hall: mesonephros and mullerl\n duct in amphibia. 109 



Mammalia. 



Kip : The ostium and tlie auterior end of the duct are from two or 

 three separate peritoneal evagiuations- A portion of the duct takes cells 

 from the Wolffian duct. There is a cephalic displacement of the ostial 

 opening similar to that described by me in Amblystoma and by various 

 authors in Anura. 



E. Theoretical Coxsideratioxs. 



Since Balfour and Sedgwick ('79) first suggested a homology between 

 the evaginations in the chick which form the anterior end of the Miillerian 

 duct, and the pronephros, there has been a tendency to derive the ostium 

 in some way from the pronephric nephrostomes. Semon ('92) suggested 

 that the eggs were originally emptied into the pronephric duct through 

 canals homologous with the vasa efFerentia of the testis, but that, on in- 

 creasing in size, they fell directly into the body cavity and a pronephric 

 tubule was specialized to transmit them to the pronephric duct, or to a 

 duct derived from the pronephric by a splitting process. 



In trying to trace the probable origin of the ostium and Miillerian 

 duct, we are confronted by two very different conditions : In elasmo- 

 branchs the ostium is derived directly from the pronephric nephro- 

 stomes; in Amphibia the ostium coexists with the pronephric nephrostomes 

 and is independent of them. It seems to me that these two conditions 

 can be reconciled by supposing a diverging differentiation, somewhat 

 after the following manner : 



In the elasmobranchs, the large amount of yolk which serves for the 

 nourishment of the young is, of course, a secondary acquirement. The 

 ancestral form we may picture as having lived as a free larva, and as 

 having possessed a pronephros which, in addition to functioning as an 

 excretory organ, had to prepare itself for the function of carrying off, in 

 the adult, the eggs set free in the body cavity. This was accomplished 

 by the pronephric duct dividing^ to form two potentially separate ducts, 

 much as the hermaphroditic duct of the pulmonate gastropods is divided 

 by a longitudinal infolding. The halves of the duct diverged from each 

 other posteriorly and opened separately. Phylogenetically, this splitting 

 and separation was carried cephalad until a stage of nearly complete 



^ That this division would separate the excretions of the pronephros from those 

 of the mesoncpliros is of no consequence. The best explanation seems to me to be 

 that of Van Wijhe ('89), who sees in it a means of preventing self-fertilization, 

 — the early vertebrates having been, probably, hermaphroditic. 



VOL. XLV. — NO. 2 6 



