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Chromosomal analyses were made of 463 rabbit blastocysts drawn from thirty-nine superovulated does of four strains. Semen containing 1\m=.\5%diploid spermatozoa was taken from one strain only. Artificial insemination was conducted with untreated semen or with centrifuged fractions containing either 0\m=.\4% or 2\m=.\9%diploid spermatozoa. The results indicate that diploid spermatozoa are not a major cause of triploidy among embryos. The sex ratio scored chromosomally among 434 diploid blastocysts was 48\m=.\62(\m=+-\2\m=.\40)% males. With data in the literature incorporated, the average sex ratio among 1077 blastocysts (consistent over three species of mammal) became 50\m=.\42 (\m=+-\1\m=.\52)% males. These figures may be taken to estimate the primary sex ratio. Sex ratio among blastocysts is extremely stable over various biological and experimental conditions. Twenty-three (5%) of the 463 analysable blastocysts were heteroploid. They comprised eight triploids (5 XXX, 3 XXY), four trisomics, four diploid/trisomic mosaics, one diploid/monosomic mosaic, four diploid/ tetraploid mosaics, one diploid/triploid mosaic and one triploid/ hexaploid mosaic.`C lustering' of heteroploids occurred in particular dams. Superovulation had no demonstrable effect on the incidence of heteroploidy. There is some evidence of genetic (dam strain) effects on the incidence.

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Chromosomal analysis was made from 3744 chick embryos derived from reciprocal matings between parents, one of which was heterozygous for a pericentric inversion, a centric fission, or one of three translocations. The objectives were to determine the types and frequencies of genetically balanced and unbalanced gametes transmitted to early embryos by rearrangement heterozygotes and to ascertain if some gametic types were preferentially produced or utilized. The array of embryos obtained from the heterozygous sires did not deviate significantly from the expected, except in the case of the centric fission group. Among embryos derived from heterozygous dams, however, significant deviations from the expected 1:1 ratio for complementary gametic products, resulting from specific balanced and unbalanced segregation types, were found consistently in all three translocation groups and the centric fission group (P < 0.025). It was concluded that differences exist between heterozygous sires and dams in the frequencies at which some gametic types are produced. The deviations from the expected ratios among progeny of heterozygous dams may be the result of anaphase lagging during the meiotic division of oogenesis.

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