DISCUSSION. The finding of a blastocyst with a
high percentage of tetraploid cells suggests that
chromosome abnormalities may be associated with
early embryonic mortality in the bovine. Although
it is unknown to what extent, if any, the tetra-
ploid cells might have influenced the future
development of the blastocyst, we have not seen
such a high percentage of tetraploidy in over 800
cows we have examined and from this concluded that
this condition would probably be lethal to the
embryo. A much larger survey must be made, how-
ever, before we can know the importance chromosome
abnormalities have in early embryonic death in the
bovine.

The mosaic condition most 1ikely arose as a
result of the failure of an early cleavage divi-
sion after chromosome replication with the subse-
quent development of a diploid cell line and also
a tetraploid cell line. The sex chromosomes are
compatible with this hypothesis. The presence of
only 1 corpus luteum tends to rule out fusion of
2 blastocysts, 1 diploid and the other tetraploid.
However, one possibility is that there was fusion
of a fertilized egg with a fertilized polar body,
1 diploid, the other tetraploid. The complexity
of this mechanism tends to rule it out. Another
mechanism which can be postulated is that the
blastocyst was originally a pure tetraploid and
subsequently a diploid set of chromosomes were
lost from 1 cell in mitosis. This also appears
to be a complex mechanism which is difficult to
explain. Unfortunately, there is no way that this
problem can be solved at the present time.

From studies on pigs (5) and rabbits (3), it
is known that delayed fertilization causes an
increase in the number of chromosome abnormalities
in embryos. It is interesting to speculate about
the increased possibility of delayed fertilization
in species where artificial insemination is preva-
lent and timing of insemination is not controlled
by receptivity of the female. It is possible that
chromosome abnormalities could play a significant
role in reproductive failure in the bovine where
artificial insemination is widely practiced.

906