346 GEORGE A. BATES 



in origin, while others claim that the cells concerned in its develop- 

 ment arise wholly from the mesoderm. 



It has been generally assumed that in the phylogenetic history 

 of vertebrates the pronephric " duct once opened directly to the 

 exterior through the ectoderm, which assumption had its origin in 

 the theory of the phylogenetic relationship of annelid and verte- 

 brate. This is presented very clearly by Haddon ( '87) and also 

 in his "Introduction to the study of embryology" ('87, p. 239). 

 In these publications he recapitulates the discussion from the 

 time ('75), when Hensen determined for the rabbit that the 

 pronephric duct is of ectodermal origin, to Perenyi ('87) who 

 claimed the same thing for the frog. 



He quotes Spee ( '84) for the guinea-pig and Flemming ( '86) as 

 confirming Hensen for the rabbit; also Van Wiihe ('86) as saying 

 that the duct arises from the ectoderm in the thornback ray 

 (Raja clavata). He says: "The origin of the segmental duct 

 from the epiblast being now known to occur in Elasmobranchs, 

 Anura, Lacertilia and Rodents we are justified in assuming that 

 this is the general fact." Upon this assumption he proceeds to 

 present an hypothesis by which Jie homologizes the pronephric 

 duct of vertebrates with the nephiidia of the chaetopod worms. 

 He says: "There can be little doubt that the segmental duct 

 arises from the epiblast. This discovery will necessarily lead to 

 a modification in our views concerning the morphology of the 

 vertebrate excretory organs." He quotes Hatschek as having 

 described a single nephridium in Amphioxus which is in all re- 

 spects comparable with a vermian nephridium and then makes the 

 following statement : 



We have, then, only to assume that a pair of similar vermian ne- 

 phridia occurred in each body segment of the ancestral vertebrate, and that 

 the nephridia of each side of the body opened externally into a lateral 

 groove [which he has previously described as developed in the epiblast 

 much as the neural tube is developed]. It would further only be neces- 

 sary for the groove to deepen and next to form a canal (just as does the 

 neural groove) to bring about the vertebrate arrangement. Thus, in 

 vertebrates, as in invertebrates, the nephridia open by epiblastic pores, 

 but in the former the area into which they open is precociously converted 

 into a canal which subsequently acquires a secondary opening to the ex- 



