BY JAS. P. UILL AND C. J. MARTIN. 65 



enclosed between the two a small space. Behind tlie point Avhere 

 the connection of the edges of the Wolthan duct with the 

 ectoderm is first seen, the duct rapidly becomes reduced in size 

 and approaches closer to the ectoderm (fig. 2S, 7v.a). 



Finally it is reduced to a single cell, which passes directly over 

 into the ectoderm (Mg. 29, u-.d.). 



From these observed facts we are inclined to believe that the 

 Wolffian duct in Platypus has an ectodermal origin. We cannot 

 assert this dogmatically from the examination of one stage; yet 

 the balance of e^ idence is in favour of this view, and indeed from 

 the facts at our disposal it is the only view we can put forward. 



The duct certainly does not grow backwards by jDroliferation 

 from its posterior end as Martin states to ])e the case in the 

 rabbit, for as opposed to the condition in that animal, where 

 according to Martin the Wolffian duct at its extreme posterior 

 end is thicker than just anterior to that point, in Platypus the duct 

 gradually becomes thinner posteriori}^, and as we have described, 

 passes directh^ over into the ectoderm. Nor can the duct grow 

 backwards Iw the addition of cells from the mesoderm, for as w^e 

 have shown the Wolffian duct is quite distinct posteriorly from the 

 Anlage of the tul3ules and from the adjacent mesoderm. We are 

 therefore inclined to believe that the Wolffian duct in Platypus 

 grows backwards by separation or delamination of cells from the 

 ectoderm. 



Just as the difierentiation of the Anlage of the Wolffian duct 

 from the ectoderm is lost as it is traced posteriorly, so the 

 differentiation of the Anlage of the tubules from the intermediate 

 cell mass is also lost. The Anlage of the tubules can, however, 

 be traced behind the termination of the" Wolffian duct as a narrow 

 strand of rounded cells readily distinguishable from the looser 

 branching cells of the rest of the intermediate cell mass. The 

 relations of the Anlage of the tul^ules to the intermediate cell 

 mass in Platypus is thus essentially the same as Martin has 

 described for the rabljit. 



ijj LIBRARY ^\ 



