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The effects of the oestrogens and progestins in some common oral contraceptives on the cleavage of fertilized rabbit ova were studied in vitro. Three concentrations (5, 10 and 25 µg./ml.) of each of four progestins (norethynodrel, norethindrone, norethindrone acetate and 17-hydroxy-6-methylprogesterone acetate) and of two oestrogens (mestranol and ethinyloestradiol) were tested. All but norethindrone and norethynodrel had some effects on the cleavage of ova. Three effects were seen: complete inhibition of cleavage, slowing or retardation of cleavage, and fragmentation (division of the egg into numerous spherical or ovular bodies).

Differences between conditions in vivo and in vitro and the mechanism of these actions of the steroids are discussed.