WOLFFIAN DUCT AND BODY IN THE CHICK. 447 
the point of its original attachment, and does not unite with 
the segmental tubes till considerably after its first development. 
The difference in the development of the segmental duct in 
the forms possessing a pronephros and in Elasmobranchs is only 
one of degree. 
In both cases it at first arises as a projection, either solid or 
containing a diverticulum from the body cavity, from the 
parietal peritoneum just ventral to the muscle plates ; but in the 
one case this groove has a greater longitudinal extension than 
in the other. In all probability the hinder part of the seg- 
mental duct is in all cases formed by an independent growth 
from the hind end of this groove. 
Amongst the Amniota the chick is the type in which the 
development of the segmental duct has been most carefully 
examined. 
In the chick it arises as in Amphibia as a projection (solid in 
the chick) from the parietal mesoderm just ventral to the 
muscle plates; and the extent of the ridge is the space occupied 
by five segments. 
This ridge is constricted off at intervals from the intermediate 
cell mass, but remains attached at certain points. The hind end 
of the duct is formed by a growth back from the hind end of 
this ridge, which takes place independently of adjacent 
structures. 
The question now presents itself: are these structures at the 
anterior end of the segmental duct in the chick, which so closely 
resemble in development the openings of the Ichthyopsidan 
head-kidney, homologous with that head-kidney ? 
To a consideration of this question I shall return: 
Mesonephros. 
The mesonephros obtains a large development in all the 
groups of the Vertebrata; but it does not persist as an excre- 
tory organ in the adult of the Amniota. 
It develops in three very markedly distinct ways. 
The first of these characterises the Elasmobranchii. 
The second the Amphibia, Teleostei, Ganoidei, Marsipo- 
branchii. | 
The third the Amniota. 
The Development of Mesonephros in Elasmobranchii. 
The segmental tubes of Elasmobranchii were originally de- 
scribed by Balfour as arising as solid diverticula of the peritoneal 
epithelium. An examination of Balfour’s specimens led me, 
however, to conclude that they originated as specialised parts 
of the body cavity, viz. from the canals in the intermediate 
