Transplantation of Living Nuclei of Late Gastrulae into Enucleated Eggs of Rana pipiens

I N previous papers it has been shown that living nuclei of animal hemisphere cells of frog blastulae and early gastrulae can be transferred into enucleated eggs (Briggs & King, 1952,1953a). Approximately 20 per cent, to 40 per cent, of the recipient eggs cleave normally, and the majority of these develop into complete embryos, demonstrating that the transplanted nuclei are undifferentiated (i.e. still capable of participating in all types of differentiation). This result is more or less expected since the nuclei in question come from undifferentiated parts of the blastula and early gastrula. The next step in this analysis was to test the nuclei from areas of the late gastrula which are regionally determined, namely, the chorda-mesoderm and the presumptive medullary plate. Embryological evidence indicates that the chorda-mesoderm is determined to differentiate into mesodermal structures, while the overlying presumptive medullary plate has at least a labile determination in the direction of neural differentiation. However, it is not known whether these regional determinations involve stabilized differentiation of the constituent cells. This question will be discussed after we have presented the results of experiments in which nuclei from chorda-mesoderm and presumptive medullary plate were transferred into enucleated eggs. The areas from which nuclei were taken for transplantation are indicated in Text-fig. 1. Donor cells were obtained from the median plane, in a region approximately midway along the longitudinal axis of the embryo. In the case of the chorda we used only cells from the two uppermost layers, i.e. the layers nearest the overlying presumptive medullary plate. Presumptive neural nuclei were provided by the two lower cell-layers of the presumptive medullary plate, these being the layers adjacent to the chorda. In all cases the nuclei were transplanted by means of the technique previously described (Briggs & King, 1952, 1953 a, b). The procedure involves first enucleating the recipient egg, and then transferring into it a single nucleus from a donor cell. The nucleus must be protected by its own cytoplasm in the course of the transfer, which means that donor