HALL : MESONEPHROS AND MULLERIAN DUCT IN AMPHIBIA. 117 



To account for the two very different modes of origin of the Mtillerian 

 ducts found in elasmobranchs on the one hand and in the Amphibia 

 (and probably the Amniota) on the other, it is suggested that the 

 Miillerian evaginations in the Amphibia studied by me represent a ven- 

 tral set of pronephric tubules comparable if not homologous with the 

 ventral set in Amphiuma. This ventral set, which originally possessed 

 a secretory function, has become, like the pronephric tubules of elasmo- 

 branchs, specialized to subserve a sexual function. Instead of opening 

 into the pronephric duct, they form a duct of their own which, by fusing 

 with the pronephric duct for a short distance in Amblystoma and many 

 Amniota, still ])etrays the fact that it had its origin in a splitting of that 

 duct. 



New Haven, May, 1902. 



Addendum. 



Too late to be incorporated in the body of the text, there came to hand 

 an extremely important paper by Brauer (:02). I shall refer to such 

 parts only as have a direct bearing on points treated in my own paper. 

 His material consisted of specimens 6f Hypogeopliis rostratus, belonging 

 to the Gymnophiona. 



The raesonephros is very long, extending throughout seventy-six seg- 

 ments, — from the 24th to the 100th. Two greater divisions may be 

 distinguished, that comprising the region of segments 24 to 29, and that 

 including segments 30 to 100. In the former division the mesonephric 

 units remain rudimentary and finally degenerate ; in the latter both 

 primary and secondary are formed, the primary reaching a functional 

 stage. This posterior division also shows a differentiation into two 

 regions; in segments 50 to 100, the secondary tubules become fully 

 developed ; in segments 30 to 50 they degenerate. The degeneration of 

 the secondary units brings about a strictly raetameric arrangement of 

 the tubules in this region of the adult kidney, for the entire organ has 

 from the beginning a metameric structure to the extent that there is 

 but one ]3rimary unit in each segment. 



As the units of the last ten mesonephric segments (somites 90 to 100) 

 are peculiarly modified, typical development is present in only forty 

 segments (somites 50 to 90). The development of a mesonephric unit 

 in this region is as follows : the somites are constructed essentially as in 

 Amblystoma, their lumina being, however, larger. The ventral (lateral) 

 portion of the somite, corresponding to what I have called the mesomer, 



