Printed in Great Britain Experimental studies on the organization of the preimplantation mouse embryo I. Fusion of asynchronously cleaving eggs

The interaction between mouse eggs conjoined in pairs of different ages and stages of development has been examined: 1. Forty-two per cent of chimaeras formed from 8-cell eggs paired with late morulae or early blastocysts produced morphologically normal, but large, blastocysts. 2. Fusion of pairs of vitally labelled and unlabelled eggs has shown that presumptive trophoblast derived from as late as the early blastocyst stage can become incorporated into the inner cell mass of chimaeras. These observations suggest that the preimplantation chimaeric embryo can regulate for chronological differences in its constituent cells and that the trophoblast up to the early blasto-cyst stage may still be developmentally labile