EXCRETORY ORGANS. 603 



Before leaving the mesonephros it may be worth while putting forward 

 some hypothetical suggestions as' to its origin and relation to the pro- 

 nephros, leaving however the difficult questions as to the homology of the 

 segmeiital tubes with the segniental organs of Chsetopods for subsequent 

 discussion. 



It is a peculiarity in the development of the segmeiital tubes that they 

 at first end blindly, though they subsequently grow till they meet the seg- 

 meiital duct with which they unite directly, without the latter sending 

 out any offshoot to meet them 1 . It is difficult to believe that peritoneal 

 infimdibula ending blindly and unprovided with some external orifice can 

 have had an excretory function, and we are therefore rather driven to 

 suppose that the peritoneal infuiidibula which become the segmeiital tubes 

 were either from the first provided each with an orifice opening to the 

 exterior, or were united with the segmeiital duct. If they were from the 

 first provided with external openings we may suppose that they became 

 secondarily attached to the duct of the proiiephros (segmeiital duct), and 

 then lost their external openings, 110 trace of these structures being left, 

 even in the ontogeny of the system. It would appear to me more probable 

 that the proiiephros, with its duct opening into the cloaca, was the only 

 excretory organ of the unsegmented ancestors of the Chordata, and that, 

 on the elongation of the trunk and its subsequent segmentation, a series 

 of metameric segmeiital tubes became evolved opening into the segmeiital 

 duct, each tube being in a sort of way serially homologous with the primi- 

 tive pronephros. With the segmentation of the trunk the latter structure 

 itself may have acquired the more or less definite metameric arrangement 

 of its parts. 



Another possible view is that the segmeiital tubes may be modified 

 derivates of posterior lateral branches of the proiiephros, which may at 

 first have extended for the whole length of the body cavity. If there is 

 any truth in this hypothesis it is necessary to suppose that, when the nil- 

 segmented ancestor of the Chordata became segmented, the posterior 

 branches of the primitive excretory organ became segmentally arranged, 

 and that, in accordance with the change thus gradually introduced in them, 

 the time of their development became deferred, so as to accord to a certain 

 extent with the time of formation of the segments to which they belonged. 

 The change in their mode of development which would be thereby intro- 

 duced is certainly not greater than that which has taken place in the case 

 of segmeiital tubes, which, having originally developed on the Elasmobranch 

 type, have come to develop as they do iu the posterior part of the mesoiie- 

 phros of Salamandra, Birds, etc. 



Genital ducts. So far the origin and development of the excre- 

 tory organs have been considered without reference to the modifications 

 introduced by the excretory passages coming to serve as generative 

 ducts. Such an unmodified state of the excretory organs is perhaps 

 found permanently in Cyclostomata 2 and transitorily in the embryos 

 of most forms. 



1 As mentioned in the note on p. 600 Sedgwick maintains that the anterior 

 segmental tubes of the Chick form an exception to this general statement. 



2 It is by 110 means certain that the transportation outwards of the genital products 

 by the abdominal pores in the Cyclostomata may not be the result of degeneration. 



