94, 92, 100, and 103 mm. These sizes corresponded to ovulational ages of approximately 8-13 weeks \[[@B9]\].

All the specimens were part of the large collection kept at the Embryology Institute of the Universidad Complutense, Madrid, and were the products of miscarriages and ectopic pregnancies managed at the Department of Obstetrics of the University. Approval for the study was granted by the university ethics committee (approval number, B-08/374). Because of the nature of the specimens, we were unable to rule out the possibility that they contained pathology. However, no pathology was found in the developing umbilical vessels, liver, intestine, adrenal, and kidney in the specimens examined. Identification of gender was based on observations of the urogenital fold, including the primitive uterus.

After routine procedures for paraffin-embedded histology, most of theadult-like course of the urethra as well as the striated muscle extending superiorly along the prostate. Thus, the change may occur at a stage slightly later than the present specimens. In contrast, a well-known diagram of the female perineum by Oelrich \[[@B6]\] was based on a 27-year-old woman, and not on observations of fetuses. However, Kato et al. \[[@B8]\] described the urethrovaginal sphincter at 20 weeks (CRL, \>180 mm). During the hypothetical topographical change at the late stage, mechanical stress is likely to be responsible for differentiation of the membranous structure between the crus and the striated sphincter (see above) into the male urogenital diaphragm or the female perineal membrane. Subsequently, the female urethral striated muscle seems to extend posterolaterally along the superficial surface of the developed perineal