ABSTRACT
During normal development of the mouse embryo, primordial germ cells (PGCs) differentiate in the root of the allantois and in the hind region of the embryo, then pass to the hind gut and through the mesentery to reach the germinal ridges (Chiquoine, 1954; Bennett, 1956; Mintz & Russell, 1957; Mintz, 1957; Ożdżeński, 1967). The number of PGCs increases greatly during migration (Chiquoine, 1954; Mintz & Russell, 1957). After penetrating into the gonads PGCs continue to divide mitotically for a certain time, then undergo changes which differ in each sex: the meiotic prophase begins in the ovaries (Brambell, 1927; Borum, 1961), gonial divisions are arrested in the testes and the chromatin in the nuclei of the germ cells undergoes characteristic dispersion. The behaviour of germ cells in the male embryos of the mouse is similar to that described by Clermont & Perey (1957) and Beaumont & Mandl (1963) in the rat.