4.62 ADAM SEDGWICK, 
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 Axolotl’s 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, | 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, 
and in the present state of our knowledge no satisfactory ex- 
' «Comp. Embryology,’ vol. ii. 
