24 ADAM SEDGWICK. 



I have often met with it even larger and more developed, and 

 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 

 protovertebrse : 



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. 



In 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. 41 — 43; 47). 



Part 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 



