Amniote

“ Outside the embryo there now remains nothing but the highly vascular [Pg 199] allantois and the bloodless serous membrane and amnion. The amnion, whose fluid during the later days of incubation rapidly diminishes, is continuous at the umbilicus with the body-walls of the embryo. The serous membrane (or outer primitive amniotic fold) is, by the completion of the cleavage of the mesoblast and the withdrawal of the yolk-sack, entirely separated from the embryo. The cavity of the allantois, by means of its stalk passing through the umbilicus, is of course continuous with the cloaca. ”

“ Nor does an abundance or want of amniotic fluid affect the development of the embryo or ovum during the earlier stages. It is no more a cause of abortion than the slight changes occasionally found in the amnion itself.
The Umbilical Cord.—The navel-string, however—the sheath stretching from amnion to foetus, enclosing the umbilical vessels—is subject to quite [p. 492] a number of changes, frequently the cause of abortion, occasionally mere results of other complications. ”

“ If the disturbance causing the abortion has been of rapid progress, the cavity is large, the embryo approximating in development the period of expulsion; whilst if the changes have taken place slowly, the walls are thick, the cavity small, and the embryo may appear merely as a small mass pendent from the navel-string, or may have entirely disappeared, and can be traced only by the fine detritus in the amniotic fluid, the cord itself perhaps only in part remaining, and even this may have disappeared. ”

“ The Umbilical vessels unite the fœtus to the chorion before any regular placenta is formed outside of the membrane. Up to the tenth week the decidual vessels are as yet slender, straight, and insulated; while the umbilical vessels begin already to arrange themselves into minute cotyledons [21] . The reason of these arrangements is obvious. The embryo needs growth. This it gets through the maternal blood. But as its gossamer frame, for the space of two and three months, requires but little assistance from such a source for the promotion of growth ”

“ The amniotic fluid performs many important duties. Perhaps the first, in point of time, is to provide sufficient room for the embryo to grow in. Later, as the fluid increases, it permits the fetus to move freely, and yet renders the movements less noticeable to the mother. ”

“ The chorion is the more external of the two fetal membranes; it early surrounds the ovum and throws out processes (villi) toward the decidua reflexa and serotina. These processes are at first formed equally all over the chorion, but those which lie in connection with the decidua serotina grow more rapidly in size and complexity, and remain to form the main part of the placenta. The membrane in which the fetus lies is the amnion. Contained within the amnion is more or less fluid, the liquor amnii, in which the fetus rests as in a water-bed. ”

“ The function of the placenta is partly to supply nutrition, partly to serve as an organ of respiration for the embryo, whose lungs are, of course, not functional, and partly it acts in the same way as the kidney does in after life, by excreting certain products. From the placenta the embryo derives those food elements at first provided by the secretion of the uterine glands. Afterwards these elements are supplied by cells which lie between the fœtal villi and the blood of the mother. ”

“ That the embryo probably lives in utero in virtue of its own life-principle, even through the entire period of gestation, (51, 56, and 58,) is rendered probable by many facts related by unimpeached authorities, which go to prove that when the whole intact Ovum has been expelled at an advanced period of gestation, or at the natural termination of that period, the fœtus has continued to live, and the circulation of the blood has not ceased for an instant during a space of time of from nine minutes to a quarter of an hour. ”

“ The Ovum.—Pathological changes of the ovum itself, of the embryo, of the surrounding membranes are less frequent as primary causes of abortion, and they are of less importance to the practitioner as being in no way amenable to treatment. When they do occur they usually lead to expulsion in the earlier months.
Those conditions liable to lead to abortion are especially diseases of the [p. 490] chorion, placenta, and umbilical cord, rarely of the amnion, the embryo itself, or the amniotic fluid. ”

“ Blizard presented to the Royal College of Surgeons a very instructive specimen of a human Ovum having a sarcomatous and calculous placenta, which had been expelled at three months and a half, by a patient who had miscarried three times within the three previous months.
There is also in the same college a magnificent specimen of a solid Ovum, of the size of a hen’s egg and shape. A small portion of the shell of this Ovum having been cut off, the embryo is seen, of the size of a common fly, within the cavity, which may be just large enough to admit the top of a man’s thumb. ”

