In ten days no portion of the yolk is observable on the outside of the vesicle.

The embryo being taken out of the amnion,—now become full

of water, the thorax is found

to be completely formed, and the roots of the feathers very distinct.

The contents of the egg, during the formation of the embryo, become much diminished in quantity, and the void space is gradually occupied by a gas, which was examined by Mr. Hatchett, and found to be atmospheric air, deposited at the great end of the egg between the layers of the membrane lining the shell. Even prior to incubation, there is always a small portion of air in this place, which is supposed to be employed in aerating the blood, from the time of its

Embryo of the egg, Fig. 147(6), showing the opening in the abdomen, from which portions of the vesicular and areolar

first acquiring a red colour, membranes and turns of the intestines are protruding. till superseded in that office Magnified two diameters.

by the external air acting through the eggshell upon the blood in the vessels of the vesicular membrane,

with which it is lined.

Between the period of fourteen and eighteen days, the yolk becomes completely inclosed by the areolar membrane; and at the expiration of the latter period the greater part of the yolk is drawn into the body, as in the marginal figure. At twenty days, the chick is completely formed, the yolk is entirely drawn in, and only portions of the membrane belonging to the vesicle are seen externally. The yolkbag has a narrow tube, half an inch long, connecting it to the intestine, eight inches above the openings of the cæca into the gut.

The whole of these changes, which in the viviparous animal are effected within the womb of the mother, take place in the incubated chick

by virtue of its own powers; and Embryo eighteen days old.-Half the natural

VOL. II.

44

size.

without any assistance, except that of the atmospheric air and of a certain degree of warmth. In the course of incubation the yolk becomes constantly thinner and paler by the admixture of the white; and at the same time innumerable fringe-like vessels, with flocculent extremities, of a singular structure, form on the inner surface of the yolk-bag, and hang into the yolk. The office of these is presumed to be, to absorb the yolk and to convey it into the veins of the yolk-bag, where it is assimilated to the blood and applied to the nutrition of the new being. Blumenbach states, that in numerous and varied microscopical examinations of the yolkbag, in the latter weeks of incubation, he thinks he has observed the actual passage of the yolk from the yellow flocculent vessels of the inner surface of the bag into the blood-vessels which go to the chick. He has, at all events, seen manifest yellow streaks in the red blood contained in those veins. When the chick has escaped from the shell, the yolk, we have seen, is not exhausted, but is received into the abdomen, and as it communicates with the intestinal tube, it is for some time a source of supply to the young animal, until its strength is equal to the digestion of its appropriate food. The highly vascular chorion is manifestly an organ of aeration, like the placenta of the mammalia.

I. DEPENDENCIES OF THE FŒTUS-These are the parts of the ovum, that form its parietes, attach it to the uterus, connect it with the foetus, and are inservient to the nutrition and development of the new being.

They consist,—First, of two membranes, which constitute the parietes of the ovule and which are concentric; the outermost called the chorion, the innermost, filled with a fluid, in which the fœtus is placed, and called the amnion or amnios. Secondly, of a spongy, vascular body, situate without the chorion, covering about one-quarter of the ovule, and connecting it with the uterus, -the placenta. Thirdly, of a cord of vessels,—extending from the placenta to the foetus, the body of which is penetrated at the umbilicus, by the vessels,—called the umbilical cord or navel string. Besides these dependencies, anatomists have described a vesicle, filled with a fluid supposed to be of a nutritious character, and which has been assimilated to the yolk of the egg of oviparous animals,—called the umbilical vesicle; and another vesicle, which is only presumed to exist in the human ovum, because it is found in that of other mammalia, called the allantois. It is a sort of elongated bladder, between the chorion and amnion and communicates with the urinary bladder by the urachus. These last organs consequently belong more particularly to comparative anatomy; and were this not the case, the great dissidence, that prevails amongst authors, regarding their structure and uses, would incline us to pass them over without much notice. The other dependencies will require a more detailed description.

1. The chorion or endochorion is the outermost of the membranes of the ovule. About the twelfth day after conception, according to Velpeau, it is thick, opaque, resisting, and flocculent at both surfaces. These flocculi, in the part of the ovum that corresponds to the tunica decidua reflexa, aid its adhesion to that membrane; but, in the part where the ovum corresponds to the uterus, they become developed to constitute the placenta. At its inner surface the chorion corresponds to the amnion. These two membranes are, however, separated during the earliest period of fœtal existence, by a serous fluid; but at the expiration of three months, the liquid disappears and they are afterwards in contact.

By many anatomists, the chorion is conceived to consist originally of two laminæ; Velpeau, however, denies this, and asserts, that he has never been able to separate them, even by the aid of previous maceration.

As the placenta is formed on the uterine side of the chorion, the membrane is reflected over the foetal surface of that organ, and is continued over the umbilical cord, as far as the umbilicus of the fœtus, where it is confounded with the skin, of which it consequently appears to be a dependence. As pregnancy advances, the chorion becomes thinner, and less tenacious and dense, so that at the full period, it is merely a thin, transparent, colourless membrane, which is much more delicate than the amnion.

Haller and Blumenbach affirm it to be devoid of vessels; but, according to Wrisberg, it receives some from the umbilical trunks of the foetus, and, according to others, from the decidua. DUTROCHET conceives it to be an extension of the foetal bladder.

