Bundahishn 16

On the nature of generation it says in revelation, that a woman when she comes out from menstruation, during ten days and nights, when they go near unto her, soon becomes pregnant.

When she is cleansed from her menstruation, and when the time for pregnancy has come, always when the seed of the man is the more powerful a son arises from it; when that of the woman is the more powerful, a daughter; when both seeds are equal, twins and triplets.

If the male seed comes the sooner, it adds to the female, and she becomes robust; if the female seed comes the sooner, it becomes blood, and the leanness of the female arises therefrom.

The female seed is cold and moist, and its flow is from the loins, and the colour is white, red, and yellow; and the male seed is hot and dry, its flow is from the brain of the head, and the colour is white and mud-coloured (hasgun).

All the seed of the females which issues beforehand, takes a place within the womb, and the seed of the males will remain above it, and will fill the space of the womb; whatever refrains therefrom becomes blood again, enters into the veins of the females, and at the time any one is born it becomes milk and nourishes him, as all milk arises from the seed of the males, and the blood is that of the females.

These four things, they say, are male, and these female the sky, metal, wind, and fire are male, and are never otherwise; the water, earth, plants, and fish are female, and are never otherwise; the remaining creation consists of male and female.

As regards the fish it says that, at the time of excitement, they go forwards and come back in the water, two and two, the length of a mile (hâsar), which is one-fourth of a league (parasang), in the running water; in that coming and going they then rub their bodies together, and a kind of sweat drops out betwixt them, and both become pregnant.