230 
difficult for me to conceive it possible for all the spermatogonia to 
disappear, and be later replaced by new ones derived from the vege- 
tative cells during the intervals elapsing between these stages, espe- 
cially in the face of the fact that these stages show a wholly con- 
sistent plan of development. Furthermore, the spermatogonia of the 
adult are identical in appearance with those observed at the period 
of emergence from the egg. Previous to that time, the stages are so 
numerous as to perfectly link that stage with that of the freshly laid egg. 
It is proverbially difficult to prove a negative. It would seem 
that the burden of proof lies with him who would claim that there is 
at any stage of development of the vertebrate testis a total degener- 
ation of the sex-cells or of their derivitaves, the spermatogonia. 
Loiser ’02 claims that in the sparrow (Passer domesticus) the 
sex-cells, “ovules males”, present in the adult testis during the winter, 
do not give rise to spermatozoa, but that the ‘actual spermatogonia 
arise at about the last of February and first of March by transform- 
ation of vegetative “germinative” cells. This is coupled with the 
assertion that the vegetative cells undergo amitotic division immedi- 
ately before undergoing this transformation. The instructive and 
thorough work of ErzoLp ’91 upon the same form, presents views 
quite the reserve of those held by LoıseL. He holds that the sperma- 
togonia persist through the winter, and in the early spring begin to 
rapidly divide by mitosis, giving rise eventually to the spermatozoa. 
According to him, then, there is a strict dualism in the cells of the 
seminiferous tubules throughout adult life. 
Figs. 12—14 show certain stages of the seasonal changes in the 
testis of Chrysemys, from the period of active spermatogenesis in the 
fall (Sept. 10th) through the winter period of inactivity, in which 
spermatogonia and vegetative cells are alone found (Oct. 20th etc.), 
up to the stages of late winter and early spring, in which sperma- 
togenesis is beginning. Fig. 14 represents the condition observed in 
a turtle killed on January first. It must be said, however, that the 
lot of turtles from which this specimen was taken, had been kept in a 
fairly warm room since November first. This must have materially 
hastened spermatogenesis, since a specimen not kept in this manner 
but killed on January twentieth, did not show a single spermatocyte, 
nor had the spermatogonia materially increased in number. It is not 
my intention to lay great stress upon this acceleration of development 
as such, because so many of our domestic animals furnish sufficient 
proof of the influence exerted by the environment in modifying or 
breaking down the seasonal periodicity of sexual activity. 
