/6 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



They seem to arise either (1) from a splitting of the first set, or (2) from 

 independent evaginations of the peritoneum, or from both. 



4. In regard to the order of appearance of the primary nnits, the 

 usual statement that it progresses from behind forward, is incorrect (at 

 least for Rana sylvatica). The majority of the primary units, those 

 occupying all except the extreme ends of the kidney, appear simulta- 

 neously, but they develop from behind forward. Later, a few are added 

 at both posterior and anterior ends of the series. 



5. As in Aniblystoma, the antero-posterior extent of the germ cells 

 becomes much reduced as development proceeds. 



III. Development of the MuUerian Duct. 



The point of greatest phylogenetic interest in the development of the 

 Miillerian duct, its relation to the Wolffian duct, is still in dispute. 

 There seems no doubt that in elasmobranchs the greater part of it is 

 derived from the Wolffian duct, and a similar origin has been claimed by 

 the latest investigators for a portion of it in birds and mammals. In 

 regard to the Amphibia, statements have been very contradictory, the 

 Miillerian duct being described sometimes as developing independently 

 of, sometimes in connection with, tlie AVolffian duct. Consequently my 

 attention has been especially directed to this point. 



Another problem, the solution of which may lielp in deciphering the 

 phylogenetic history of the Miillerian duct, is the mode of formation of 

 the ostium ahdominale. In elasmobranchs, the ostium is said to be 

 formed by a fusion of several of the pronephric nephrostomes. In birds 

 and mammals, on the contrary, it would seem that it is formed by the 

 coalescence of several evaginations of the coelomic epithelium, indepen- 

 dently of the nephrostomes. In the Amphibia it has not been claimed, 

 to my knowledge, that more than one coelomic evagination precedes 

 the formation of the ostium. I, therefore, have given this question 

 special attention in my investigations, in order, if possible, to throw some 

 liglit on the suggested homology between the ostial evaginations of 

 tlie higher vertebrates and the nephrostomes of the elasraobranch 

 pronephros. 



A. Ambltstoma. 



The Miillerian ducts of adult Amblystoma are rather simple struc- 

 tures, consisting of a pair of tubes, each having a single ostium ahdomi- 

 nale opening into the body cavity, and a posterior outlet opening into 