“ A new human being is the ultimate result of conception, but the chorion, amnion, placenta and umbilical cord must also be created to serve as aids in building and protecting the developing child during its uterine life. The part played by these accessory structures is so vital, in spite of being temporary, that it will be well for us to look into their origin and functions before considering the fetus itself which they serve. ”

“ The latter succeeds immediately to the former; beginning at the moment when the new being is grafted on the maternal womb, and continuing until its expulsion from thence at the full period of gestation. It follows, therefore, that the Embryonic or former state is that in which the new being is as yet, without any direct or indirect communication with the mother, and still less so with external objects. This state persists for about two weeks after fecundation, during which the Embryo continues to derive its nourishment from the cortical membrane of the Ovum. (Boer, Soemmering, Plagge, myself.) ”

“ The three embryonic layers are indicated by broken lines, dots, and black lines, just as they are in the frog diagrams. Not only is the embryo slowly pinched off from the yolk sac (y.s.) , but, as the yolk is absorbed beneath it, and it grows in size, it sinks into the space thus made, the extra-embryonic somatopleur and epiblast rise up round it as two folds, which are seen closing in 5, and closed in 6, over the dorsal side of the young chick. In this way a cavity, a., lined by epiblast, and called the amniotic cavity, is formed. ”

“ Again, the amniotic fluid prevents injuries that might otherwise befall the child in case the mother wears her clothing too tight. Harmful as the practice of tight-lacing during pregnancy is, it does not, thanks to the presence of the amniotic fluid, result in the disfigurement of the child. For the same reason a blow struck upon the abdomen, as in a fall forward, is not so serious as might be thought, since the fluid, not the child, receives the force of the impact. Some physicians believe that the fetus swallows the amniotic fluid and thus secures nourishment. ”

“ By the end of the third month, or twelfth week, centres of ossification have appeared in most of the bones, the fingers and toes are separated and bear nails in the form of fine membranes; the umbilical cord has definite form, has increased in length and begun to twist. The neck is longer, teeth are forming and the eyes have lids. The amnion and chorion are now in contact, and the villi have disappeared excepting at one point where a small, but complete placenta has developed. The embryo is about 9 centimetres long and weighs about 30 grams. ”

“ This gentleman having observed an ovum still alive in the uterus of a ewe, which was a short time advanced in gestation, removed it and placed it upon a warm glass plate exposed to the rays of the sun, and attentively examined it with the microscope. The beatings of the heart became more lively. He noticed the blood arise to the surface of the chorion from the fœtus, there ramify plentifully, and by anastomosing vessels, return to two of the larger trunks which were the veins of the embryo. ”

“ The life of the fetus, therefore, is absolutely contingent upon an uninterrupted, two-way flow of blood through the cord.
The Fetus. In tracing the development of the ovum after its implantation in the uterine lining, we begin, as previously stated, with a shaggy-looking vesicle, containing fluid, with a clump of cells hanging toward the centre from their point of attachment on the inner surface of the sac. This clump develops into the embryo. ”

“ 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. ”

“ 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. ”

“ In exchange for the nourishing stream of life-giving fluid by which growth and development take place, the embryo gives off its poisonous excretions which are carried back to the placenta, from which they are absorbed into the veinous circulation of the mother; so, while the mother does, through the process of nutrition, influence growth and development in the embryo, she is wholly unable to produce specific changes and such definite developmental errors as birthmarks and other deformities. ”

“ In a more technical sense, however, the use of the term embryo is limited to the first six weeks of pregnancy and designates the condition of the young creature before it has acquired the form and the organs of the infant; after that time the unborn child is called a fetus. Embryonic development, therefore, in the strictest sense of the term, chiefly involves the shifting of various groups of cells and the bestowal upon them of different kinds of activity. ”

“ The general assembly finds that the method or technique of abortion known as saline amniocentesis whereby the amniotic fluid is withdrawn and a saline or other fluid is inserted into the amniotic sac for the purpose of killing the fetus and artificially inducing labor is deleterious to maternal health and is hereby prohibited after the first twelve weeks of pregnancy. ”

“ If such be the case, can it have the power of retaining the germ in the womb, when on the most minute examination of the young, I could not detect, by cicatrice or line of abrasion on any part of the abdomen, that they ever possessed umbilical vessels, or had been in any way nourished by a placenta? Let us take into consideration the small size of the animal found in the pouch, its utter helplessness, its slight power of motion, and its firm attachment to the nipple. The more it is in the embryonic state the firmer is its attachment to the mother ”