MUSEU.M OF COMPARATIVE ZOOLOGY. 321 



up of such remote parts ; and the hypothesis must fall to the ground as 

 soon as it is proved that the Analschlauche are in reality only modified 

 nephridia opening into a proctodasum. In addition to the auatomical 

 evidence favoring such an interpretation, Hatschek ('80, pp. 60-62) has 

 recently advanced strong evidence from an embryological standpoint for 

 believing that they are nephridia which primitively opened directly to 

 the exterior. 



Among the attempts to find a homologue of the segmental duct already 

 present in the "Worms may be mentioned the longitudinal canal described 

 by Hatschek ('78, p. 117 seq.) in the larva of Polygordius. Were Hat- 

 schek's account accurate, it would doubtless waiTant great changes in 

 our conceptions of the interrelationships of the Yermian nephridia, and of 

 the origin of the Vertebrate kidneys ; but his statements have not been 

 confirmed by any subsequent observations, though several investigators 

 have concerned themselves with this interesting form (Fraipont, '87, 

 p. 83 ; Eisig, '87, p. 662 ; E. Meyer, '87, p. 59i j Bergh, '85, p. 27, foot- 

 note). 



The remaining views in respect to the nature of the duct agree in re- 

 garding it as a secondary growth.^ Beard ('87, p. 651) and Haddon 

 ('87) accept the ectodermal origin of the segmental duct, and endeavor 

 to give it phylogenetic significance by assuming that the ontogenetic 

 connection of the dixct with the ectoderm indicates that it was repre- 

 sented in the phylogeny at first by a groove into which the nephridia 

 opened, and that this groove gradually became closed and was cut off 

 as a tube extending from the most anterior tubule backwards to the 

 cloaca. Riickert ('88) and van Wijhe ('89, pp. 507, 508), on the 

 other hand, assert a gradual backward growth of a primitively anterior 

 opening. 



None of these views are compatible with the fact that in many Verte- 

 brates the duct is demonstrably of mesodermal origin. They also seem 

 to me to give insufficient significance to the mode in which the pronephric 

 diverticula unite to form a longitudinal canal : and the first two authors 

 neglect it wholly. 



Finally, the abandoned view of Balfour ('76, pp. 25, 26), according to 

 which tlie duct arises hy the fusion of the distal ends of the several nephridia, 

 has been revived by Eisig ('87, p. 649), and was accepted by Riickert 

 ('88, p. 264) for the loronephric portion of the duct. This view seems to 

 me much the most probable, and I am inclined to accept it. 



^ Here belongs also the view of Boveri ( '90), which has already been discussed 

 (page 265). 

 VOL. XXI. — xo. 5 21 



