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 foetus. 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 foetal 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 foetus with that portion of the chorion which forms the foetal 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 20 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 begins to be formed toward the end of the second month, and then increases in size until the end of gestation, when it assumes an oval or rounded shape, and measures from 7 to 9 inches in length, 6 to 8 inches in breadth, and weighs from 15 to 20 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 foetal and a maternal.

The Fatal 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 interior 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

uterus.'

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 foetal 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 a fusion of the walls of the blood vessels and the walls of the villi and uterine sinuses.

The function of the placenta 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 foetus; at the same time permitting the carbonic acid and other waste products, the result of nutritive changes in the foetus, 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.

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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 surface 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 from 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.

The Internal Ear is developed from the auditory vesicle, budding from the third cerebral vesicle; the membranous vestibule appears first,

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and from it diverticula are given off, which become the semi-circular canals and cochlea.

The cavity of the tympanum, the Eustachian tube, and the external auditory canal are the remains of the first branchial cleft; the cavity of this cleft being subdivided into the tympanum and external auditory meatus by the membrana tympani.

The Skeleton. The chorda dorsalis, the primitive part of the vertebral column, is a cartilaginous rod situated beneath the medullary groove. It is a temporary structure, and disappears as the true bony vertebræ develop. On either side are the quadrate masses of the mesoblast, the primitive vertebræ, which send processes upward and around the medullary groove, and downward and around the chorda dorsalis, forming in these situations the arches and bodies of the future vertebræ.

More externally the outer layer of the mesoblast and epiblast arch downward and forward, forming the ventral laminæ, in which develop the muscles and bones of the abdominal walls.

The true cranium is an anterior development of the vertebral column, and consists of the occipital, parietal and frontal segments, which correspond to the three cerebral vesicles. The base of the cranium consists, at this period, of a cartilaginous rod on either side of the anterior extremity of the chorda dorsalis, in which three centres of ossification appear, the basi-occipital, the basi-sphenoidal, and the pre-sphenoidal. They ultimately develop into the basilar process of the occipital bone and the body of the sphenoid.

The entire skeleton is at first either membranous or cartilaginous. At the beginning of the second month centres of ossification appear in the jaws and clavicle; as development advances, the ossific points in all the future bones extend, until ossification is completed.

The limbs develop from four little buds projecting from the sides of the embryo, which, as they increase in length, separate into the thigh, leg and foot, and the arm, forearm and hand; the extremities of the limbs undergo subdivision, to form the fingers and toes.

Face and Visceral Arches. In the facial and cervical regions the visceral laminæ send up three processes, the visceral arches, separated by clefts, the visceral clefts.

The first, or the mandibular arches, unite in the median line to form the lower jaw, and superiorly form the malleus. A process jutting from its base grows forward, unites with the fronto-nasal process growing from above, and forms the upper jaw. When the superior maxillary processes

fail to unite, there results the cleft-palate deformity; if the integument also fails to unite, there results the hare-lip deformity. The space above the mandibular arch becomes the mouth.

The second arch develops the incus and stapes bones, the styloid process and ligament, and the lesser cornu of the hyoid bone. The cleft between the first and second arches partially closes up, but there remains an opening at the side which becomes the Eustachian tube, tympanic cavity, and external auditory meatus.

The third arch develops the body and greater cornu of the hyoid bone.

Alimentary Canal and its Appendages. The alimentary canal is formed by a pinching off of the yelk sac by the visceral plates as they grow downward and forward. It consists of three distinct portions, the fore gut, the hind gut, and the central part, which communicates for some time with the yelk sac. It is at first a straight tube, closed at both extremities, lying just beneath the vertebral column. The canal gradually increases in length, and becomes more or less convoluted; at its anterior portion two pouches appear, which become the cardiac and pyloric extremities of the stomach. At about the seventh week the inferior extremity of the intestine is brought into communication with the exterior, by an opening, the anus. Anteriorly the mouth and pharynx are formed by an involution of epiblast, which deepens until it communicates with the fore gut.

The Liver appears as a slight protrusion from the sides of the alimentary canal, about the end of the first month; it grows very rapidly, attains a large size, and almost fills up the abdominal cavity. The hepatic cells are derived from the intestinal epithelium, the vessels and connective tissue from the mesoblast.

The Pancreas is formed by the hypoblastic membrane. It originates in two small ducts budding from the duodenum, which divide and subdivide, and develop the glandular structure.

The Lungs are developed from the anterior part of the œsophagus. At first a small bud appears, which, as it lengthens, divides into two branches; secondary and tertiary processes are given off these, which form the bronchial tubes and air cells. The lungs originally extended into the abdominal cavity, but became confined to the thorax by the development of the diaphragm.

The Bladder is formed by a dilatation of that portion of the allantois remaining within the abdominal cavity. It is at first pear-shaped, and communicates with the intestine, but later becomes separated, and opens