Premeiosis and premeiotic DNA synthesis in the left ovary of the female chick embryo.

Premeiotic DNA synthesis in the germ cells of the female mouse embryo has been studied by Peters, Levy & Crone (1962) and Crone, Levy & Peters (1965). In this species, with a close synchronization in germinal development, the process of oogenesis which is the period of multiplication by mitotic divisions of the germ cells (oogonia) seems to be followed rapidly (within 24 h) by the first step of the prophase of meiosis (see Borum, 1961). In previous work (Callebaut & Dubois, 1965; Callebaut, 1967) we have investigated DNA synthesis in the ovarian germ cells of the chick embryo, both in vitro and in vivo, by autoradiography following the incorporation of tritiated thymidine. In a new series of experiments with a modified technique, it has been possible to demonstrate that the nuclei of the germ cells in premeiotic S phase in the female chick embryo have a distinctive structure.