MUSEUM OF COMPARATIVE ZOOLOGY. 291 



in the Rabbit the Wolffian duct arises from a solid rod-like thickening of 

 the ectoderm near the middle protovertebne. In a second short commu- 

 nication ('67, p. 502), Hensen merely reaffirmed his confirmation of His; 

 but finally he (75-76, pp. 369-372) published a fuller account of his 

 observations, accompanied with figures. These, however, are far from 

 conclusive, and it does not seem surprising that this single observation 

 was distrusted by subsequent writers. 



In 1884 Graf Spee published an account of his very careful investiga- 

 tion of the subject, and reasserted the ectodermal origin of the Wolffian 

 duct. 1 Following this publication have appeared a large number of con- 

 firmatory papers, which have moreover extended the observations of 

 Graf Spee; so that at present the ectodermal origin of the duct has 

 been asserted for every class of Vertebrates, with the single exception of 

 the little known Dipnoi. 



As stated in the Introduction to the present paper, it was my hope 

 in undertaking these studies to find in Amphibia results confirmatory of 

 Graf Spee's position. If, then, a contrary result has been reached, it 

 has been because I have been driven to that conclusion by evidence 

 brought out in the course of the investigation. In my opinion, the 

 entire excretory system of the forms I have studied unquestionably 

 develops without any participation of the ectoderm in its formation. 

 The duct develops from mesoderm throughout its entire length, and 

 at its posterior end, in Rana and Bufo at least, comes in contact with 

 one of the entodermal cornua of the hind gut ; so that nowhere in its 

 development does it come into organic union with the outer germ 

 layer. 



I must in this case distinctly disavow the suggestion of Hertwig ('88, 

 p. 280), who endeavors to harmonize the accounts by assuming that 

 only the posterior end of the duct is formed from the ectoderm. This 

 explanation would by no means be admissible, unless it be granted that 

 the ectodermal constituent might in this case be reduced to nothing 

 at all. On the other hand, it must be confessed that a fundamental 

 opposition in the mode of development of an organ in two closely related 

 groups is at present hardly reconcilable with our general conceptions 

 of embryological processes. 



1 Graf Spee, and subsequently Flamming ('86), did not clearly recognize the fact 

 that the Wolffian duet and the mesonophros develop in different ways, and were 

 led to defend an ectodermal origin for the excretory system. This interpretation is in 

 evident opposition to the accounts of others, and, in my opinion, is not justified by 

 their own observation, even should these prove to be accurate in every particular. 



