STUDIES IN THE EXPERIMENTAL ANALYSIS OF SEX. 441 
beginning of September. From September until the breeding 
season in March no increase in size or alteration of the 
cellular structure occurs, the testes apparently remaining in 
a state of complete inactivity during this period. About 
March the shedding of the sperm normally occurs, and the 
cycle begins again. These alterations in size are accompanied 
by corresponding changes in the cellular activity of the 
organs. After the shedding of the spermatozoa at the breed- 
ing season the diminished testes consist of the germinal 
tubes lined with spermatogonia, and containing in their 
cavities remaius of spermatozoa, that have not been shed and 
the debris of the supporting cells to which the spermatozoa 
were attached in bundles. The spermatogonia soon begin 
to divide and proliferate, and to form groups of cells known 
as spermatocysts. The rapid increase of these spermatocysts, 
or nests of spermatogonia, continues through the summer 
months, and is responsible for the active growth of the testes. 
In June and July spermogenesis, or the formation of spermato- 
cytes from the proliferating spermatogonia, is active, and 
from August to October the formation of ripe spermatozoa 
from spermatids is definitely finished. In November, 
December, January, and February the testes are crammed 
with ripe spermatozoa, and no apparent changes occur in the 
cellular structure or activities of the testes. We may, there- 
fore, say that the activity of the testes coincides with the 
months of active feeding, namely, May to September, and 
that during the months when the frogs are hibernating at 
the bottom of a pond the testes remain in a fully mature and 
quite inactive state (see Table on p. 468). 
The thumb-pads of the male frog also undergo a cyclical 
change, but their periods of growth and rest do not obviously 
correspond at all to those of the testes. Immediately after 
the breeding season the thumb-pads, with their deeply 
pigmented papillae, are cast off, and the thumb remains 
comparatively smooth from late April or May until August or 
September. The comparative smallness of the pad at this 
period is due to the reduction of the glands and to the 
