The fertilization of the mature ovum—maturation having occurred within the ovarian follicle before its rupture—has been shown by numerous researches on the ova of other animals to consist in the fusion of the male and the female nuclear substance; and it appears that of the enormous number of spermatozoa, estimated by _Lode_ at 226 million at a single ejaculation, that enter the female genital passage, but a single one penetrates the ovum. Towards the head of this spermatozoön there extends from the surface of the ovum a process, flat at first, but becoming more and more prominent, until it surrounds the head, and fuses with it. The motile tail of the spermatozoön disappears, whilst the head, which has now passed through the vitelline membrane and entered the ovum, assumes the appearance of a nucleus, and is called the _male pro-nucleus_. The original nucleus of the ovum has previously prepared itself for fertilization by the extrusion through the vitelline membrane of portions of its substance (known as _polar globules_), and now constitutes the _female pro-nucleus_. Towards this latter, situated somewhere near the centre of the cell, the male pro-nucleus continues to move, the vitelline granules meanwhile being disposed round about it in radiating lines, forming a star-shaped figure. Having come into contact, the two pronuclei fuse completely to form a new nucleus, the nucleus of the now fertilized egg-cell. The result of fertilization is the formation of the first _segmentation-sphere_, from which, by further subdivision, the new individual is formed. Thus is effected that which _Hippocrates_ describes in the words: “The seed possessed both by man and by woman, flow together from all parts of the body; the fruit is formed by the mingling of the two seeds.” [Illustration: FIG. 55A.—First Stage. FIG. 55B.—Second Stage. Entrance of a spermatozoon into the ovum of ascaris megalocephala. After preparations by M. Nussbaum. (Half of the ova only are depicted.) ] [Illustration: FIG. 56.—Ovum of Asterakanthion ten minutes after fertilization. ] [Illustration: FIG. 57.—Fusion of male pro-nucleus and female pro-nucleus to form the segmentation nucleus of the fertilized ovum. ] The most favourable period for the occurrence of fertilization appears to be when intercourse takes places from eight to ten days after the termination of the menstrual flow. In 248 instances in which the date of the fruitful coitus was exactly known, it was ascertained by Hasler that in 82½ per cent. of all cases, conception was effected in the fourteen days succeeding the menstrual period. In general it may be stated that the theory of the periodicity of ovulation and of the causal relation of this process to menstruation, has not been shaken by the result of researches recently undertaken by opponents of that theory; hence it appears that the fertilized ovum is the ovum of the last completed menstruation.