Further, the blood-vessels of women stand out less than those of men, and women are rounder and smoother because the secretion which in men goes to these vessels is drained away with the catamenia.We must suppose, too, that the same cause accounts for the fact that the bulk of the body is smaller in females than in males among the vivipara, since this is the only class in which the catamenia are discharged from the body.And in this class the fact is clearest in women, for the discharge is greater in women than in the other animals.Wherefore her pallor and the absence of prominent blood-vessels is most conspicuous, and the deficient development of her body compared with a man's is obvious.
Now since this is what corresponds in the female to the semen in the male, and since it is not possible that two such discharges should be found together, it is plain that the female does not contribute semen to the generation of the offspring.For if she had semen she would not have the catamenia; but, as it is, because she has the latter she has not the former.
It has been stated then that the catamenia are a secretion as the semen is, and confirmation of this view may be drawn from some of the phenomena of animals.For fat creatures produce less semen than lean ones, as observed before.The reason is that fat also, like semen, is a secretion, is in fact concocted blood, only not concocted in the same way as the semen.Thus, if the secretion is consumed to form fat the semen is naturally deficient.And so among the bloodless animals the cephalopoda and crustacea are in best condition about the time of producing eggs, for, because they are bloodless and no fat is formed in them, that which is analogous in them to fat is at that season drawn off to form the spermatic secretion.
And a proof that the female does not emit similar semen to the male, and that the offspring is not formed by a mixture of both, as some say, is that often the female conceives without the sensation of pleasure in intercourse, and if again the pleasure is experience by her no less than by the male and the two sexes reach their goal together, yet often no conception takes place unless the liquid of the so-called catamenia is present in a right proportion.Hence the female does not produce young if the catamenia are absent altogether, nor often when, they being present, the efflux still continues; but she does so after the purgation.For in the one case she has not the nutriment or material from which the foetus can be framed by the power coming from the male and inherent in the semen, and in the other it is washed away with the catamenia because of their abundance.But when after their occurrence the greater part has been evacuated, the remainder is formed into a foetus.Cases of conception when the catamenia do not occur at all, or of conception during their discharge instead of after it, are due to the fact that in the former instance there is only so much liquid to begin with as remains behind after the discharge in fertile women, and no greater quantity is secreted so as to come away from the body, while in the latter instance the mouth of the uterus closes after the discharge.When, therefore, the quantity already expelled from the body is great but the discharge still continues, only not on such a scale as to wash away the semen, then it is that conception accompanies coition.Nor is it at all strange that the catamenia should still continue after conception (for even after it they recur to some extent, but are scanty and do not last during all the period of gestation; this, however, is a morbid phenomenon, wherefore it is found only in a few cases and then seldom, whereas it is that which happens as a regular thing that is according to Nature).
It is clear then that the female contributes the material for generation, and that this is in the substance of the catamenia, and that they are a secretion.