors to substantiate the fact that, in several 

 classes of vertebrates, the duct in process of growth is in close 

 union with the outer germ-layer and that it is prolonged back- 

 ward by means of cell-proliferation in that layer, while in front it 

 is being constricted off from that layer or, as he terms it, the par- 

 ent-tissue. He further states that ''the pronephric duct grows 

 at the expense of the outer germ-layer and moves, as it were, along 

 the latter, with its terminal opening behind, as far as the hind 

 gut." This interpretation led, later, to the assumption by vari- 

 ous authors of the view that not only the pronephric duct but the 

 entire urinary system was derived from the ectoderm (Hertwig 

 '92, p. 358). 



Hertwig, later, shows that such views cannot be made to har- 

 monize entirely with conditions found in the lower vertebrates 

 and, making allowance for all observations, he summarizes the 

 subject thus: "The pronephros is developed from the middle 

 germ plate, and that then its posterior end comes into union with 

 the outer germ-layer and in conjunction with" the latter gi'ows 

 farther backward as the pronephric duct." He quotes Van 

 Wijhe and Riickert in support of this explanation and then inter- 

 prets the pronephric duct "at its first appearance as a short canal- 

 like perforation of the wall of the body, which begins in the body- 

 cavity with one or several ostia and opens out upon the skin by 

 a single orifice. Originally the outer and inner openings lay near 

 together, later they moved so far apart that the outer opening of 

 the canal united with the hind gut. " 



