The ultrastructure of germ cells in foetal and neonatal male rats
The Gonads of the rat undergo sex differentiation on the 14th day post coitum (p.c.), the testis becoming clearly distinguishable by the presence of an incipient tunica albuginea. The male germ cells become incorporated into medullary cords (the precursors of seminiferous tubules). In contrast, the germ cells in ovaries are scattered in cortical nests. Recent quantitative studies have shown that at 14·5 days p.c., the number of germ cells is somewhat greater in the male than the female (Beaumont & Mandl, 1963; cf. Beaumont & Mandl, 1962). In both sexes mitotic activity ceases at about 18·5 days p.c. Thereafter, the male germ cells remain at prolonged interphase; a proportion of them show histological changes frequently associated with degeneration. Quantitative estimates, on the other hand, indicate that none are eliminated from the testis. In the coeval female, the germ cells enter the prophase of meiosis, whereafter no further mitotic divisions are possible.