Recent embryological research has demonstrated conclusively that the concept of the initial anatomical bisexuality or equipotentiality of the embryo is erroneous. All mammalian embryos, male and

female, are anatomically female during the early stages of fetal life. In humans, the differentiation of the male from the female form by the action of fetal androgen begins about the sixth week of embryonic life and is completed by the end of the third month. Female structures develop autonomously without the necessity of hormonal differentiation. If the fetal gonads are removed from a genetic female before the first six weeks, she will develop into a normal female, even undergoing normal pubertal changes if, in the absence of ovaries, exogenous hormones are supplied. If the fetal gonads are similarly removed from a genetic male, he will develop into a female, also undergoing normal female pubertal changes if exogenous hormones are supplied. The probable relationship of the autonomous female anatomy to the evolution of viviparity is described.