In addition to the above problems with the ‘scale’ of fertility, we have the problem of
the inherent variation in the hen’s response to insemination, in terms of the proportion of
fertile eggs that she subsequently lays. This is, in turn, a function of the number of
spermatozoa which she retains in her oviduct and transfers to the egg at fertilization and
is true not only with respect to individual hens, but with respect to different insemina-
tions made into the same hen within a period of a few days Wishart et al., 1992.. This
means that large numbers of hens must be inseminated to demonstrate significant
differences in the fertilizing ability of two samples of frozen-thawed semen