STUDIES IN THE EXPERIMENTAL ANALYSIS OF SEX. 465 
absolutely smooth unpapillated condition of the summer frog. 
It is entirely unfounded to regard this smooth condition as 
the reduced phase to which thumbs are reduced as the result 
of castration ; rather must it be regarded as an active phase 
brought about by the proliferation of the epidermis between 
the papillae, which causes the depressions to be filled up and 
the papillae to disappear. It is surely not without significance 
that this acquisition of the smooth thickened epidermis on the 
thumb coincides with the period of the greatest activity of 
the testes, when they are attaining their full size and are 
engaged in the maturation of the genital products (see Table, 
p. 468). The conclusion which we draw from the whole series 
of experiments, including our own and those of other authors, 
is that the cycle of changes in the thumb of the male frog is 
conditioned by the presence of testicular tissue in a normal 
functional condition, but that it has hitherto proved impossible 
to replace the action of the normal testicular tissue by means 
of the injection of testis-extracts, or by the implantation of 
testes or pieces of testis from other frogs. The fate of such 
introduced tissues is invariably degeneration, destruction and 
absorption, or replacement by fibrous tissue, and there is no 
experimental evidence, according to our view, to justify the 
conclusion that the introduced testicular matter, while it is 
being broken up and absorbed, gives rise to any substance 
which influences the secondary sexual characters in the same 
way as the presence of the normal, living testicular tissue of 
the individual itself. 
The deduction, therefore, which has been widely based on 
Nussbaum's experiments, that the testis of the frog contains 
an internal secretion, which, on being circulated in the blood, 
calls forth the development of the secondary sexual cha- 
racters, either with or without the mediation of the nervous 
system, is without experimental foundation. 
The fact that the developmental cycle of the thumb 
depends for its normal course on the presence of normal 
living testicular tissue can be equally well explained on the 
theory that the testicular cells enter into a chain of metabolic 
