52 WILLIAM MORTON WHEELER, 



pieces of the coelomic walls. Rückert's work has been fully confirmed on 

 nearly all important points in a recent elaborate paper by Rabl (1896). 

 The wide gap between Amphioxus, which has an extensive pronephros 

 but no mesonephros, and the Selachian with its vestigial pronephros 

 but highly developed metameric metanephros, has been at least partially 

 filled by Maas' valuable researches on Myxine. This form has a 

 pronephros, at least in its younger stages, which resembles the pro- 

 nephros of Amphioxus, and a mesonephros of a simpler and more 

 primitive structure than that of any other Craniote. 



We should expect to find in the renal system of Tetromyzon a 

 connecting link between the renal system of Myxine on the one hand 

 and the Gnathostomes on the other. This, however, is the case only 

 to a limited extent and only in the development of the pronephros. 

 The mesonephros, on the contrary, presents highly coenogenetic modi- 

 fications. Petromyzon is, therefore, interesting mainly on account of 

 the strange commingling of primitive and highly specialized characters 

 which it presents, a commingling which is by no means confined to 

 the nephric system of the animal. In this portion of my paper I shall 

 consider first the pronephros and mesonephros separately and then 

 their relations to each other, leaving for the conclusion a few general 

 statements concerning the development of the reproductive organs. 



a) The Pronephric Tubules and the Pronephric Duct. 

 There are three main theories concerning the phylogenetic origin 

 of the pronephric duct and these have been based on three very dif- 

 ferent series of facts. 1) According to one theory the pronephric 

 system was inherited by the primitive Chordate from some ancestral 

 Invertebrate form which was either a Plathelminth (Haeckel, 1874, 

 Gegenbaur, 1878, Fürbringer, 1878) or an Annelid (Semper, 1876, 

 Balfour 1881, and many others). Those who seek the origin of the 

 pronephros among Plathelminths lay most stress on the duct which 

 is so well developed in these worms, whereas those who believe in 

 the Annelid ancestry of the Chordata, emphasize the metameric tubules 

 and attempt to derive the duct either from surviving Plathelminth 

 conditions in larval Annelids (Semper, 1876, Balfour, 1881), or from 

 a condition like that of Lanice conchilega, an Annelid in which the 

 nephridia are connected by means of a longitudinal duct (E. Meïer, 

 1886, Cunningham, 1887 a and 1887 b). 2) The views of the authors 

 above cited, at least so far as the pronephric duct is concerned, were 

 severely shaken by the observations of Boveri on Amphioxus. In 



