In most fishes the ova are impregnated externally to the body of the mother. Thus the female "roe", or "spawn", of many fish e.g., the codfish contains many mUlions of eggs which are "spawned" into the water and fecundated by the "milt", or spermatic secretion of the male, without the act of copulation, the meeting of the male and female reproductive elements being left to chance.
In the process of fish-culture the spawn and the milt are artificially stripped out of the female and male fish, and mixed together in a specially constructed jar filled with water, when in the course of time myriads of fish are hatched.
Among frogs the male embraces the female, and when the latter discharges ova, the male ejects sperm on them.
So also, veterinarians, when they have difficulty in mating animals, sometimes inject semen with a syringe into the female genitalia. A royal scion of France is reputed to have owed his existence to the application of this device, while times without number this procedure has been successfully followed in women hitherto sterile.
A woman at each menstrual period experiences a sort of "mimic labor", discharging a sanguineous fluid with which she "lays a little egg" he ripe ovum.
"The menstrual and gravidital changes follow the same cycle, and differ from one another essentially only in two points: 1, the time occupied, and 2, the extent of the changes. In fact the alterations, though of the same character, are greater in extent and occupy a longer period during gestation than during menstruation.
These considerations force us to the conclusion that the gravid uterus is passing through the menstrual cycle prolonged and intensified. The function of gestation is a direct modification of the function of menstruation, and the two are physiologically homologous".
If one of her ova be fertilized by a spermatozoid, there is at once initiated in the woman a series of astonishingly pronounced and rapid changes, which continue throughout the whole period of gestation and lactation.
Within the short space of nine months, corresponding to the growth of the embryo, there is an enormous increase in the size and power of the uterus, so that it is both adapted to give lodgement to a full sized babe, and to expel it by tremendous contractions through the "birth-passages" at the termination of pregnancy.
While performing the functions of gestation and suckling, she normally ceases to menstruate, and all her primary and secondary sexual organs undergo marked changes, while her heart and blood vessels are rendered more powerful, for the increased work which is demanded of them.
The secretion of semen is largely controlled by the mental condition of the male, and by his surroundings and habits; and he can perform the sexual act at one season as well as another, or remain absolutely continent indefinitely without impairing his procreative ability.
Most of the lower animals have a "rutting season", or a time of periodical sexual excitement, being without desire at other times; but man is entirely independent of this, and maintains the power to found his family in accordance with reason and prudence at any time during his sexual life.