The nuclei from four- and eight-cell mouse embryos were transplanted into enucleated two-cell embryos. It was found that such embryos not only developed to the blastocyst stage in vitro (72% and 35%), but also developed to full term (22% and 8%) after transfer to recipient mice. However, development of embryos which contained nuclei from the inner cell mass was not observed. Since the development of enucleated zygotes which contain advanced nuclei is limited (the present study; McGrath and Solter: Science, 226:1317-1319, '84; Robl, Gilligan, Critser, and First: Biol. Reprod., 34:733-739, '86), it appears that cytoplasmic factors are important for the development of nuclei from advanced cells.

A new and reliable pronuclear transplantation procedure for the mouse egg has been developed by McGrath and Solter ('83). To overcome the technical difficulties of such a procedure, especially in uniformly preparing enucleation pipettes and in reducing damages during micromanipulation, we have examined the effect of cutting the zona pellucida of the eggs. By making a slit in the zona of an egg, the time for pipetting and exchange of pronuclei between eggs was shortened because the sharp tip of the pipette was not necessary. Although the proportion of pregnant recipients and young obtained after transfer of pronuclear transplanted eggs cultured for 1 day or 3 days was quite low, it was significantly increased (70% for pregnancy rate and 32% for the young) following transfer of eggs cultured for 4 days. These values were comparable with those after transfer of unoperated eggs cultured to morulae and blastocysts.

Summary. The

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