Matter, which is only a capacity of receiving forms, takes in generation a form like that of the individual which furnishes it; and with respect to the generation of animals that have sexes, he thinks that the male alone furnishes the prolific principle, and that the female affords nothing that can be looked upon as such.[BM] For though he says elsewhere, speaking of animals in general, that the female emits a seminal fluid within herself, yet he does not regard that as a prolific principle: nevertheless, according to him, the menstrual blood serves for the formation, growth, and nutriment of the foetus, but the efficient principles exist only in the seminal fluid of the male, which does not act like matter, but as the cause. Averrhois, Avicenna, and other philosophers, who followed the sentiments of Aristotle, have sought for reasons to prove that females have no prolific fluid; they urge, that as females have a menstrual fluid that was necessary and sufficient for generation, it does not appear natural to suppose they possess any other; particularly because it begins to appear, like the seminal fluid in the males, at the age of puberty; besides, continue they, if females have really a seminal and prolific fluid, why do they not produce without the approach of the male, since they contain the prolific principle as well as the matter necessary for the nutriment and growth of the embryo? This last reason seems to be the only one which merits any attention. The menstrual blood seems to be necessary for the support, nutriment, and growth of the foetus, but it can have no part in the first formation, which is made by the mixture of two fluids alike prolific. Females therefore may have, as well as the males, a prolific fluid for the formation of the embryo, besides the menstrual blood for its nutriment and expansion; and certainly a female being possessed of a prolific fluid, extracted from all parts of her body, as well as the necessary means of nourishment and expansion, it is no impossible imagination that she would produce females without any communication with the male. It must be allowed, that this metaphysical reasoning which the Aristotelians adopt to prove that females have no prolific fluid, may become the most considerable objection that can be made against all systems of generation, and particularly against our explanation. [BM] See Aristotle, de gen. lib. i. cap. 20 and lib. xi. cap. 4.