WOLYFIAN DUCT AND BODY IN THE CHICK, 443 
it appears to me after its separation from the internal glome- 
rulus to get an independent growth, and while the latter is 
undergoing atrophy to become larger and extend itself posteriorly, 
so as almost to overlap the external glomerulus of the next 
tubule. 
With regard to the number of the external glomeruli in the 
chick and the exact limits of their occurrence, the following is 
briefly what I have been able to make out in a chick with thirty 
protovertebree : 
In the 11th segment there are two rudimentary tubules 
running from the Wolffian duct to the peritoneal epithelium. 
At the point of attachment of these there is a small rudiment of 
the external glomerulus, visible for only one section in each 
case. 
In the 12th segment there is at the beginning a Wolffian 
tubule and a well-marked external glomerulus extending through 
three sections. At the hind end of the 12th segment and 
beginning of the 13th there is an external glomerulus for three 
sections continued into part of the segmental tube behind, in 
which an internal glomerulus will subsequently be developed. 
In the 13th segment there is an external glomerulus for three 
sections. 
In the 14th segment there are two segmental tubes with 
developing external glomeruli. 
In the 15th segment no external glomeruli appear to be 
developed, the segmental tubes being already separated from 
the peritoneal epithelium. 
Tn later stages only the three or four hindermost of the above 
external glomeruli appear to develop further. The anterior 
glomeruli soon atrophy with the adjoining tubules and duct. 
In the duck a much greater number become developed, 
and they may be seen in the anterior segments after their 
respective tubules have entirely atrophied. 
The bearing of the developmental processes above recorded on 
any hypothesis as to the phylogenetic history of the vertebrate 
excretory system I propose to examine in the second part of this 
paper (pp. 460—462 ; 464). 
Parr II. A Discussion of the Vertebrate Excretory System in 
General. 
The most peculiar feature of the excretory system of the 
vertebrata is the presence of three more or less distinct parts, the 
pronephros, the mesonephros, and the metanephros or kidney 
proper. In the following pages my object will be to explain the 
relation of these parts, more especially those of the pronephros 
VOL, XXI.—NEW SER. GG 
