tion of the embryo is male, the gonadal ridge
begins to produce TESTOSTERONEand a hormone
unique to embryonic development, müllerian-
inhibiting hormone. In response to testosterone
the inner layer, the medulla, begins to take shape
as the testes and the outer layer, the cortex,
degenerates. The genital tubercle becomes the
glans PENIS, the urogenital groove and folds
enlarge to form the rest of the penis, and the
labioscrotal swellings fuse to form the SCROTUM.
If the embryo’s genetic composition is female
there is no secretion of testosterone and müller-
ian-inhibiting hormone. So instead the outer
layer, the cortex, begins to develop into the
ovaries and the medulla deteriorates. The genital
tubercle becomes the CLITORIS, the urogenital
groove and folds become the VAGINA and labia
minora, and the labioscrotal swellings form the
labia majora. Primordial germ cells from the yolk
sac migrate into the evolving testes or ovaries to
become gametes (ova or sperm) as development
continues. By 12 weeks gender differentiation is
complete, though gender does not become
detectable with ULTRASOUNDimaging until the 20th
week or later.
THE MYTHOLOGY OF THE HYMEN
The hymen, a narrow ring of membranous tissue
that extends across the opening of the VAGINA,
derives from the Greek god of the wedding feast,
Hymen (also Hymenaeus). Hymen was the prog-
eny of Dionysus, the god of FERTILITY, and
Aphrodite, the goddess of love. Though the belief
persists today that an intact hymen is evidence of
a woman’s virginity, in truth numerous activities
(tampon use, horseback riding, bicycling, and
gymnastics to name a few) can rupture the
hymen. As well, the structure of the hymen varies
widely among women and may be so insignifi-
cant as to not impede the penetration of the erect
PENIS.Transition to fertility: puberty Despite genital
differences, boys and girls are fairly much alike
physiologically for a dozen years or so after birth.
Then hormonal signals trigger the onset of
puberty, the transition from childhood to sexual
and reproductive maturity. Though researchers do
not know what activates the hormonal signals, the
consequences are very familiar: the emergence of
SECONDARY SEXUAL CHARACTERISTICS. The ovaries and
testicles again become active, initiating the
anatomic and physiologic changes that transform
boys to men and girls to women. With sexual and
reproductive maturity complete, SEXUAL INTER-
COURSEand pregnancy become possible.
New life: conception, pregnancy, and childbirth
On the surface of it, reproduction is an astonish-
ingly simple premise, and its organs uniquely
suited to its purpose. During sexual intercourse
the erect penis fits precisely within the vagina,
reaching to the CERVIX. EJACULATIONdeposits mil-
lions of sperm in the upper vagina, a short swim
from the cervical os (opening through the cervix
into the UTERUS). When conditions and timing are
right, the sperm make their way through the
cervix and uterus and into one of the FALLOPIAN
TUBES, encounter the ovum (egg), and one of the
millions penetrates the ovum to fertilize it. The
resulting ZYGOTEtravels down the fallopian tube,
tumbles into the uterus, and implants itself into
the dense, spongy endometrium: CONCEPTION.
The woman’s body nourishes and shelters the
developing FETUS, expanding and changing to
accommodate its needs. The uterus stretches up to
10 times its normal size, pushing the abdominal
wall outward. Again it is hormones that facilitate
and support these processes, and hormones that
bring the pregnancy to its conclusion: childbirth.
In the process of it, of course, there is nothing
simple about any dimension of reproduction.
Reproduction represents one of the most intri-
cately choreographed experiences the human
body can accommodate. Hundreds of hormones
direct countless interactions, each of which spurs
other events. Numerous factors, internal and
external, influence reproduction to make it possi-
ble or not possible. Among the most significant
advances in reproductive medicine are technolo-
gies to assist the process at various points along
the reproductive continuum, from fertilization
through childbirth.
Completing the cycle: menopause Men remain
fertile nearly the rest of their lives after puberty,
though sperm production and quality tend to
diminish in later life as testosterone levels
decrease. FERTILITY ends for women with
MENOPAUSE, the cessation of OVULATIONand men-236 The Reproductive System