The development of the urinogenital organs of the lamprey. 51 



Part. III. Comparative and Theoretical. 



In the following general consideration of the nephric system of 

 Vertebrates, I shall have little to say concerning the Amniota. The 

 conditions in these forms can only be interpreted satisfactorily when 

 we understand the renal organs of the A crania and Anamnia. A 

 further reason for excluding the Amniota from the discussion lies in 

 the fact that no satisfactory observations on the basic class of the 

 Amniota, the Reptilia, have appeared up to date, for I cannot attribute 

 much value to Wiedersheim's observations (1890) and the theoretical 

 conclusions he has deduced from them. From work now in progress 

 in my laboratory I feel confident that Wiedersheim has failed to 

 comprehend the conditions in the Testudinata, and Rabl (1896, p. 708 

 and 714) even claims that Wiedersheim did not see pronephros of 

 the Reptiles he studied, as the embryos were too old. The nephric 

 systems of birds and Mammals do not interest us in this place, except 

 in so far as they have a bearing on the conditions in the Anamnia, 



And even of the works on the development of the renal organs 

 of the Ichthyopsida, those on the Amphibia and Teleostei have per- 

 haps furnished more smoke than flame, more obscurity than hght. I 

 do not wish to deprecate the splendid achievements of Mollier (1890), 

 Field (1891) and Semon (1891) on Amphibia, and Felix (1897) on 

 Teleostei. The obscurity is produced not so much by the authors as 

 by the highly specialized forms which were mostly the objects of their 

 study, and by the difficulties that must always attend the too exclusive 

 application of embryological methods, difficulties which have been 

 again and again emphasized and by no one more fully and condignly 

 than by Gegenbaur (1898, passim). 



Of the works which mark epochs in the investigation of the renal 

 organs of Vertebrates, there are two, to which a third has been 

 only recently added. I refer to the works of Boveri (1892) on the adult 

 Araphioxus, Rückert (1891) on the embryo Selachian and Maas on the 

 young Myxine (1897). Boveri demonstrated the existence of an arche- 

 typal raetameric nephric system in the lowest of typical Chordata and its 

 close resemblance, in all essential morphological details, to the nephridia 

 of Annelids on the one hand and to the Craniote pronephros on the 

 other. Rückert had previously pointed out the resemblance of 

 the Selachian pronephros to Annelid nephridia, and had, moreover, 

 given weighty evidence for the view that the pro- and meso- 



nephros are not serial homologues but originate from dyshomologous 



4* 



