WOLFFIAN DUCT AND BODY IN THE CHICK. 35 



Elasmobranchii) may be considered as in some respects primitive, 

 the other must be regarded as very much modified. 



Whatever may have been the phylogenetic origin of the 

 Wolffian tubules, the ontogenetic origin, as seen in Amphibia, 

 Teleostei, Ganoids, Marsipobranchii, cannot possibly be re- 

 garded as in any way approaching the former. We cannot 

 suppose that a definite serial organ like the mesonephros de- 

 veloped in phylogeny as a series of independent cavities in a 

 mass of mesoblastic cells. At any rate, I think I am justifi.ed, 

 in the present state of our knowledge, in making this statement. 

 It is completely opposed to our ideas, and cau only be accepted 

 when all other hypothesis as to the origin of the mesonephros in 

 phylogeny, based on the facts of embryology, have been shown 

 to be untenable. 



The tubules of the mesonephros in Elasmobranchii, however, 

 i'n which group they arise from parts of an organ previously 

 developed, present a method of development which is not at all 

 at variance with our a priori views as to their phylogenetic 

 origin. Erom considerations of this kind it seems to me a fair 

 assumption that the development of the tubules in Elasmo- 

 branchs from parts of the body cavity more nearly resembles 

 the method by which the organ arose in phylogeny than does 

 that of the Wolffian tubules of the remaining Ichthyopsida. 



In Elasmobranchs the Wolffian tubules have a segmental 

 arrangement ; one is found in each segment. In all probability 

 this also is a primitive condition. 



The arrangement of the tubules in the other vertebrata, 

 although it does not actually afford support to this view, still it 

 does not disprove it. It is a well-known fact that the segmental 

 tubes have very rarely a segmental arrangement in the adult or 

 even in the embryo. But in this connection it must be remembered 

 that the tendency of development always seems to be to render 

 that part of the mesonephros, which is going to function in the 

 adult as an excretory organ, more compact, i.e. to bring its 

 constituent parts closer together. I need only refer to the kid- 

 neys of the Urodele Amphibia. Here the posterior part of the 

 mesonephros, which is going to function in the adult as kidney, 

 becomes distinguished by its size and the course of its ducts 

 from the anterior part, and in the female by its size only from 

 the anterior part. And Eiirbringer has shown, in Salamandra 

 maculatttj that in correspondence with the increasing size of the 

 posterior region there is found an increased number of primary 

 tubules in a segment, as well as of dorsal secondary tubules.^ 



* Spengel however asserts, that in the female of those Amphibia he has 

 investigated, the kidney (mesonephros) contains an uniform number of 



