702 CLARENCE L. TURNER 
cells are starting through the procession of changes and divisions 
which occur before mature spermatozoa are formed. The mi- 
gration of new germ cells into the testis ceases about the first 
or the middle of August. The end of this period of migration 
may or may not have something to do with the decline in the 
volume of the testis which starts at the same time. It is certain 
that the sharp decline occurring from March to June is caused 
by the expulsion of the spermatozoa. 
It is not satisfactory merely to point out internal changes in 
the testis and to correlate them with periodical variations in the 
volume of the testis unless it be argued that the germ-cell cycle 
is an automatic and inherent one which will take place inde- 
pendently of the factors of external environment. The very 
fact that there are seasonal changes justifies a closer scrutiny of 
the seasonal environment of the fish. 
The perch is a bottom dweller. A gill net suspended in the 
water a few feet above the bottom will trap only a few fish, while 
one suspended at the bottom in the same locality will trap great 
numbers. It has been found by Prof. A. 8. Pearse that the 
perch occur in the greatest numbers in Lake Mendota at the 
bottom of the lake at a depth of from 20 to 35 feet. The rec- 
ords of the Wisconsin Geological and Natural History Survey 
show that the variation in the temperature between the surface 
and the bottom of the lake at any given date is a matter of only 
two or three degrees. The variation in the temperature at the 
bottom of Lake Mendota at a depth of 6 meters is shown graphi- 
cally in figure 1B. The curve is based on the mean temperature 
for the last fifteen years at a depth of 6 meters. The base line 
represents 0 degrees, centigrade, and each space an interval of 
5 degrees. The spaces between the ordinates represent inter- 
vals of thirty days. During the winter the temperature at the 
bottom remains slightly above zero. During the latter part of 
March there is a rise in temperature, and the rise continues till 
about the third week in August. It will be seen at a glance 
that the two critical points are early in the spring and late in 
August and-a comparison of these two periods with the critical 
points on the curve in figure la shows a coincidence in the criti- 
