A METHOD of testing the potentialities of nuclei from embryonic cells has been described by Briggs and King¹. The method consists of transferring a nucleus from an embryonic cell into an enucleated and unfertilized egg of the same species. King and Briggs², who have performed their experiments on Rana pipiens, found that normal tadpoles resulted from eggs with transplanted nuclei in about 35 per cent of cases in which the nuclei were taken from blastulæ, but in only about 6 per cent of cases in which the nuclei were taken from late gastrula endoderm; they have not reported normal development from nuclei of post-neurulæ or later stages.