WOLFFIAN DUCT AND BODY IN THE CHICK. 43 



that though perfectly well suiting the requirements of the larva, 

 its position is unsuitable for the satisfactory performance of 

 its functions in the adult. Balfour has suggested^ that the 

 atrophy of the pronephros is due to its position in that part of 

 the body cavity which eventually becomes the pericardium ; 

 and has pointed out, as a confirmation of this view, that it 

 only persists in the adult of those animals in which it is com- 

 pletely shut off from the body cavity, e.g. Teleostei. 



(The enormous size which the pronephros attains in adult 

 Teleostei is peculiar, but, coupled with the remarkably feebly 

 developed mesonephros in the adult, is not astonishing. The 

 pronephros seems capable of carrying on all the excretory work 

 in some adult Teleostei, in which the mesonephros is not present. 

 The absence of the mesonephros in these cases is probably purely 

 secondary, and, no doubt, traces of it would be found if a close 

 examination were made. The survival of a larval character into 

 the adult state is paralleled by the AxolotFs gills,) 



A second feature of difference between this anterior part of 

 the Avian excretory system and the Amphibian pronephros, is 

 the absence in the former of a continuous glomerulus. This 

 may be abortion from disuse, and does not really present a 

 serious difficulty. 



A third feature of difference is that the Avian pronephros 

 extends over a much greater area than that of the Ichthyopsida, 

 but when I draw attention to the fact that this difference is 

 found amongst the various members of the Ichthyopsida them- 

 selves, I think it can hardly be looked upon as a difficulty. In 

 Teleostei the head-kidney is distinguished by one peritoneal 

 opening and a correspondingly short glomerulus. From this we 

 have all stages to the five peritoneal openings of Petromyzon. 



Finally, even if the Avian pronephros did differ in certain 

 features from the Ichthyopsidan pronephros, this can hardly be 

 regarded as a serious difficulty. 



The pronephros of Teleostei with its Malpighian capsule 

 containing the isolated glomerulus, and with its one peritoneal 

 opening, surely differs considerably from the pronephros of the 

 frog with its three peritoneal openings and its glomerulus lying 

 free in the body cavity. 



Again, without laying too much stress upon it, I point to the 

 pronephros of Myxine, which differs still more remarkably from 

 that of other types. 



The difficulty presented by the Elasmobranchii, in which the 

 tubules, though retaining certain primitive features of develop- 

 ment, do not develop in continuity with the duct, is very great, 



* ' Comp, Embrjology,' vol. ii. 



