222 G. C. PRICE, 



word pronephros was used, whether the entire embryonic kidney was 

 meant, or only the specialized part which has been described as pro- 

 nephros in the adult. I would suggest, therefore, that the word 

 "holonephros" be employed to designate the entire embryonic kidney, 

 while the names pronephros and mesonephros are used as formerly to 

 designate the two structures that have been derived from this common 

 anläge. 



Is the pronephros of Bdellostoma, using the word in its restricted 

 sense, homologous with the pronephros of other vertebrates? It has 

 just been shown that the entire holonephros develops in the same way 

 as a pronephros, and this of course applies to the comparatively small 

 part which gives rise to the pronephros, so that in a general sense, 

 at least, the question must be answered in the affirmative. But the 

 relation of the pronephros here to the specialized organ which serves 

 as the larval excretory organ in the Amphibia, for example, is not so 

 easily determined. This is a subject I shall not discuss. 



It is not the purpose of the present paper to make any extended 

 comparisons between the excretory system of Bdellostoma and the 

 excretory system of other Vertebrates, but the facts here given seem 

 to have a very obvious phylogenetic signification, and in closing, I 

 wish, as briefly as possible, to point this out. 



From a study of the excretory system of the Selachians, Rîjckert 

 ('88) was led to formulate the following theory in regard to the phylo- 

 geny of the excretory system of the vertebrates: 



1) Originally the excretory system consisted of a series of seg- 

 mental tubules, which led from the coelom, and opened independently 

 of one another along the side of the body. 



2) The segmental duct was formed by the tubules losing their 

 independent opening to the exterior, and the distal end of each be- 

 coming united with the one just posterior. 



3) As the segmental duct extends through a much greater portion 

 of the body than the pronephric tubules, the latter must formerly 

 have been much more numerous than at present, or, in other words, 

 they must have been co-extensive with the segmental duct. 



4) Through the region of the present mesonephros, the primary 

 pronephric tubules entirely disappeared, but a second generation of 

 pronephric tubules persisted as the mesonephric tubules. 



At the time of the publication of this theory, no case was known 

 in the Vertebrates where excretory tubules opened to the exterior in- 

 dependently of one another, but since then Boveri ('92) has shown 



