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Whitten (1956) showed that 8-cell mouse embryos develop into blastocysts in a simple chemically defined medium containing glucose, and McLaren & Biggers (1958) demonstrated that blastocysts cultured in this way produce normal mice when transferred into uteri of foster mothers. Whitten (1957) also showed that late 2-cell mouse embryos developed into blastocysts if lactate was incorporated in the medium but earlier stages did not cleave under these conditions. Thus the observation that mouse zygotes develop into normal blastocysts in the lumen of oviducts in organ cultures (Biggers, Gwatkin & Brinster, 1962) suggested that special conditions for initial development are provided by the tube. Recently, Whittingham & Biggers (1967) demonstrated that 1-cell embryos cleave to the 2-cell stage in a simple medium containing lactate and
Whitten et al. (Fri,) studied this question.