28 ADAM SEDGWICK. 



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 Masmolranchn, 



The segmental tubes of Elasmobranchii were originally de- 

 scribed by Balfour as arising as solid diverticula of the peritoneal 

 epithehum. 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 



