A. S. PARKES 
electrophoresis or differential centrifugation have been made. 
In the case of rabbits and cattle, there have been claims, sup- 
ported by breeding experiments, that the two groups separated 
were in fact X- and Y-spermatozoa. All this work, however, is 
in serious need of repetition and confirmation on a large scale. 
Immunological methods are also being investigated for the 
possible separation or differential destruction of one or other 
type of spermatozoa. 
Assuming that the separation of X- and Y-spermatozoa will 
be effected one day, and I have no doubt it will, what are the 
chances of preserving the separated gametes for future use as 
required? At present, the only practicable approach to the 
long-term preservation of mammalian cells is so to reduce the 
ambient temperature that the biochemical and _ biophysical 
processes of life are attenuated sufficiently to cause the cell or 
tissue to pass into a state of suspended animation. Up to about 
12 years ago, such a method had been effective with only one 
or two types of normal vertebrate cells, but it was then dis- 
covered that the otherwise fatal effects on cells of freezing and 
thawing could in many cases be very largely avoided by the use 
of protective substances, of which glycerol is the best known. 
This particular technique was first evolved on the spermatozoa 
of domestic fowl, not on mammalian cells, but it was very 
quickly adapted to the spermatozoa of the bull because of the 
economic importance of efficient use of bull semen in artificial 
insemination centres. In man, of course, there have been 
attempts over this period, and long before, to preserve human 
spermatozoa at low temperatures. Some success has been 
achieved: there have been reports in America of one or two 
children being produced as a result of artificial insemination 
with preserved spermatozoa. The whole subject, however, is at 
a very elementary stage; it is nothing like so far developed for 
man as for farm animals, especially the bull, for which post- 
humous paternity is now possible many years after death. 
Obviously a great deal more work will have to be done before 
the long-term preservation of human spermatozoa, whether or 
not separated into X- and Y-types, becomes practicable. ‘There 
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