WOLITIAN DUCT AND BODY IN THE CHICK. 25 



and mesonephros, and to show that they have arisen as differen- 

 tiations of a primitively uniform structure. 



Tor 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 

 Wolfiian 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- 



i Urodela. ^ Auura. ' Csecilia. 



