E. A.
WATSON
253
The cytometer is then allowed to stand for five
minutes to allow the sperm to settle in position. At
this dilution, in cold normal saline, they rapidly become
immotile.
The count .is made under a 1/6 objective and a
mechanical stage greatly facilitates the work.
If the dilution is about right (approximately five
sperm per small square), an accurate estimate is obtain-
ed by counting the number of spermatozoa in three
rows of 16 small squares, one in the middle, and one
at each end of the cytometer, and taking the average
in 16 small squares.
As the volume of each small square is 1/80 cu. mm.,
the number of spermatozoa per cu. mm. of the semen
may be calculated from the following equation, where
= mean number of spermatozoa in 16 small squares,
d = dilution :
No. of sperm cu. mm. = 80x × d/16
„
„ c. c.
= 5,000dx
B. USING THE THOMA-ZEISS HAEMOCYTOMETER
A dilution of 1 in 200 is suitable. The dilution is.
made as follows : Thoroughly mix the semen. Then
with the Thoma-Zeiss pipette, draw semen, previously
well mixed, up to the 0.5 mark. (As the background
of the capillary tube is white it will be difficult to
observe the semen. This can. be overcome by viewing
it from a little to one side.) The pipette end is then
introduced into normal saline and suction applied
until the diluting fluid is drawn up to the 101 mark.
No air must enter the pipette during these operations.
The mixture is then shaken well in the bulb. A little
of the fluid is decarded to ensure that the drop used
for the count comes from the pipette bulb. A single
drop is placed on the central, ruled area of the count-
ing slide. It is important that the size of the drop
shall be such that there are no air bubbles present when
the cover slip is applied and also that none of the
fluid runs into the channel surrounding the ruled area.
With a little practice this is not difficult. The ruled
area comprises 16 large squares, each divided into 16
small ones. The volume of each small square, each
cover-slip is applied, is l/4,000th cu. mm.
The number of spermatozoa in 100 small squares is
counted (six large squares plus four small ones) using
the l/6th objective. Of those spermatozoa crossing the
lines bounding the squares, only those lying on two
sides should be included, so that none is counted
twice. In good samples the number in each large
square will vary from 15 to 25.
The calculation is as follows : x being the number
of spermatozoa in 100 small squares, and the dilution
being 1 in 200.
No. of spermatozoa per cu. m
= 8,000x
The number of abnormal spermatozoa may be estima-
ted at the same time. These appear in all samples of
semen and from animals of all grades of fertility, and
it is extremely doubtful whether the percentage of
abnormals is of any value in semen evaluation if the
total numbers of normal spermatozoa are sufficiently
high.
For good fertility, the density should be 700,000 per
cu.mm. or higher; frequently, samples from some bulls
contain up to 1,500,000 spermatozoa per cu.mm.
* * * *
NOTE
Certain Breed Societies have drawn up rules govern-
ing the artificial insemination of pedigree cows; in view
of the temporary nature of some of these rules, any
veterinary surgeon wishing to carry out artificial inse-
mination of pedigree cows should obtain from the
Secretary of the Breed Society concerned a copy of the
current regulations. The names and addresses of those
Societies which already have regulations are as fol-
lows :—
Ayrshire Cattle Herd Book Society,
58, Alloway Street, Ayr.
British Friesian Cattle Society,
Aldwych House, Aldwych, London, W.C.2.
English Jersey Cattle Society,
51, Hillcrest Gardens, Esher, Surrey.
English Guernsey Cattle Society,
Clevedon, Wokingham, Berks.
Shorthorn Cattle Society,
Westfield, Medmenham, Great Marlow, Bucks.
GENETICS OF VIRUSES PATHOGENIC TO ANIMALS*
By E. A. WATSON, Science Service, Department of Agriculture, Ottawa, Ontario, Canada
THE question: ' What is a virus?', is a familiar
one, frequently put and still awaiting answer.
Are viruses living or dead, animate, in-
animate, particulate, biological or biochemi-
cal elements, agents, transmitters, enzymes,
ferments, catalysts, or what? Without know-
ing what a virus truly is, considerable know-
ledge regarding the properties and characteristics
of some of the viruses has been gained. It can
be said that a virus is a filterable, infective
agent, invisible under the microscope; hence the
term ultramicroscopic, filterable viruses. "Fil-
terable" can have only a relative sense and.
meaning, being dependent upon the nature of
the filter and its density, the physical proper-
ties of the fluid material, electric charges, and
other factors. Moreover,
some micro-organi-
sms have filterable phases or forms in their
life cycles. The belief that viruses are living
agents rests mainly on the fact that they are
capable of multiplication and adaptation in
the living body and, in vitro, in the presence
*(Reprinted from Publication No. 12 of the American Association for the Advancement of Science, pages 18—31)