MOEPHOLOGY OF CYCLOSTOMATA. 387 



These nephric tubules receive, according to Bovert, the blood 

 from the aorta, which gives two branchlets to each nephric 

 segment. These branchlets form in each sesrment a network in 

 the neighbourhood of, and winding around, the nephric tubule ; 

 it is this network that Boveei calls <jlomerulus. 



From the structure, the position, the segmental arrangement, 

 the physiological function, and the relation of the blood- vascular 

 system to this system of organs, Boveri regards the latter 

 as a primitive form of Vertebrate nephric organ and horaologised 

 it particularly with the pronephros of Cran iota. The points 

 of difference which exist between the " Nierencanälchen " of Ä}ii- 

 phloxus and the pronephros of Craniota, have been smoothed away 

 by the author's masterly arguments. The first of these points 

 is the want of the segmental duct in Amjjhioxiis ; but this is re- 

 presented, according to Boveri, by a part of the peri branchial 

 chamber. The second is the relation of the nephric segments to 

 other systems of organs. The " Nierencanälchen " is branchio- 

 meric while the pronephros of Craniota is myomeric, in arrange- 

 ment. But this difference is looked upon by him as only apparent ; 

 for the number of gill-slits first formed agrees with that of the 

 muscle-segments in the same region ; this is sufficiently demons- 

 trated by the figure given by Weiss {foco cit., fig. 3). 



Thus the author has brought the " Nierencanälchen " of 

 Amphioxus into perfect harmony with the pronephros of Craniota. 

 Some additional light is now, I believe, thrown from the side of 

 Craniota by the facts obtained in Cyclostomata, the lowest 

 class of Craniota. This harmony will be brought out more in 

 discussing the development of the pronephros in Selachia, Teleostei, 

 and Amphibia, which will be treated further on. 



