DE SECRET. MUL.

behoves it to subenter the matter of the fetus and the substance, because it seems that the fetus sometimes is assimilated to the father in the genitals and in many other things; this would not be so unless it entered the substance of the fetus. Furthermore, physicians say that in the man's semen there is a certain generating spirit that penetrates the whole matter of the joined seeds, and that spirit is the formative principle of all members. Just as a smith disposes iron with a hammer, so that spirit disposes and softens all the members, and through that spirit the efficient principle is saved. But philosophers posit that the man's sperm exhales vaporably, because the womb is a very porous member, and thus after the formation of the fetus, the heat of the sun causes the man's sperm to exhale and recede from the womb through the pores it uses for nourishment. It is also evident, because it is a skin, and every skin in an animal is porous, because otherwise sweat could not exit without pain, which is false; because in heat the pores open, and then the sweat exits, and then also hairs exit through the pores. Note that an embryo is a certain fleshy mass, gathered from these seeds, and that fleshy mass is altered for the formation of the fetus.

These things being seen and one part of the opinion of Aristotle or the physicians being accepted, it must be seen by what manner and how those seeds are received in the woman. Whence, when a woman has been in intercourse with a man, she similarly emits at the same time the menstrual