48 ADAM SEDGWICK. 



phros, tLen, is to be explained on the supposition that the larva 

 of the chick^s ancestor used it for a considerable period of its 

 early life as an excretory organ, so that it may be said that the 

 pronephros holds the same general relation to the mesonephros 

 in the Ichthyopsida as does the mesonephros to the metane- 

 phros in the Amniota. 



I do not mean to affirm that the above explanation of the 

 lateness of the development of the metanephros is absolutely 

 valid, for I think that a careful consideration of the develop- 

 ment of the hind part of the mesonephros in Amphibia and Elas- 

 mobranchii might necessitate a slightly different explanation. 



But an explanation of that kind must be sought to explain the 

 remarkably late development in the chick of an organ which 

 phylogenetically must be assumed to have had an origin simul- 

 taneous with that of the mesonephros. 



With regard to the relation which the testes enters into with 

 the mesonephros, it is interesting to notice the modified develop- 

 ment which always characterises this connection. 



Here it can be definitely affirmed that the lateness and conse- 

 quent modification of the process is due to the fact that the 

 apparatus has not been required in the larvse of the Ichthyopsida 

 and of the Amniote ancestors, and consequently has been put off 

 and modified in development. The explanation is exactly similar to 

 that given for the modification in development of the Amphibian 

 mesonephros, except that here we are supposed to be able to 

 assert with greater reason that the putting off and consequent 

 modification is due to the fact that the connection between the 

 testes and mesonephros was not wanted sooner, and so was not 

 developed. 



Summary of the Hypothesis and main Arguments used. 



The whole of the Vertebrate excretory system, including pro- 

 nephros, mesonephros, and metanephros, are derived from a 

 primitive organ possessed by the ancestral Vertebrate. This 

 organ had a segmental character, and consisted of a duct, the 

 segmental duct opening in every segment into the body cavity, 

 close to a continuous structure, known now as the glomerulus, 

 which was placed close to the main vascular channels and acted 

 as an excretory organ. 



The anterior end of this organ was used by the larva, and 

 developing more or less with regard to other structures at the 

 normal time, retained many primitive features of development 

 originally characterising the whole organ, and is known to us as 

 the pronephros. The posterior part of the organ had its develop- 

 ment delayed with regard to other structures, particularly those 

 in connection with which it primitively developed ; the develop- 



