[Chronology of development of the genital tract of the calf fetus].

The chronology and the modalities of the differentiation of the genital apparatus were studied in 187 calf fetuses whose insemination age was exactly known (between 32 and 110 days). Male. In the male, the first seminiferous cords form around days 41-42, the interstitial cells appear 2 or 3 days later. From days 60-70 on, the rete testis is made of tubules with an open lumen, which connect the seminiferous cords. The masculinisation of the external genitalia begins as early as day 47 by a rapid increase of the anogenital distance: on day 60, the penis opens under the umbilicus and the scrotum is well differentiated. The regression of the Müllerian ducts starts at the level of their anterior (tubal) part on day 50, when their diameter decreases. From day 58 on, the oviducts become discontinuous and they are almost completely absent by day 63; the uterine horns and the vagina have dissappeared by day 80. The masculinization of the internal genitalia occurs during two phases: 1) between days 56 and 58: the early buds of the seminal vesicles and of the prostate appear, as well as the first differences at the level of the urogenital connections and of the Cowper's glands; a supra-urethral diverticulum develops at the level of the posterior prostatic urethral flexure; the non sexual urethra remains short. 2) After day 70 take place: the differentiation of the epididymides the branching of the seminal vesicles and the stabilization of the Wolffian ducts (after a transitory diminution of their diameter between days 60 and 80). Female. In the female, sexual organogenesis proceeds later than in males. The Müllerian ducts (which show a transitory reduction of their diameter at their tubal level between days 50 and 60) develop steadily after day 60 at the level of their uterine horns and of the vagina; simultaneously the non sexual urethra lengthens rapidly. A suburethral diverticulum, which is absent in the male, develops at the level of the urogenital connections, between days 60-70. The regression of the Wolffian ducts takes place after day 70 when the mesonephros disappear; they first become discontinuous in their median part (at the uterine level) between days 77-80, but important remains of these ducts still persist on day 110, in the posterior part of the vagina. The first primordial ovarian follicles form only after day 100 approximately.