result. Sexual differences exist from the first. Nussbaum made
experiments on frogs (_Rana fusca_), which go through a yearly
cycle of secondary sexual changes at the period of heat. These
changes cease on castration, but, if the testes of other frogs
are introduced beneath the skin of the castrated frogs, Nussbaum
found that they acted as if the frog had not been castrated. It
is the secretion of the testes which produces the secondary
sexual changes. But Nussbaum found that the testicular secretion
does not work if the nerves of the secondary sexual region are
cut, and that the secretion has no direct action on the organism.
Pflüger, discussing these experiments (_Archiv für die Gesammte
Physiologie_, 1907, vol. cxvi, parts 5 and 6), disputes this
conclusion, and argues that the secretion is not dependent on the
action of the nervous system, and that therefore the secondary
sexual characters are independent of the nervous system.

Steinach has also in later experiments ("Geschlechtstrieb und
echt Sekundäre Geschlechtsmerkmale als Folge der
innerskretorischen Funktion der Keimdrusen," _Zentralblatt für
Physiologie_, Bd. xxiv, Nu. 13, 1910) argued against any local
nervous influence. He found in _Rana fusca_ and _esculenta_ that
after castration in autumn the impulse to grasp the female
persisted in some degrees and then disappeared, reappearing in a
slight degree, however, every winter at the normal period of
sexual activity. But when the testicular substance of actively
sexual frogs was injected into the castrated frogs it exerted an
elective action on the sexual reflex, sometimes in a few hours,
but the action is, Steinach concludes, first central. The
testicular secretion of frogs that were not sexually active had
no stimulating action, but if the frogs were sexually active the