Already in the writings of the old Indian physician _Susruta_, we find expression of the view that the period that immediately succeeds the cessation of the menstrual flow is one most favourable to conception. “The time of generation,” he says, “is the twelfth night after the commencement of menstruation.” In the Jewish _Talmud_, the day before the onset of menstruation, and the days immediately succeeding the cessation of the flow, are indicated as those most favourable to the occurrence of conception; moreover, in the _Talmud_, notwithstanding the fact that intercourse during menstruation is prohibited on pain of death, and that coitus is not regarded as permissible until the lapse of twelve clear days after the cessation of the flow, nevertheless the assertion is made that intercourse during menstruation may lead to conception. [Illustration: FIG. 58. —Passage of spermatozoon through the zona pellucida of the ovum of asterakanthion. ] [Illustration: FIG. 59.—Ovum of scorpæna scrofa thirty-five minutes after fertilization. ] [Illustration: FIG. 60.—Male pro-nucleus and female pro-nucleus in fertilized ovum of frog, prior to the formation of the segmentation nucleus. ] _Hippocrates_ writes: _Hae nempe post menstruam purgationem utero concipat_. _Aristotle_ says: _Plerasque post mensum fluxum nonnullas vero fluentibus adhuc menstruis_. _Galen_ writes: _Hoc autem conceptionis tempus est vel incipientibus vel cessantibus menstruis_. _Soranus_ writes to a similar effect: Just as the soil is suitable only at certain seasons for the reception of the seed, so also in the human race intercourse does not always take place at a time suited for the reception of the semen. To be effective, coitus must occur at the proper time.... The act of intercourse that is to lead to conception may best occur either just before or just after the menstrual flow, when, moreover, there is strong desire for the sexual embrace, and neither when the body is fasting, nor when it is full of drink and undigested food. The time before menstruation is, however, unsuitable, for then the womb is heavy from the flow of blood, and two conflicting tendencies will come into operation, one for the absorption of material and the other for its outflow. During menstruation, again, conception is unlikely to occur, for then the semen is wetted and washed away by the flowing blood. The sole proper time is that immediately after the flow, when the womb has freed itself from its humours, and warmth and moisture stand in harmonious relationship.