732 GENERAL CONCLUSIONS. 



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

 segmental tubes with the segmental organs of Chaetopods for subsequent 

 discussion. 



It is a peculiarity in the development of the segmental tubes that they at 

 first end blindly, though they subsequently grow till they meet the segmental 

 duct with which they unite directly, without the latter sending out any 

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

 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 infundibula which become the segmental tubes were either from 

 the first provided each with an orifice opening to the exterior, or were united 

 with the segmental duct. If they were from the first provided with external 

 openings we may suppose that they became secondarily attached to the duct 

 of the pronephros (segmental duct), and then lost their external openings, no 

 trace of these structures being left, even in the ontogeny of the system. 

 It would appear to me more probable that the pronephros, 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 segmental tubes became 

 evolved opening into the segmental duct, each tube being in a sort of way 

 serially homologous with the primitive 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 segmental tubes may be modified 

 derivatives of posterior lateral branches of the pronephros, 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 un- 

 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 

 segmental tubes, which, having originally developed on the Elasmobranch 

 type, have come to develop as they do in the posterior part of the mesone- 

 phros of Salamandra, Birds, etc. 



Genital ducts. So far the origin and development of the 

 excretory 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 



1 As mentioned in the note on p. 729 Sedgwick maintains that the anterior 

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



