Indifferent anlagen of sex differentiation

The gonads (1), the efferent ducts (mesonephric
and paramesonephric) (2), and the external
genitalia (3) all develop from an indifferent
stage. At about the end of the sixth week of
pregnancy in humans, after the primordial
germ cells of the embryo have migrated into the
initially undifferentiated gonads, an inner portion
(medulla) and an outer portion (cortex) of
the gonads can be distinguished. When a normal
Y chromosome is present, early embryonic
testes develop at about the 10th week of pregnancy
under the influence of a testis-determining
factor (TDF). If a normal Y or TDF (SRY) is not
present, ovaries develop. The wolffian ducts,
the precursors of the male efferent ducts (vas
deferens, seminal vesicles, and prostate),
develop under the influence of testosterone, a
male steroid hormone formed in the fetal testis.
At the same time, the müllerian ducts—precursors
of the fallopian tubes, the uterus, and the
upper vagina—are suppressed by a hormone,
the Müllerian Inhibition Factor (MIF; also
known as anti-müllerian hormone, AMH).
When testosterone is absent or ineffective, the
wolffian ducts degenerate. The müllerian ducts
develop under the influence of estradiol, a hormone
produced by the fetal ovaries. The external
genitalia (3) in humans do not develop until
relatively late, starting in the 15th to 16thweek.
Full development of male external genitalia depends
on a derivative of male-inducing testosterone,
5-dihydrotestosterone, a metabolite of
testosterone produced by the enzymatic action
of 5!-reductase.