456 ADAM SEDGWICK. 
body are capable of shifting their position according to the 
wants of the particular species. 
We know very well other organs can do this, and I need only 
mention the anus placed so near the head in frogs, and so far 
off in Ceecilia, and it seems only probable that an important 
gland like the kidney should be capable of acquiring a position 
and arrangement of its constituent parts different from the posi- 
tion of their development, if it is advantageous for the per- 
formance of the function of the organ. 
The evidence which at the first look appeared so strong 
against the primitiveness of the Elasmobranch arrangement of 
one primary tubule to each segment proves on examination to 
lose a great part of its force. 
I now come to a difficulty which apparently at present presents 
an insuperable obstacle to a successful solution of the question 
under consideration, viz. What was the structure and deve- 
lopment of the excretory system of the ancestral Vertebrate ? 
Assuming that the development of the Elasmobranch mesone- 
phros presents primitive features in the two details already con- 
sidered, its development in a third particular can by no means 
be assumed to be primitive. The fact that the segmental 
duct develops independently of the tubules cannot, in the 
present state of our knowledge, be regarded as primitive. 
Objections of precisely the same kind as those used in arguing 
against the development of the tubules in Amphibia, &c., being 
primitive present themselves here. 
Any phlyogenetic hypothesis which presents difficulties from 
a physiological standpoint must be regarded as very provisional 
indeed. The physiological difficulty present in the conception 
that in the evolution the mesonephros has arisen by the fusion of 
two distinct parts, viz. the duct and tubule, is so great that 
until facts are brought forward to show a different origin we 
must consent to admit our total ignorance on this point. I 
think that the observations recorded in the first part of this 
paper on the development of the Avian Wolffian duct and 
anterior tubules are of great interest in this relation. Here 
we have the Wolffian duct and tubules developing in continuity 
in the anterior part of the excretory system, which has been 
always admitted to present the most primitive development. 
But this point I must again keep for later consideration. 
So far, then, the following conclusions have been reached—the 
development of the mesonephros of Hlasmobranchii is in part 
primitive (tubules), and in part very much modified, while the 
development of the mesonephros of Amphibia, Teleostei, &c., 
is in all respects modified. 
Turning to the development of the segmental duct, we find 
