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It rests on that most basic of distinctions between the sexes, the centripetal and centrifugal forces of the universe. In the very nature of the sperm-cell and the germ-cell we find this difference: the one attracts, gathers, draws in; the other repels, scatters, pushes out.
Sir Clifford Allbutt's view, referred to in the following letter, probably had reference to the fact that the sperm-cell goes, or is carried, to the germ-cell, never vice versa. In this letter Darwin gives the reason for the "law" referred to. It is to these articles, as well as to my letters, that Darwin chiefly refers." Down, Bromley, Kent, S.E. April 30, 1868.
Can the fatherless brood be reared to maturity and again made to yield a fatherless generation? What is the precise structure of the nuclei of the cells which originate from the nucleus of the egg-cell only, and not from a nucleus formed by the fusion of that with a sperm-cell nucleus? These and similar questions are the motive of further careful study now in progress.
The two most primary sexual organs are the testis and the ovary; it is the object of conjugation to bring into contact the sperm from the testis with the germ from the ovary. There is no reason to suppose that the germ-cell and the sperm-cell are essentially different from each other.
With regard to the physiology of the female organs, the following topics should be considered: The meaning of puberty as the beginning of a long fertile period of about thirty years; the nature of menstruation as a periodical process preparing the lining of the uterus for reception and attachment of an embryo if a sperm-cell meets a liberated egg-cell near an ovary, and not as a season of illness invented by the powers of darkness; the possibility of fertilization following sexual relations at any time during the fertile life of a woman; the essential facts of sexual relation as a method of depositing sperm-cells so that they can swim on the way to meet an egg-cell; and the nature of the close blood relationship of mother and embryo.
We now know that this process essentially consists simply in the copulation or fusion of two microscopical cells, the female egg-cell and the male sperm-cell. The fusion of the nuclei of these two sexual cells indicates with the utmost precision the exact moment at which the new human individual arises.
The important conclusion is forced upon us by these experiments with a needle, that even in so typical and highly organised a creature as one of the higher or five-fingered, air-breathing vertebrates, the egg-cell does not require any material admixture from the sperm-cell in order that it may successfully germinate and develop, but only a disturbance of equilibrium, which can be administered as well by a needle's point as by a sperm-filament!
In the higher animals the egg or ovum is produced by the female, and is fertilized by the sperm-cell produced by the male. The necessary union between these two essential elements is attained in various ways. Thus the female salmon deposits her eggs on a convenient spot in the bed of a stream and the attendant male salmon then projects over them the spermatozoa.
Whether, as among the lowest forms of life, these two cells are seemingly identical in nature; or whether, as among higher forms, they are distinguishable into sperm-cell and germ-cell; it remains throughout true that from their combination results the mass out of which is evolved a new organism or new series of organisms.
In order, however, that the substance of the female germ should reach that condition of chaos suitable for embryonic development, there is still necessary the influence of the supra-lunar astral forces. Entry for these is provided by the union of the germ-cell with the male sperm-cell.7
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