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The Chorion, the external investment of the embryo, is formed by a fusion of the original vitelline membrane, the external layer of the amnion, and the allantois. The external surface now becomes covered with villous processes, which increase in number and size by the continual budding and growth of club-shaped processes from the main stem, and give to the chorion a shaggy appearance. They consist of a homogeneous granular matter, and are penetrated by branches of the blood-vessels derived from the aorta.
The presence of villous processes in the uterine cavity is proof positive of the previous existence of a fetus. They are characteristic of the chorion, and are found under no other circumstances.
At about the end of the second month the villosities begin to atrophy and disappear from the surface of the chorion, with the exception of those situated at the points of entrance of the fetal blood vessels, which occupy about one-third of its surface, where they continue to grow longer, become more vascular, and ultimately assist in the formation of the placenta; the remaining two-thirds of the surface loses its villi and blood-vessels, and becomes a simple membrane.
The Umbilical Cord connects the fetus with that portion of the chorion which forms the fetal side of the placenta. It is a process of the allantois, and contains two arteries and a vein, which have a more or less spiral direction. It appears at the end of the first month, and gradually increases in length, until, at the end of gestation, it measures about twenty inches. The cord is also surrounded by a process of the amnion.
Development of the Decidual Membrane.-The interior of the uterus is lined by a thin, delicate mucous membrane, in which are imbedded immense numbers of tubules, terminating in blind extremities, the uterine tubules.. At each period of menstruation the mucous membrane becomes thickened and vascular, which condition, however, disappears after the usual menstrual discharge. When the ovum becomes fecundated, the mucous membrane takes on an increased growth, becomes more hypertrophied and vascular, sends up little processes, or elevations from its surface, and constitutes the decidua vera.
As the ovum passes from the Fallopian tube into the interior of the uterus, the primitive vitelline membrane, covered with villosities, becomes entangled with the processes of the mucous membrane. A portion of the decidua vera then grows up on all sides and encloses the ovum, forming the decidua reflexa, while the villous processes of the chorion insert themselves into the uterine tubules, and in the mucous membrane between them.
As development advances the decidua reflexa increases in size, and at about the end of the fourth month comes in contact with the decidua vera, with which it is ultimately fused.
The Placenta.–Of all the embryonic structures, the placenta is the most important. It is formed in the third month, and then increases in size until the seventh month, when a retrogressive metamorphosis takes place until its separation during labor, at which time it is of an oval or rounded shape, and measures from seven to nine inches in length, six to eight inches in breadth, and weighs from fifteen to twenty ounces. It is most frequently situated at the upper and posterior part of the inner surface of the uterus.
The placenta consists of two portions, a fetal and a maternal.
The fetal portion is formed by the villi of the chorion, which, by developing, rapidly increase in size and number. They become branched and penetrate the uterine tubules, which enlarge and receive their many ramifications. The capillary blood vessels in the anterior of the villi also enlarge and freely anastomose with each other.
The maternal portion is formed from that part of the hypertrophied and vascular decidual membrane between the ovum and the uterus, the decidua serotina. As the placenta increases in size, the maternal blood-vessels around the tubules become more and more numerous, and gradually fuse together, forming great lakes, which constitute sinuses in the walls of the
As the latter period of gestation approaches, the villi extend deeper into the decidua, while the sinuses in the maternal portion become larger and extend further into the chorion. Finally, from excessive development of the blood-vessels, the structures between them disappear, and as their walls come in contact, they fuse together, so that, ultimately, the maternal and fetal blood are only separated by a thin layer of a homogeneous substance. When fully formed, the placenta consists principally of blood vessels interlacing in every direction. The blood of the mother passes from the uterine vessels into the lakes surrounding the villi; the blood from the child flows from the umbilical arteries into the interior of the villi; but there is not at any time an intermingling of blood, the two being separated by a delicate membrane formed by fusion of the walls of the blood vessels and the walls of the villi and uterine sinuses.
The function of the placenta, besides nutrition, is that of a respiratory organ, permitting the oxygen of the maternal blood to pass by osmosis through the delicate placental membrane into the blood of the fetus; at the same time permitting the carbonic acid and other waste products, the
result of nutritive changes in the setus, to pass into the maternal blood, and so to be carried to the various eliminating organs.
Through the placenta also passes all the nutritious materials of the maternal blood which are essential for the development of the embryo.
At about the middle of gestation there develops beneath the decidual membrane a new mucous membrane, destined to perform the functions of the old when it is extruded from the womb, along with the other embryonic structures, during parturition.
DEVELOPMENT OF THE EMBRYO. Nervous System.—The cerebro-spinal axis is formed within the medullary canal by the development of cells from its inner surfaces, which as they increase fill up the canal, and there remains only the central canal of the cord. The external surface gives rise to the dura mater and pia
The neural canal thus formed is a tubular membrane ; it terminates posteriorly in an oval dilatation, and anteriorly in a bulbous extremity, which soon becomes partially contracted, and forms the anterior, middle and posterior cerebral vesicles, from which are ultimately developed the cerebrum, the corpora quadrigemina, and medulla oblongata, respectively.
The anterior vesicle soon subdivides into two secondary vesicles, the larger of which becomes the hemispheres, the smaller, the optic thalami ; the posterior vesicle also divides into two, the anterior becoming the cerebellum, the posterior the pons Varolii and medulla oblongata.
About the seventh week the straight chain of cerebral vesicles becomes curved from behind forward and forms three prominent angles. As development advances, the relative size of the encephalic masses changes. The cerebrum developing more rapidly than the posterior portion of the brain, soon grows backward and arches over the optic thalami and the tubercula quadrigemina; the cerebellum overlaps the medulla oblongata.
The sursace of the cerebral hemispheres is at first smooth, but at about the fourth month begins to be marked by the future fissures and convolutions.
The Eye is formed by a little bud projecting from the side of the anterior vesicle. It is at first hollow, but becomes lined with nervous matter, forming the optic nerve and retina; the remainder of the cavity is occupied by the vitreous body. The anterior portion of the pouch becomes invaginated and receives the crystalline lens, which is a product of the epiblast, as is also the cornea. The iris appears as a circular membrane without a central aperture, about the seventh week; the eyelids are formed between the second and third months.