728 GENERAL CONCLUSIONS. 



General conclusions and Summary. 



Pronephros. Sedgwick has pointed out that the pronephros 

 is always present in types with a larval development, and either 

 absent or imperfectly developed in those types which undergo 

 the greater part of their development within the egg. Thus it 

 is practically absent in the embryos of Elasmobranchii and the 

 Amniota, but present in the larvae of all other forms. 



This coincidence, on the principles already laid down in a 

 previous chapter on larval forms, affords a strong presumption 

 that the pronephros is an ancestral organ ; and, coupled with 

 the fact that it is the first part of the excretory system to be 

 developed, and often the sole excretory organ for a considerable 

 period, points to the conclusion that the pronephros and its duct 

 the segmental duct are the most primitive parts of the 

 Vertebrate excretory system. This conclusion coincides with 

 that arrived at by Gegenbaur and Fiirbringer. 



The duct of the pronephros is always developed prior to the 

 gland, and there are two types according to which its develop- 

 ment may take place. It may either be formed by the closing 

 in of a continuous groove of the somatic peritoneal epithelium 

 (Amphibia, Teleostei, Lepidosteus), or as a solid knob or rod of 

 cells derived from the somatic mesoblast, which grows backwards 

 between the epiblast and the mesoblast (Petromyzon, Elasmo- 

 branchii, and the Amniota). 



It is quite certain that the second of these processes is not a 

 true record of the evolution of 'the duct, and though it is more 

 possible that the process observable in Amphibia and the 

 Teleostei may afford some indications of the manner in which 

 the duct was established, this cannot be regarded as by any 

 means certain. 



The mode of development of the pronephros itself is ap- 

 parently partly dependent on that of its duct. In Petromyzon, 

 where the duct does not at first communicate with the body 

 cavity, the pronephros is formed as a series of outgrowths from 

 the duct, which meet the peritoneal epithelium and open into 

 the body cavity ; but in other instances it is derived from the 

 anterior open end of the groove which gives rise to the segmental 

 duct. The open end of this groove may either remain single 



