"You know, I just came across a medical article, which I know will interest you. It seems that men are not of course all male, as women are not all women. Well, the proof that men are not all male goes something like this: remove the ovaries of a female mouse. Let the gal recover. Then take a pint of male piss. Extract it with alcohol or acetone or ether or what not — I've forgotten. Do this again. Then again. Finally evaporate the stuff down until it is a thin jelly. Of this jelly take a tenth of a cubic centimeter and inject it into the muscle of said mouse. The mouse starts to menstruate!
"Now this is due to the fact that men excrete female hormones in their urine. Ain't that something. So always take a piss before or after you jerk off or you might end some day by finding yourself pregnant — with a castrated mouse perhaps.
"Anyhow this is all true."
(letter to Kenneth Burke, July 29, 1931, quoted in East, The Humane Particulars: The Collected Letters of William Carlos Williams and Kenneth Burke, 53)