Transplantation of living nuclei from blastula cells into enucleated frogs’ eggs

The role of the nucleus in embryonic differentiation has been the subject of investigations dating back to the beginnings of experimental embryology. At first it was supposed by Roux, Weismann and others that differentiation is the result of qualitative nuclear divisions, different blastomeres thereby receiving the different kinds of nuclei which determine their subsequent differentiation. Later on this theory was disproved by numerous experiments showing that, during early cleavage at least, the distribution of the nuclei can be changed at will without altering the pattern of development. The cleavage nuclei have, therefore, been regarded as identical, and differentiation has been ascribed primarily to the well-known localizations in the egg cytoplasm. This evidence, it should be emphasized, relates only to the early phases of development. During this time it is definitely true that the nuclei in the various blastomeres are equivalent. However, whether they remain equivalent or become differentiated as the various parts of the embryo differentiate has never been tested. The possibility that nuclei might differentiate in response to regional differences in the cytoplasm, and that such nuclear changes might have reciprocal effects on the cytoplasm during cell differentiation, was suggested by Morgan.1 More recently Schultz2– 4 has discussed the problem more fully, indicating the known cytogenetical mechanisms that could account for nuclear differentiation, and Weisz5 has reviewed it in relation to ciliate morphogenesis. Obviously this problem can be solved only by the development of a method for testing directly whether nuclei of differentiating embryonic cells are or are not themselves differentiated. This sort of test could be obtained, as suggested to us several years ago by Schultz, if it were possible to transplant nuclei. Ideally, this type of experiment should be carried out by transplanting the nucleus …