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In Drosophila melanogaster the primordial germ cells are normally formed at the posterior tip of the egg during the preblastoderm stage. In order to determine whether the posterior polar plasm is capable of inducing the formation of primordial germ cells in another region of the embryo, portions of this cytoplasm were transferred from wild-type embryos of the early cleavage stage to the anterior tip of mwh e embryos of the same age. At various times after the injection (15-200 min), embryos were fixed for histological analysis. Alternating thick and thin sections were examined for the presence of experimentally induced pole cells. In more than half of the embryos analyzed in this way, one to six cells were found containing the polar granules as well as round nuclear structures, both of which are characteristic of normal pole cells and are not present in blastoderm cells. In order to determine whether these "pole cells" function normally, i.e., develop further into germ cells, the cells induced at the anterior tip of mwh e blastoderm embryos were introduced into the posterior region of y w sn(3) hosts of the same age. The flies resulting from these embryos were mated to y w sn(3) partners. In addition to the expected y w sn(3) progeny, wild-type flies heterozygous for mwh e and, therefore, descended from the experimentally induced pole cells were found in 4% of the crosses. Such flies did not appear in the control experiments after transfer of normal anterior cells from noninjected blastoderm embryos. These results demonstrate that the posterior polar plasm can be transferred to the anterior tip of the embryo and that in this presumptive somatic region it still retains its capacity to determine the formation of the primordial germ cells.

Transplantation of Posterior Polar Plasm in Drosophila . Induction of Germ Cells at the Anterior Pole of the Egg