MOEPnOLOGY OF CYCLOSTOMATA. 350 



during the larval life of Petronyzon. E>^pecially it mud he noticed 

 that the foreiiiod i^air of the pershteiit tubules {the third pair) is- in 

 close contact with the hind border of the hind uudl of the hi-anchial 

 chamber ivhc re, in the foregoing stage, the second pair of the tubules 

 was found, this latter Jiaring d isapjwarcd in the course of the 

 last stage. It follows that the two somites, to which the first and 

 the second, pair of the tubules have belonged, hare now entered into 

 the formation of the branchial region. 



The development of the pronephros after this consists only 

 in tlie prolongation and the convolution of the tubules, no further 

 change taking place with reference to the number of the tubules 

 or to their histological structure, until the system undergoes 

 degeneration to be replaced by the mesonephros, which functions 

 as the excretory organ for the whole subsequent life of Petro- 

 myzon. 



The convolution of the tubules is hard to make out. I 

 have reconstructed them from a number of sections ; some of 

 these are diagramatically given in the annexed woodcut 

 (Woodcut 2). 



With the growth of the muscle-segments the collecting duct 

 is prolonged, so that the points of connection of the tubules 

 with that duct become farther apart from one another, while the 

 nephrostomal portions of the tubules retain more or less their 

 original positions ; in this Avise, the tubules are laid in oblique 

 positions directed anteriorly and posteriorly [A) and have no other 

 curvature than the ventro-lateral bending (the frontal projection 

 of the curvature is shown in F). Then the antero-posterior 

 bending begins to take place. The foremost tubule is curved 

 forwards in its whole length, while a small curvature in the distal 



