DOUBLE CONCEPTION.
If two germs be evolved simultaneously, each may be impregnated by spermatozoa, and a twin pregnancy be the result. This is by no means a rare occurrence. It is very unusual, however, to have one birth followed by another after an interval of three or four months, and each babe present the evidences of full maturity. Perhaps such occurrences may be accounted for on the supposition that the same interval of time elapses between the impregnation of the two germs as there is difference observed in their birth; that after the act of insemination, sperm was carried to each ovary; that one had matured a germ ready for fecundation, then impregnation and conception immediately followed, and the decidua of the uterus hermetically sealed both Fallopian tubes, and thus securely retained the sperm within the other Fallopian canal. The stimulus of the sperm so pent up causes that ovary to mature a germ, although it may do so slowly, and after two or three months it is perfected, fertilized, and a second conception occurs within the uterus. If each embryo observe a regular period of growth and each be born at maturity, there must be an interval of two or three months between their births. But it is far more common for the parturition of the first, displaying signs of full maturity, to coincide with the birth of a second which is immature and which cannot sustain respiratory life. The birth of the latter is brought about prematurely, by the action of the uterus in expelling the matured child.
UTERINE PREGNANCY.
There are many who manifest a laudable desire to understand the physiology of conception, the changes which take place, and the order of their natural occurrence. When impregnation takes place at the ovaries or within the Fallopian tubes, there is exuded upon the inner surface of the womb a peculiar nutritious substance. It flows out of the minute porous openings surrounding the termination of the Fallopian tube within the uterine cavity, and, thus, is in readiness to receive the germ, and retain it there until it becomes attached. Undoubtedly, the germ imbibes materials from this matter for its nurture and growth. This membranous substance is termed the decidua, and disappears after conception is insured. Two membranes form around the embryo; the inner one is called the amnion, the outer one the chorion. Both serve for the protection of the embryo, and the inner one contains the liquor amnii, in which it floats during intra-uterine life. Immediately after conception, the small glands in the neck of the uterus usually throw out a sticky secretion, filling the canal, or uniting its sides, so that nothing can enter or leave the uterine cavity.
The fertilized ovum rapidly develops. After its conception it imbibes nourishment, and there is a disposition in fluids to pass into it, through its delicately-organized membranes. If this process is not involuntary, it is, at all events, at the convenience and use of the developing germ. After three months the embryo is termed the foetus. Its fluids are then so much more highly organized, that some of them are tinged with sanguine hues, and thenceforward acquire the characteristics of red blood. Out of red blood, blood-vessels are formed, and from the incipient development of the heart follow faint lines of arteries, and the engineers of nutrition survey a circulatory system, perfecting the vascular connections by supplementing the arteries with a complete net-work of veins and capillaries.
THE PLACENTA OR AFTERBIRTH.
Whenever conception occurs, a soft, spongy substance is formed between the uterus and the growing ovum, called the placenta. It is composed of membrane, cellular tissue, blood-vessels, and connecting filaments. The principal use of this organ seems to be to decarbonate the blood of the foetus, and to supply it with oxygen. It performs the same function for the foetus that the lungs do for the organism after birth. It allows the blood of the foetus to come into very close contact with that of the mother, from which it receives a supply of oxygen, and to which it gives up carbonic acid. This interchange of gases takes place in the placenta, or between it and the uterus, through the intervening membranes. This decarbonating function requires the agency of the maternal lungs, for the purpose of oxygenating the mother's blood.
The placenta is attached to the uterus by simple adhesion. True, in some instances, morbid adhesion takes place, or a growing together in consequence of inflammation, but the natural junction is one merely of contact, the membranes of the placenta spreading out upon the cavity of the uterus, so that, finally, the former may be entirely removed without a particle of disturbance or injury to the latter. Formerly, it was supposed that the placental vessels penetrated into the substance of the uterus. We know now there is no such continuation of the vessels of the one into the other. The decarbonation of the blood requires the placental and uterine membranes to be in contact with each other.
If the union were vascular, the mother's blood would circulate in the foetal body, and the impulses of the maternal heart might prove too strong for the delicate organism of the embryo. Besides, the separation of the placenta from the uterus might prove fatal to both parent and offspring. The placenta is only a temporary organ, and when its functions are no longer required, it is easily and safely removed.