Vle CONG. INTERN. REPROD. ANIM. INSEM. ARTIF., PARIS, 1968, VOL. I

one, corresponding with a lower specific gravity. These
observations could be reproduced and corroborated in
model experiments under the correct hydrodynamical
conditions.

DISCUSSION. Apparently there is strict correspondence
between the rate of passive sedimentation in a gravitat-
ional gradient with a carrying medium of low viscosity,
and the active speed of spermatozoa, but not between
active swimming speed and head size, as follows from the
reversed behaviour of cock and bull spermatozoa.

FPor bull spermatozoa there is agreement with recent
observations by SCHILLING et al. (7) that specimens with
smaller heads swim faster, but under their experimental
conditions with separating media of much higher
viscosity (8-12 CP§ and density (1.038-1.044) (6) they
found the smaller heads in the upper instead of in the
lower fraction. Since SCHILLING did not yet publish full
details of the composition of his medium, whereas in his
sedimentation procedure no gravitational gradient occurs,
it is not possible to analyse the reasons for the
reversed& relationship between head size and settling
velocity, but it does not seem urnlikely that it may be
due to differences in the osmotic condition of the cyto-
plasmic parts.

Since passive and active velocities correspond to
each other both in bull and cock spermatozoa, whereas
head sizes do not, the present results contradict the
hypothesis that differences of shape, size, mass and
active velocity as such have anything to do with the
presence of either an X- or a Y-chromosome (2,3,6,7),
the more so where it is obviously possible to reverse
the relative weight ratio between smaller and greater
sperm heads in a population by changing the medium, and
by the fact that avian spermatozoa apparently behave in
a similar manner as mammalian ones. Results indicate
that both active and passive moving velocitiess depend
primarily on drag instead of on mass differences.

It is considered more likely that any shift in the
sex ratio obtained after fractionating procedures with
mammalian semen would rather be due to physiological
than to biometrical factors.

(1) C. VAN DUIJN Jr., 1960.Mikroskopie 15:142-156.

(2) D. JORDANO, 1952.Arch.Zoot. 1:5-12; 520-325.

(3) P.E. LINDAHL, 1960.Z.Tierziicht.Zlichtbiol. 74:181-197.

54; R. RIKMENSPOEL, 1957.Experientia 13: 124.

5) R. RIKMENSPOEL, 1957.Thesis, Utrecht University.

(6) E. SCHILLIKG, 1966.Zuchthyg. 1:5-14.

(7) E. SCHILLING, 1. JAZBEK, P. SCHMID, 1967.Z.Tier-
ziicht.Zlichtbiol. 83:331-339.

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