THE IMPREGNATED OVUM.

The changes which the ovum undergoes after impregnation have been necessarily examined only in the lower animals. The time which elapses between impregnation and the escape of the ovum into the Fallopian tube varies much, being sometimes but a few hours, and in the deer being four months, according to Pockels. The cells of the membrana granulosa become club-shaped and radiated round the ovum as it leaves the ovary, and disappears as it enters the uterus. While passing through the Fallopian tube the ovum enlarges, loses its vitelline membrane, and gets a covering from the mucous surface which is developed into the chorion.

The yolk undergoes remarkable rotation by means of cilia, and then becomes divided by dichotomous cleavage into many cell-like bodies with a light centre. After this segmentation, and about the time the ovum enters the uterus, the germinal membrane, or blasdodermic vesicle of Bischoff, forms within the zona pellucida.— From the surface of this membrane, at a spot termed the "area germinitiva," the embryo becomes developed-the nervous system and organs of animal life from its outer surface, and the digestive and other vegetative organs. from its inner. These two surfaces are called respectively the serous layer and the mucous layer.

The Decidua is meanwhile forming in the uterus by an alteration in its tissue, as Sharpey believes, assuming a soft, thick, and vascular condition. Into this the ovum and its chorion become firmly set. On its surface are many openings of tubular glands which dip into the uterine wall, and which receive the tufts of the chorion. Tho ovum becomes covered by the "decidua reflexa," which Hunter described as pushed before the ovum as it left the Fallopian tube, the mouth of which he stated was subsequently closed by the "decidua serotina." If the decidua be an altered state of the mucous membrane of

the uterus, this view cannot be correct. Both decidua vera and reflexa incorporate about the 3rd month, and are expelled at parturition.

The tufts of the chorion next accumulate at the side of the ovum next the decidua vera, and here form the placenta, by which the human foetus and that of other placentalia derive blood in the later months of utero-gestation. According to Goodsir, the following structures separate the maternal and foetal blood-the wall of the venous sinus, a thin layer of decidua, a stratum of fluid, the cells and membrane at the end of the tufts, and the walls of the foetal capillaries.

a, b. Embryo. c. Vitelline membrane. d. Yolk. e. Umbilical duct. fg. Amnion. h. Point of union of cephalic and caudal involucra. i. Cavity filled with liquor amnii. 1. Urachus. m. Allantois. o, p. Chorion.

To return to the ovum, Bischoff found that of the dog about 3 lines long, and half as wide between the 12th and 16th days. The area germinativa becomes surrounded by a less clear space, termed the area vasculosa, and in it appears the first trace of the embryo as the "primitive streak." The edges of this groove are termed

up

the "dorsal lamina," and they rise and arch together to form the spinal canal. Two "ventral lamina" also project downwards and enclose the viscera. The anterior or cephalic end of the embryo enlarges, and three vesicles, the rudiments of the brain, become apparent. About 24 hours after the formation of the primitive streak the amnion begins to form, and soon contains liquor amnii which bathes the embryo. The amnion is continued along the cord to the integument of the embryo.

The relations of the embryo and its coverings will be understood from a careful study of the annexed figures.

Human Embryo, after the figure in Müller's Physiology, from a preparation by Prof. Sharpey.

1. Chorion. 2. Amnion. 3. Umbilical vesicle. 4, 5, 6. Intestine. 7. Umbilical vein. 8. Vena porta. 9. Liver. 10. Filament of umbilical vesicle, probably omphalo-mesenteric artery. 11. Filament, probably omphalomesenteric vein. 12. Umbilical artery. 13. Sheath of cord. 14. Eyeball. 15. External ear.

The liquor amnii flows into the œsophagus and trachea of the foetus. It is a yellowish acid fluid containing chloride of sodium and other salts, a trace of albumen, urea, and casts of the uriniferous tubes, which prove that the foetal urine is poured into this fluid. The umbilical vesicle is very small in the human embryo, lying between the chorion and amnion, its duct communicating with the intestine, but it disappears early.

The Allantois is a sac formed on the front of the lower end of the embryo. It holds the secretion of the Wolffian bodies and conducts the umbilical vessels to the placenta, and disappears as soon as they unite to it. The allentoic fluid contains chlorides, lactates, and phosphates, and a peculiar substance, analogous to uric acid, termed allantoin.

The development of the most important organs and tissues have been before alluded to, but here the Wolffian body may be noticed. It is an oval body which lies in front of the kidney and supra-renal body, being so large as to conceal them. Several tubes exist in them which secrete a fluid, poured by a duct into the sinus urinogenitalis, or foetal cloaca. They act as temporary kidneys and disappear, with the exception of the parovarium—a tubular mass in the hilus of the ovary-and the organ of Giraldés, above the testis, which Kobelt has proved may become the seat of encysted hydrocele.

The Umbilical Cord connects the foetus and the placenta, and is covered by the amnion. It contains a soft, protective structure, which Wharton described, in 1659, as gelatin, but which has a beautiful cellular arrangement like the vitreous body; one umbilical vein carrying blood from the placenta, and two umbilical arteries returning it; the urachus, or remains of the allantois; the omphalo-enteric duct; and the omphalomesenteric vessels.

The Fœtal Circulation may be here briefly sketched. The umbilical vein (1) carries pure blood from the pla

centa and enters the fœtus through the navel, runs to the liver, and divides into branches to left side (2), and

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1. Umbilical vein.

2. Its left division.

3. Vena portæ.

4. Ductus venosus.

5. Hepatic veins.

6. Vena cava.

8. Arch of aorta.

9. Descending aorta.

10. Superior vena cava.

11. Right ventricle.

12. Pulmonary artery.

13. Ductus arteriosus.

14. Bifurcation of aorta.

15. Iliacs ending in umbilical arteries.

The Fatal Circulation.

one to the right, which joins the porta (3), as has been