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Summary. Sex chromatin is present in 67 to 90% of the ectodermal cell nuclei in the female mouse amnion. A similar body is found in 2 to 15% of these nuclei in the male. This criterion is a reliable indicator of the sex of the foetus from the 11th day of gestation onwards. Sex chromatin is found in many of the cell nuclei of females, but is absent from the nuclei of males. In a suitable tissue, the presence or absence of nuclear sex chromatin is a reliable indicator of the genetic sex of the individual. The nuclei of mouse cells usually contain so much heterochromatin that sex chro¬ matin cannot be detected (Moore, 1966); nevertheless, sex chromatin should be present in suitable tissues, because the behaviour of the X chromosomes in the mouse is similar to that of other species. One X chromosome in each cell of the female mouse is genetically inactive (Lyon 1961), replicates its dna late in the S-phase of the cell cycle (Galton 8c Holt, 1965), and is hetero- chromatic in the prophase stage of mitosis (Ohno 8c Hauschka, 1960). These features are associated with the appearance of sex chromatin in other species and, therefore, provide indirect evidence for sex chromatin formation in the mouse. Kenney & Mittwoch (1965) found sex chromatin in cultured fibro- blasts from female mice, but only in 10% of'clear' cells with few chromocentres; these 'clear' cells constituted about half of the total cell population. Bianchi & de Bianchi (1966) and Farias, Kajii & Gardner (1967) sexed rat foetuses by observation of sex chromatin in the amnion; Bianchi & de Bianchi (1966) state, although without further details, that the method can be used in the mouse. The amnion is known to be one of the most favourable tissues for nuclear chromatin analysis: the human female amnion contains 80 to 96% sex chromatin positive nuclei (Klinger, 1966). It therefore seemed worthwhile to examine the mouse amnion to discover firstly whether sex chromatin was visible, and secondly whether it could be used for sexing foetuses. Pregnant mice were killed 10£, 11^, 13^ and 17^ days after ovulation. The amnion was dissected out, transferred in a pipette to a clean, albumen-coated slide, and flattened with a paintbrush and fine forceps; care was needed in handling the amnion from early foetuses. The amnion was fixed in 95% alcohol and coated with celloidin (Geneva Conference, 1966), stained for 15 min in 2% lactic-acetic-orcein, dehydrated and mounted in Canada balsam.

AMNIOTIC SEX CHROMATIN AND FOETAL SEXING IN THE MOUSE