Nine phenotypically normal purebred Hampshire and crossbred Hampshire x Yorkshire gilts were mated to purebred Hampshire boars to produce 25-day embryos for chromosomal analysis. Blood samples were drawn from five of these gilts and one of the boars for leucocyte cultures and a comparison was made between karyotypes prepared from the dam with those prepared from her 25-day embryos.

The mean ovulation rate of the nine gilts was 11.8 ± 0.4. A total of 75 embryos were recovered, of which six were degenerating. In addition to the regressing embryos, several sites of complete embryo reabsorption were found. Prenatal losses were calculated to be 35.9%.

The diploid number of 38 chromosomes were found in 85.9% and 90.2% of the cells counted in the leucocyte and embryo cultures, respectively. A normal chromosomal complement was displayed in all cells examined from the leucocyte culture obtained from the boar.

Karyotype analyses showed only one abnormal embryo, which was monosomic for group 16. Other variations in chromosome number were described as the mechanical limitations of the technique used. If chromosomal abnormalities are constant in nature, these results would indicate that most abnormal embryos are unable to survive implantation, which is in agreement with findings of other researchers. Although the monosomic embryo had survived implantation, it is highly unlikely that it would have survived through parturition.