Mouse embryos developed from the eight-cell stage to expanded blastocysts in a defined medium in which glycine was the only source of fixed nitrogen (Whitten, 1957). These findings were confirmed by Bunim (1960), who also showed that glycine could be replaced by Versene and suggested that the function of the glycine was not for protein synthesis but to chelate ions of copper or zinc which might have contaminated the medium. However, Brinster (1965) found that only an occasional two-cell embryo developed in the presence of single amino acids, though he subsequently demonstrated that 80% formed blastocysts when glutathione was the only source of fixed nitrogen (Brinster, 1968).
Incorporation of labelled leucine by early mouse embryos in vitro was reported by Mintz (1964) and by Monesi & Salfi (1967) but neither Greenwald & Everett (1959), nor Weitlauf & Greenwald (1967), could demonstrate uptake of methionine in vivo. It is not clear if this