BY JAS. P. HILL AND C. J. MARTIN. 
65 
enclosed between the two a small space. Behind the point where 
the connection of the edges of the Wolffian duct with the 
ectoderm is first seen, the duct rapidly becomes reduced in size 
and approaches closer to the ectoderm (fig. 28, w.d). 
Finally it is reduced to a single cell, which passes directly over 
into the ectoderm (Hg. 29, w.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 evidence 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 proliferation 
from its posterior end as Martin states to be 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 posteriorly, and as we have described, 
passes directly over into the ectoderm. Nor can the duct grow 
backwards by the addition of cells from the mesoderm, for as we 
have shown the Wolffian duct is quite distinct posteriorly from the 
Anlage of the tubules 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 differentiation 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 tubules to the intermediate cell 
mass in Platypus is thus essentially the same as Martin has 
described for the rabbit. 
E 
