83 



remains of the larval pronepliros. This identification is 

 confirmed by a study of various early stages. In the 

 youngest forms examined (symmetrical larvae one to two 

 weeks hatched), the mesonephric tuhules are absent, or are 

 only just forming, while the segmental duct extends 

 anteriorly as a straight line which becomes convoluted in 

 its anterior extremity forming the pronephros. In a later 

 stage (asymmetrical fish 3/5th inch long) the three regions 

 of the kidney described above are well marked, the latter 

 portion presenting all the characters of a mesonephros, 

 while the nephrostomes and glomi of the pronephros are 

 still recognisable, though much reduced. In an asym- 

 metrical form about one inch in length, the intermediate 

 lymphatic portion is relatively shorter, the mesonephric 

 portion with its Malpighian corpuscles has extended fur- 

 ther forward, the pronephric convokitions of 1he seg- 

 mental duct are still present, but glomi, pronephric 

 chambers and nephrostomes have disappeared and lyni- 

 pliatic tissue and blood spaces are largely developed. 

 Finally in the oldest adult specimens examined the head 

 kidney contains only lymphatic tissue. 



The pronephros probably degenerates in all Teleostean 

 fishes with the exception of one or two specialised forms. 

 Organs formerly supposed to represent a persisting func- 

 tional pronephros such as are present in Lophius have 

 been shewn to be anterior extensions of the mesonephros, 

 and others such as those present in AnguiUa and Esox are 

 lymphatic organs with degenerate remains of the 

 pronephric convolutions. Onlv in Fierasfer, and perhaps 

 Zoarces, is the evidence conclusive that a functional 

 pronephros exists in adult life. In Dadijlopterus Calder- 

 wood* has described an organ which he regards as a func- 

 tional pronephros, but in this case it is still probable that 

 * Jour. Mar. Biol. Assoc, vol. ii. (N. S.), pp. 43-46, 1891-2. 



