310 BULLETIN OF THE 



ever presents either of these processes in an unambiguous manner. I 

 have already expressed my doubts in regard to the development of the 

 pronephros and duct by the incomplete closure of a groove of somato- 

 pleure. The best attested claim that has been made for such a mode 

 of origin was that made by Goette, Furbringer, HoflFmann, and Marshall 

 and Bles, for Amphibia ; but this position is distinctly contradicted by 

 my own observations. Indeed, this mode of origin has been recently 

 denied in tlie case of every class except Teleosts, a group in which it is 

 very difficult to obtain accurate evidence respecting the early history of 

 the mesoderm. 



On the other hand, numerous recent investigators have described the 

 first rudiment of the pronephros as a series of distinct evaginations. 

 Such observations have been recorded in Cyclostomes by Kupffeii ('88), 

 in Ganoids by Beard ('89), and in Amniotes by almost all writers on 

 their early development. It seems to me, therefore, that the mode of 

 formation by means of serial evaginations has a far wider distribution, 

 and is more clearly attested, than that by means of an incompletely 

 closed fold. I am of opinion that the condition in Amphibia and Sela- 

 chia is to be regarded as derived from such evaginations by means of 

 coenogenetic modification ; and that the weight of internal evidence is in 

 favor of the view that the tubules were primitively distinct. 



Typically the nephridial tubes are strictly metameric, one pair of 

 tubules being developed in each metamere. The occurrence of several 

 nephridia in a somite occurs, as we have seen, in the case of the meso- 

 nephros of certain Amphibia. This condition seems to me to be a char- 

 acter secondarily acquired. The following reasons confii-m this opinion : 

 (1.) In other forms, the strict metamerism of the nephridia is the ear- 

 liest ontogenetic condition, the duplication of the tubules appearing much 

 later. (2.) The dysmetameric arrangement seems to be coiTclated with 

 the limited number of somites w^hich are, in such cases, involved in the 

 formation of the mesonephros ; thus, in the Anura, a group in which the 

 number of trunk somites is extremely small, the mesonephros departs 

 most widely from the metameric condition ; in Urodeles, the number of 

 somites is larger, and there is an indication of metamerism in the anterior 

 tubules; and again in Coecilia, where the number of somites is still lar- 

 ger, the mesonephros has the typical metameric arrangement. (3.) The 

 pronephros, which in general represents the least modified portion of the 

 excretory system, retains a metameric condition in those forms in which 

 this arrangement is absent in the mesonephros. 



In order to ascertain the probable mode in which the metameric diver- 



