600 GESERAL COXCLUSIOXS. 



General conclusions and Sumtnari/. 



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 prone- 

 phros is an ancestral organ ; and, coupled with the fact that it is 

 the first part of the execretor}'^ system to be developed, and often the 

 sole excretory organ for a considerable period, points to the conclu- 

 sion that the pronephros and its duct — the segmental duct — are the 

 most primitive parts of the Vertebrate excretory system. This con- 

 clusion 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 development may 

 take place. It may either be formed by the closing in of a con- 

 tinuous 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, Elasmobranchii, 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 an}^ means certain. 



The mode of development of the pronephros itself is apparently 

 partly dependent on that of its duct. In Petromyzon, where the 

 duct does not at first communicate with the body cavity, the prone- 

 phros 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 (Teleostei, Ganoidei) or be divided into two, 

 three or more apertures (Amphibia). The main part of the gland in 

 either case is formed by convolutions of the tube connected with the 

 peritoneal funnel or funnels. The peritoneal funnels of the prone- 

 phros appear to be segmentally arranged. 



The pronephros is distinguished from the mcsonephros by develop- 

 (^ mental as well as structural features. The most important of the 

 /j former is the fact that the glandular tubules of which it is formed are 

 y always outgrowths of the segmental duct; while in the mesonephros 

 I > they are always or almost always^ formed independently of the duct. 



^ According to Sedgwick some of the anterior segmental tubes of Aves form an 

 exception to the general rule that there is no outgrowth from the segmental or metane- 

 phric duct to meet the segmental tubes. 



