MUSEUM OF COMPARATIVE ZOOLOGY. 295 



Flemming, but interprets their observations in a fundamentally different 

 way. In the course of a painstaking investigation, in which more than 

 forty series of sections were used, Martin never encountered conditions 

 which in his opinion demonstrated a genetic connection of the duct with 

 the ectoderm. He believes that the duct arises from a proliferation of 

 mesoderm in the region between the 9th and 11th protovertebrse, and 

 grows backward by cell division within its own mass. The posterior 

 portion of the duct, however, fuses with the ectoderm so intimately that 

 in certain regions it is quite impossible to recognize a boundary between 

 them ; but Martin believes that the fusion is wholly secondary, and that 

 the ectoderm contributes no material to the duct. Keibcd's ('88 a , p. G35) 

 studies on Erinaceus led him at first to accept Martin's attempt at har- 

 monizing the two views ; but in Cavia ('88 b , pp. 424-428) his observa- 

 tions inclined him towards the original view of Graf Spee. 



In my opinion, Martin is right in denying that an ectodermal origin of 

 the Wolffian duct has been demonstrated in Mammals. It is undoubtedly 

 true, that there is considerable evidence in favor of such a mode of origin ; 

 but it is not of a nature that would warrant one in concluding that the 

 duct arises in this way throughout all Vertebrates, or in asserting that 

 it develops in fundamentally different ways in Mammals on the one hand, 

 and in other Vertebrates on the other. AH that can be claimed, how- 

 ever, in accordance with Martin's view, is that it is possible to interpret 

 the conditions in Mammals in agreement with observations in other Ver- 

 tebrates, should these be shown to be less ambiguous. 



In Selachians, the evidence in favor of an ectodermal origin of the 

 duct is perhaps even stronger than in Mammals. In the former group, 

 besides the preliminary communications of van "Wijhe ('86, '88*) and 

 Beard ('87), there have appeared two extensive papers by Ruckert ('88) 

 and van Wijhe ('89), which seem to place the ectodermal origin of the 

 segmental duct almost beyond question ; and, so far as I am aware, no 

 recent observer has expressed doubts upon this point. It nevertheless 

 seems to me that, before accepting this result as final, we have yet to 

 inquire whether Martin's interpretation of the condition in Mammals 

 cannot be applied also in Selachii. 



It might be objected, that the latter view offers no explanation for the 

 intimate fusion which must be granted to exist between the posterior end 

 of the segmental duct and the ectoderm ; yet this argument cannot in- 

 validate the general conclusion, since a number of cases of such a union 

 of two epithelial structures in their growth have been recorded, where 

 no genetic connection is believed to exist. Such a conception is in- 



