Gauses of the changing pattern of semen fertility with length of stor-
age.

There are a number of facts which emerge from these findings on
fertility and which must be considered in developing theories for
further study of the fundamental cause or causes of the observations.
OQur experience suggests that a proper solution cannot be achieved un-
less the problemis pursued to the fundamental intracellular molecular
level and attempts made to identify the changes in the spermatozoa
which are responsible.

The observed phenomena occur at all temperatures used as a con-
trol mechanism (4°,-79 to -88°, and -196°C) and at rates positively,
though probably not directly, associated with the absolute tempera-
ture.

During the physiological aging of spermatozoa, which occurs in
semen storage, what is the reason for, first, the increase in the
functional competence of the semen sample, and, then, the straight-
line decrease in that competence? The cause of the first of these
two phenomena is the more difficult to test. We are inclined to
view the observed improvement of fertility early in storage as evi-
dence of the changing relative ability of spermatozoa within stored
samples of semen to effect fertilization. Among the fresh sperma-
tozoa inseminated, we believe there are some capable of fertiliza-
tion which contain genes or defective chromatin causing embryonic
death. Sperm cells containing them die more quickly early in stor-
age so that later theydo not fertilize as high a proportion of eggs,
quitting the field, as it were, for the spermatozoa with normal
chromatin capable of supporting normal embryogenesis (21).

The optimum fertilization and calving rate seen in the storage
curve is achieved, we believe, primarily by normal sperm cells,
After this point in time aging supervenes for all sperm cells at
rates dictated primarily by the temperature of semen storage and by
the genotype of the male producing the spermatozoa so that fertility
decreases and embryonic mortality increases relatively rapidly.

The fundamental cellular changes accompanying aging.

Soon after we had decided to concentrate our laboratory research
efforts on the nuclear chromatin in our searchfor the cause or causes
of these two fertility phenomena, we discovered interesting changes
in the intensity of the Feulgen reactionin the heads of aging bovine
spermatozoa (17). This observation and our conclusion that Feulgen-
DNA decreased during spermatozoan aging was quickly questioned by
some other cell biologists,none of whom had studied this exact prob-
lem but who well well acquainted with the vagaries of the Feulgen
reaction (13,14). For a time such comments nearly had us convinced
that the solution of the problem lay in other directions. However,

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