whose careers were cut short in the War, maintained views concerning
somatic sex-characters different from that which explains their
development as due to a hormone from the testis or ovary. Nussbaum in 1905
[Footnote: 'Ergebuisse der Anat. und Entwicklungsgesch.,' Bd. xv.;
_Pflügers Archiv_, Bd. cxxvi, 1909.] had recorded experiments on _Rana
fusca_ (which is identical with the British species commonly called _R.
temporaria_) which appeared to prove that in the male frog after
castration the annual development of the thumb-pad and the muscles of the
fore-leg does not take place, and if these organs have begun to enlarge
before castration they atrophy again. When pieces of testis were
introduced into the dorsal lymph-sac of a castrated frog the thumb-pads
and muscles developed as in a normal frog. Geoffrey Smith and Edgar
Schuster [Footnote: _Quart Journ. Mic. Sci_., lvii, 1911-12.] investigated
the subject again with results contrary to those of Nussbaum.

Smith and Schuster begin by describing the normal cycle of changes in the
testes on the one hand and the thumb-pad on the other. After the discharge
of the spermatozoa in March or April the testes are at their smallest
size. From this time onwards till August they steadily increase in size,
attaining their maximum at the beginning of September. From then till the
breeding season no increase in size or alteration of cellular structure
occurs, the testes apparently remaining in a state of complete inactivity
during this period. With regard to internal development, after the
discharge of spermatozoa in the breeding season the spermatogonia divide
and proliferate, forming groups of cells known as spermatocysts. In June
and July spermatogenesis is active, and from August to October the
formation of ripe spermatozoa is completed.

The corresponding changes in the thumb-pads are as follows. Immediately
after the breeding season the horny epidermis of the pad with its deeply