But in my opinion we can now be assured sufficiently that no animals, however small they may be, take their origin in putrefaction, but exclusively in procreation... For seeing that animals, from the largest down to the little despised animal, the flea, have animalcules in their semen, seeing also that some of the vessels of the lungs of horses and cows consist of rings and that these rings can occur on the flea's veins, why cannot we come to the conclusion that as well as the male sperm of that large animal the horse and similar animals, and of all manner of little animals, the flea included, is furnished with animalcules (and other intestines, for I have often been astonished when I beheld the numerous vessels in a flea), why, I say should not the male sperm of the smallest animals, smaller than a flea may even the very smallest animalcules have the perfection that we find in a flea.

I have divers times examined the same matter (human semen) from a healthy man... not from a sick man... nor spoiled by keeping... for a long time and not liquefied after the lapse of some time... but immediately after ejaculation before six beats of the pulse had intervened; and I have seen so great a number of living animalcules... in it, that sometimes more than a thousand were moving about in an amount of material the size of a grain of sand... I saw this vast number of animalcules not all through the semen, but only in the liquid matter adhering to the thicker part.