But when the semen from the male (in those animals which emit semen) has entered, it puts into form the purest part of the female residue (for the greater part of the menstrual flow is useless, being fluid, as is the most fluid part of the male secretion, ie in a single emission, the earlier discharge being in most cases apt to be infertile rather than the later, having less vital heat through want of concoction, whereas that which is concocted is thick and of a more material nature).