The Questions of Developmental Biology

17. Sex determination
How an individual's sex is determined has been one of the great questions of embryology
since antiquity. Aristotle, who collected and dissected embryos, claimed that sex was determined
by the heat of the male partner during intercourse. The more heated the passion, the greater the
probability of male offspring. (Aristotle counseled elderly men to conceive in the summer if they
wished to have male heirs.) Aristotle (ca. 335 b.c.e.) promulgated a very straightforward
hypothesis of sex determination: women were men whose development was arrested too early.
The female was "a mutilated male" whose development had stopped because the coldness of the
mother's womb overcame the heat of the father's semen. Women were therefore colder and more
passive than men, and female sexual organs had not matured to the point at which they could
provide active seeds. This view was accepted by the Christian Church and by Galen (whose
anatomy texts were to be the standard for over a thousand years). Around the year 200 c.e., Galen
wrote:
Just as mankind is the most perfect of
all animals, so within mankind, the
man is more perfect than the woman,
and the reason for this perfection is
his excess heat, for heat is Nature's
primary instrument . . . the woman is
less perfect than the man in respect to
the generative parts. For the parts
were formed within her when she was
still a fetus, but could not because of
the defect in heat emerge and project
on the outside.
The view that women were but poorly developed men and that their genitalia were like
men's, only turned inside out, was a very popular one for over a thousand years. As late as 1543,
Andreas Vesalius, the Paduan anatomist who overturned much of Galen's anatomy (and who
risked censure by the church for arguing that men and women have the same number of ribs),
held this view. The illustrations from his two major works, De Humani Corporis Fabrica and
Tabulae Sex, show that he saw the female genitalia as internal representations of the male
genitalia (Figure 17.1). Nevertheless, Vesalius' books sparked a revolution in anatomy, and by the
end of the 1500s, anatomists had dismissed Galen's representation of female anatomy. During the
1600s and 1700s, females were seen as producing eggs that could transmit parental traits, and the
physiology of the sex organs began to be studied. Still, there was no consensus about how the
sexes became determined (see Horowitz 1976; Tuana 1988; Schiebinger 1989).
Until the twentieth century, the environment temperature and nutrition, in particular
was believed to be important in determining sex. In 1890, Geddes and Thomson summarized all
available data on sex determination and came to the conclusion that the "constitution, age,
nutrition, and environment of the parents must be especially considered" in any such analysis.
They argued that factors favoring the storage of energy and nutrients predisposed one to have
female offspring, whereas factors favoring the utilization of energy and nutrients influenced one
to have male offspring.