The development of the urinogenital organs of the lamprey. 53 



this simple Chordate Boveri found Annelid-like nephridia but no pro- 

 nephric duct. Hence he was obliged to look for the precursor of the 

 duct in some other structure in the Amphioxus or to assume that it 

 first developed in the ancestral Craniote. He chose the former 

 alternative and developed the ingenious hypothesis that the atrial 

 chamber or some similar structure had given rise to the pro- 

 nephric ducts of the Vertebrates above Amphioxus. This hypothesis 

 has been undermined by the demonstration that the vast majority of 

 Vertebrates have purely mesodermal pronephric ducts, and that even 

 in Selachians, the forms on which Boveri built his hypothesis, the 

 evidence of a derivation of the whole duct from the ectoderm is still 

 suJ) judice. 3) A very different view from either of the preceding 

 concerning the phylogenetic of the pronephric duct is advanced by 

 PtüCKERT in his résumé (1892). He says (p. 696): "Es ist dies die 

 Vermuthung, dass die Vorniere ursprünglich weiter nach rückwärts 

 gereicht habe, als dies heute der Fall ist, und dass sie in diesem ihren 

 caudaleu Abschnitt rudimentär geworden sei, d. h. sich nur in Gestalt 

 des Ganges erhalten hat^) . . . Der heutige Vornierengang würde dann 

 nichts anderes darstellen als den übrig gebliebenen Längscanal (Sammel- 

 rohr) eines Vornierenabschnittes, dessen Quercanäle und Glomeruli zu 

 Grunde gegangen sind." This clear statement is somewhat obscured 

 in RtJCKERT's further treatment of the subject by his endeavor to 

 account for the various modes of duct formation in different Craniotes. 

 In certain forms the duct arises, as 1 have described it in Petromy0on, 

 m situ from the somatopleure and only its posterior end grows back 

 into the cloaca independently of the underlying mesoderm. In other 

 forms a portion of the duct may arise from the caudal end of the 

 pronephros as in Torpedo, or from nearly the whole pronephros, as 

 in birds, but still leaving a large portion of the duct to grow back 

 independently. Lastly, nearly the whole of the duct may grow back freely 

 without deriving material from the underlying mesoderm, as in sharks. 

 RüCKERT very consistently regards the first mode of duct formation 

 as palingenetic, the others as coenogenetic, but the importance which 

 he very naturally attaches to the way in which the duct is formed in 

 sharks, tends, nevertheless, to obscure the central idea of his theory. 

 Then, too, he appears to regard the collecting duct as a rudiment 

 independent of the tubules. I infer this from his calling attention to 

 the fact (p. 689) that in Torpedo the hindermost pronephric tubules 



1) Italicized in the original. 



