horny epidermis developed from the living epidermis beneath. The horny
layer is not shown clearly in the figures of Smith and Schuster. It seems
impossible that the horny layer or its papillae could atrophy in
consequence of castration, or be absorbed. The horny part of the frog's
thumb-pad is comparable with the horny sheath of the horns in the
mammalian Prong-buck (_Antilocapra_) which are shed after the breeding
season and annually redeveloped. Meisenheimer claims that he produced
development of papillae on the thumb-pad, not only by implantation of
pieces of testis, but also by implantation of pieces of ovary. This seems
so very improbable that it suggests a doubt whether the same investigator
was not mistaken with regard to the results of his experiments in
transplanting gonads in Moths.

Smith and Schuster conclude that the normal development of the thumb-pad
depends on the presence of normal testes, but that there is no sufficient
evidence that the effect is due to a hormone derived from the testis. It
is equally probable, according to Smith, that the testicular cells take up
some substance or substances from the blood, thus altering the composition
of the latter and perhaps stimulating the production of these substances
in some other organ of the body. These substances may be provisionally
called sexual formative substances. Smith's theory therefore is that the
action of the testes in metabolism is rather to take something from the
blood than to add something to it, and that it is this subtractive effect
which influences the development of somatic sexual organs.

Geoffrey Smith in fact, in the paper above considered, attempts to apply
to the frog the views he put forward [Footnote: _Fauna und Flora des
Golfes van Neapel_, 29 Monographie Rhizocephala.] in relation to the
effect of the parasite _Sacculina_ on the sexual organs of crabs. The
species in which he made the most complete investigation of the influence