Among 372 spontaneous abortions four had chromosome anomalies that were unique in the series. Each one presented features of interest. There was one example of monosomy‐G and this was associated with apparently normal embryonic development to about the 28‐day stage. In two specimens extra chromosome material was found. In one instance it was added to the long arm of a B‐group chromosome in 44% of cells. In the other the extra material was an abnormal element of unknown origin. The fourth specimen was a 35‐mm fetus with cyclopia and D‐trisomy associated with a D/D translocation. The woman producing this abortus was a carrier of the same translocation. The literature relevant to these four unusual cases was reviewed.

The chromosome complement and anatomical development of 10-16-day rabbit embryos were studied following delayed fertilization. Preimplantation loss was 40% in the experimental group compared with 17% in control animals. Also, a greater number of embryos retarded in growth or with structural anomalies was seen in the delayed-mated group. Cytogenetic analysis revealed a trisomy for a small metacentric chromosome in 104 control embryos. Only 1 triploid embryo was found in 84 implantation sites from rabbits that had been delayed mated. The incidence of triploidy postimplantation was much less than the 13% found previously in 6-day blastocyts. Increased embryonic loss at the time of or shortly after implantation may be responsible for the discrepancy.