Results

Day 15 of gestation: In the midline between the umbilical cord and the tail, there was a cone-like genital tubercle forwarding cephalad. The genital tubercle had an undercut groove on its ventral surface. The urogenital fold was situated on either side of this groove. Somewhat caudally to the lateral side of each genital fold, there was a vaguely outlined elevation known as the genital swelling (Fig. 1) Day 16 of gestation: The genital swelling on either side approached the midline between the urogenital folds and the anus. The genital tubercle and urogenital folds were almost the same in situation as above. Either sex of fetuses was still not distinguishable from the outer view of the external genitalia (Fig. 2). Day 17 of gestation: The genital tubercle turned to be spherical. The both genital swellings as well as the both urogenital folds were fused with each other in the midline just caudally to the genital tubercle. At this stage, the sex difference was noted from the outer view of the external genitalia; a small oval opening of the urogenital orifice appeared in the midline between the urogenital folds and the genital swellings, seemingly larger in male than in female. The male genital swellings were enlarged with the increase in the anogenital distance, but the female ones were little developed (Figs. 3 and 4).

Day 18 of gestation: Sex difference became clearer at this stage. In male, the genital swellings were more elevated, making the small urogenital orifice stretched longitudinally as the urethral groove. In contrast, in female, the urogenital orifice remained small in situ until the end of gestation (Figs. 5 and 6). Day 19 of gestation: The genital tubercle rose up in both sexes. The urogenital folds gradually ensheathed the genital tubercle. The urethral groove in male observed clearly in this stage (Figs. 7 and 8).