ABSTRACT

Stowers Institute for Medical Research, 1000 East 50th Street, Kansas City, Missouri 641102262, USA.

The propagation of life on earth fascinated people long before they understood the processes of reproduction. Personal experience along with observation of domestic species indicated that gender played a critical role. In human beings and other mammals, females gestated developing fetuses and eventually gave birth to immature offspring, but this did not happen in the absence of intact males. Similarly in domestic fowl, reliable production of eggs capable of embryonic development into viable hatchlings required a fertile male. More than two millennia ago, Aristotle emphasized the essential role of both sexes, male and female, in the reproduction of higher animals. He wrote:

“Speaking generally, however, we may say that in the case of all those animals which have the power of locomotion, whether they are adapted to be swimmers, or fl iers, or walkers, male and female are found; and this applies not only to the blooded animals but to some of the bloodless ones as well.”