444 ADAM SEDGWICK. 
and mesonephros, and to show that they have arisen as differen- 
tiations of a primitively uniform structure. 
For this purpose it is necessary briefly to recapitulate the 
more important features in the development which have a 
bearing on my argument. 
Segmental Duct and Pronephros. 
The first part of the excretory system to make its appearance 
is always a duct. This duct has received various names, but its 
homology in different forms is undisputed. I shall call it the 
segmental duct. 
In the chick the segmental duct is commonly known as the 
Wolffian duct. 
All the Ichthyopsida whose development is known, with the 
exception of Elasmobranchs, possess a structure called the head- 
kidney or pronephros. The pronephros when present always 
develops in continuity with the anterior end of the segmental 
duct. 
In the Amphibian the segmental duct arises as a groove of 
the parietal peritoneum, just ventral to the place where the body 
cavity is connected with the cavities of the muscle plates. This 
groove, which arises first of all anteriorly just behind the 
branchial region, is continued for a certain distance backward. 
It soon, however, becomes constricted into a canal lying between 
the ectoderm and parietal peritoneum. This constriction has 
been described as taking place in the following manner :—It 
first appears in the middle region of the groove, giving rise to a 
canal opening into.the body cavity in front and behind. It 
then is continued backwards until the groove is completely con- 
verted into a canal behind, which soon acquires an opening into 
the cloaca. Anteriorly the wide opening meanwhile is divided 
up into two,! three,” or four® openings, according to the species. 
The canal immediately behind the last of these openings 
becomes coiled and placed on the same level but ventral to the 
openings. The part of the body cavity into which the openings 
of the segmental duct pass widens out, a vascular projection— 
the glomerulus—from the dorsal inner wall is formed, extending 
uninterruptedly from opposite the anterior opening of the seg- 
mental duct to as far back as the posterior. The dilated section 
of the body cavity in which the glomerulus lies, and into which 
the segmental duct opens, is partially separated from the rest of 
the body cavity. The whole structure, including openings of 
duct, ventral coiled part of duct, glomerulus, and dilated part of 
body cavity, is known as the pronephros, The number of open- 
' Urodela. 2 Anura. 3 Cecilia. 
