110 bulletin: museum of comparative zoology. 



independence was reached ; then more or less of the posterior ends grew 

 back independently of each other. With the acquisition of more and 

 more yolk, the period at which the young fish began its independent 

 existence (and more active metabolism) was retarded until finally the 

 secretory function of the pronephros was superfluous, especially as the 

 mesonephros arises early in elasmobranchs and reaches forward to 

 the pronephric region. The same process — the accumulation of food 

 material in the egg — necessitated an increased development of the duct 

 which carried off the now enormous eggs, and the pronephros very early 

 in ontogeny began its transformation into an ostium abdominale. Tliis 

 is accomplished in modern elasmobranchs, according to Eabl ('96), by 

 a degeneration of some, and a fusion of the rest of the nephrostomes 

 (Figs. 0, P, p. 111). Thus we have in modern elasmobranchs a pro- 

 nephros which consists of a number of serially arranged funnels whose 

 tubules fuse to form the pronephric duct.^ This duct divides longitu- 

 dinally, one-half receiving the mesonephric excretions, the other remain- 

 ing connected with the pronephric nephrostomes, or rather the single 

 ostium derived from them. 



The ancestors of the Amphibia we can imagine as parting from those 

 of the elasmobranchs at the stage when the pronephros still had a 

 secretory function as well as a sexual one, but after the pronephric duct 

 had acquired the double structure. "With increase in cephalizatron, 

 the pronephric tubules increased in number. Gradually there arose a 

 division of labor, one set of tubules retaining the secretory function, the 

 other set taking on the task of carrying off the eggs (Fig. Q). The loss in 

 the number of secreting units thus brought about was compensated for 

 by an increase in length and accompanying coiling of the anterior end 

 of the pronephric duct to form the main bulk of the pronephros.^ An 

 the secreting action of the pronephros was retained during ontogenetic 

 development until after the sexual or oviducal set of tubules had begun 

 their transformation into an organ for carrying off the eggs, the secreting 

 tubules retained their connection with the anterior end of the pronephric 

 duct, while that part of the latter which split off to form the Miillerian 

 duct was associated exclusively with the set of sexual tubules (Fig. Q). 

 In Arablystoma (see Fig. R) we thus have the original pronephros repre- 



1 Perliaps the tubules fuse and join the pronephric duct. The origin of the 

 latter is still in dispute. 



■•2 According to Semon ('92), the anterior end of the pronephric duct in Ichthy- 

 ophis, where the pronepliric tubules number twelve on each side, does not form the 

 massive coil characteristic of the higher Amphibia. 



