At each menstrual period, one and sometimes two or three ova of mature size burst out from the ovary or ovaries, and, if unimpregnated by a spermatozoon, pass on into the uterus and are lost in the menstrual discharges. An ovum being discharged at each menstrual epoch, a woman may consequently conceive at any time of her sexual life from puberty to the menopause, i.e., until the final cessation of her menstruation. If two or three ova are discharged, and each impregnated, she will bear twins or triplets, though twins, curiously enough, are sometimes developed from a single ovum.
This menstruation is a remarkable phenomenon, which comes on in cycles, characterized not only by a periodical flow of blood from the uterine cavity, but also by constitutional disturbances; there is a shedding of the superficial layers of the mucous membrance of the uterus, and at each of the epochs an ovum is discharged from one or other oi the ovaries. It occurs in properly developed women, in temperate climates, between the ages of fourteen and forty four years, sometimes normally beginning earlier or ending later than these figures, and being observed earlier in warm and later in cold climates.
Normally this phenomenon occurs thirteen times a year, at intervals of a lunar month twenty eight days and the name is taken from the Latin word mensis, "a month". During all the period of a woman's menstrual life the function of menstruation can, in health, be interrupted only by pregnancy and suckling, so that it has been quaintly said that "woman only escapes being sick twelve times a year by having an illness pregnancy which lasts nine months".
The general public, in accordance with their usual erroneous opinions about physiological subjects, have an idea that intercourse during the first week after a menstrual period is liable to be followed by conception, but that at other times there is no danger of it.
"Experience has shown, however, that there is no single day in the intermenstrual period in which conception may not occur. Jewish women indeed, who are forbidden sexual intercourse by the Mosaic law during menstruation and the seven days following, are proverbially fruitful".
After an intercourse occuring just before a menstrual period, one might suppose that the semen and the ovum would be expelled when the flow began, but the spermatozoa pass into the distal, or remote, extremities of the Fallopian tubes within a few hours after intercourse, and the Fallopian tubes not actively sharing in the phenomena of menstruation these spermatozoids, which continue to possess life for upward of eight to ten days, may be retained and impregnate the ovum which is discharged at the menstrual period immediately following.
The law of reproduction is so strongly impressed on all animate Nature that when a healthy male and a healthy female have sexual congress the chances of insemination are very great indeed. Conception and Development of the Foetus. By conception is meant the animation of the female reproductive element by the male reproductive element so that an embryo is formed.
The ovum represents one cell and the spermatozoid one cell, and when they become fused in the process of conception there results one cell the impregnated ovum which is now the germ of an embryo. "The earlier the stage the fewer the cells, until we reach the condition when there are but few cells, then two, and finally one only. This cell is the impregnated ovum, the beginning of all development, but is itself formed of two separate parts, very different in their origin and constitution, namely, the egg cell or ovum and the spermatozoon, whose union is the act of impregnation the beginning of a new existence".