468 ADAM SEDGWICK. 
ment was consequently modified. This part is known to us as 
the mesonephros. 
The same hypothesis was applied to account for the retarda- 
tion and modification of the development of the metanephros 
with regard to the mesonephros in the Amniota. 
The main facts in favour of the hypothesis are— 
1. The development of the segmental tubes in Hlasmobranchii 
and of the pronephros and segmental duct of the Ichthyopsida as 
parts of the body cavity. 
2. The obvious modification in development of the meso- 
nephros, accompanying also the presence of a pronephros in 
most of the Ichthyopsida. 
3; The resemblance in structure between the pronephros and 
mesonephros, particular stress being laid on the fact that the 
glomerulus im both glands is developed in anatomically corre- 
sponding, i.e. homologous, parts of the body cavity 
I may point out before leaving the subject that other views con- 
cerning thenature of the pronephros have been expressed by Gegen- 
baur, Furbringer;! and Balfour.2. The two former authors look 
upon the pronephros as having an antiquity greater than that of 
Vertebrates, greater even than that of the segmented ancestors 
of Vertebrates. They regard it as being descended from the 
primitive excretory system possessed by the unsegmented 
ancestor, which has been retained in such forms as Turbellaria 
and Rotifera, the segmented posterior part having been added 
when the segmented state was reached. 
Miillerian Duct. 
Balfour’s views as to the phylogeny of the Miillerian duct 
and its homology throughout the Vertebrata are well known. 
He supposes it is one or, in the chick, more of the head-kidney 
openings which have become modified for generative purposes. 
I still adhere to the view expressed in the paper on the 
“ Rudimentary Head-Kidney of the Chick” as to the meaning 
of the peculiar structures at the anterior end of the Millerian 
duct, and I think that there are grounds, which it is not necessary 
to enter into here, for supposing that the abdominal opening or 
openings of the Miillerian duct have been derived from the 
anterior part of the excretory system after its modification to 
form the pronephros. But I quite admit that a fuller know- 
ledge of the early development of the Hlasmobranch segmental 
duct may necessitate an alteration in this view. 
1 Loe. cit. 
2 Balfour looks upon it as the most primitive part of the excretory 
system which has been retained by the larva, as so many ancestral organs 
are, long after they have been lost by the adult. ‘Comparative Embryo- 
logy,’ vol. il. 