2. The amnion lines the chorion concentrically. It is filled with a serous fluid, and contains the foetus. In the first days of fœtal existence, it is thin, transparent, easily lacerable, and somewhat resembling the retina. At first it adheres to the chorion only by a point, which corresponds to the abdomen of the fœtus; the other portions of the membranes being separated by the fluid already mentioned, called the false liquor amnii. Afterwards the membranes coalesce and adhere by very delicate cellular filaments; but the adhesion is feeble, except at the placenta and umbilical cord.

In the course of gestation, this membrane becomes thicker and tougher; and, at the full period, is more tenacious than the chorion, elastic, semitransparent and of a whitish colour.

Like the chorion, it covers the foetal surface of the placenta, envelopes the umbilical cord, passes to the umbilicus of the fœtus, and commingles there with the skin.

It has been a question whether the amnion is supplied with bloodvessels. Haller maintained the affirmative, and asserts, that he saw a branch of the umbilical artery creeping upon it. The fact of the existence of a fluid within it and which is presumed to be seereted by it, is also greatly in favour of the affirmative. But, admit

ting that it is supplied with blood-vessels, a difference has existed, with regard to the source whence they proceed; and anatomical investigation has not succeeded in dispelling it. Monro affirms, that on injecting warm water into the umbilical arteries of the fœtus, the water oozed from the surface of the amnion. Wrisberg, however, asserts, that he noticed the injection to stop between the chorion and amnion; and again, Chaussier obtained the same results as Monro, by injecting the vessels of the mother.

The amnion contains a serous fluid, the quantity of which is in an inverse ratio to the size of the new being; so that its weight may be several drachms, when that of the foetus is only a few grains. At first, the liquor amnii,—for so it is called,—is transparent; but at the full period it has a milky appearance, owing to flocculi of an albuminous substance held in suspension by it. It has a saline taste, a spermatic smell, and is viscid and glutinous to the touch. VAUQUELIN and Buniva found it to contain, water, 98.8; albumen, muriate of soda, soda, phosphate of lime, and lime, 1.2. That of the cow, according to these gentlemen, contains amniotic acid; but Prout, Dulong, and Labillardiere and Lassaigne have not been able to detect it. Prout found some sugar of milk in the liquor amnii of the human female; Berzelius detected fluoric acid in it; Scheele free oxygen; and Lassaigne, in one experiment, a gas resembling atmospheric air; in others a gas composed of carbonic acid and azote. The chymical history of this substance is, consequently, sufficiently uncertain. Nor is its formation placed upon surer grounds:—some physiologists ascribing it to the mother, others to the foetus;—opinions fluctuating, according to the presumed source of the vessels that supply the amnion with arterial blood. It has even been supposed to be the transpiration of the fœtus, or its urine. The reply to these views is, that we find it in greater relative proportion when the fœtus is small. Meckel thinks that it proceeds chiefly from the mother, but that about the termination of pregnancy, it is furnished in part by the foetus. The functions, however, to which, as we shall see, it is probably inservient, would almost constrain us to consider it a secretion from the maternal vessels.

The quantity varies in different individuals and in the same individual, at different pregnancies, from four ounces to as many pints. Occasionally, it exists to such an amount as even to throw obscurity over the very fact of pregnancy. An instance of this kind, strongly elucidating the necessity of the most careful attention on the part of the practitioner in such cases, occurred in the practice of a respectable London practitioner, a friend of the author. The abdomen of a lady had been for some time enlarging by what was supposed to be abdominal dropsy: fluctuation was evident, and the case appeared to be unequivocal. A distinguished accoucheur with a surgeon of the highest eminence were called

in consultation, and after examination, the latter declared, that "it was an Augean stable, which nothing but the trocar could clear out." As the lady, however, was even then complaining of intermittent pain, it was deemed advisable to make an examination per vaginam. The os uteri was found dilated and dilating, and, in a few hours after this formidable decision, she was delivered of a healthy child, the gush of liquor amnii being most enormous. After its discharge the lady, was reduced to the natural size, and the dropsy, of course, disappeared!

3. Placenta. This is a soft, spongy, vascular body, formed at the surface of the chorion, adherent to the uterus, and connected with the fœtus by the umbilical cord. The placenta is not in existence during the first days of the embryo state. At this period, the ovule is covered with the shaggy secretion or granulations by which the chorion is united to the decidua reflexa, where this membrane covers it, and by which it adheres to the uterus and forms the placenta, where the decidua is wanting,—as it is supposed to be in one of the views with regard to the decidua. The formation of the placenta is dif ferently accounted for according to those views. In the opinion of some, the flocculi, which are at first spread uniformly over the whole external surface of the chorion, gradually congregate from all parts of the surface into one, uniting with vessels, proceeding from the uterus, and traversing the decidua, to form the placenta; the decidua disappearing from the uterine surface of the placenta about the middle of pregnancy, so that the latter comes into immediate contact with the uterus. In the opinion of others, the placenta is formed by the separation of the layers of the chorion, and by the development of the different vessels, that creep between them. Again, Velpeau maintains, that the placenta forms only at the part of the ovule, which is not covered by the decidua, and which is immediately in contact with the uterus; and that it results from the development of the granulations that cover this part of the chorion; these granulations or villi, according to Velpeau, being gangliform organs containing the rudiments of the placental vessels.

This is an interesting topic of general anatomy, but one on which we are precluded from dwelling. In whatever manner originally produced, it is distinguishable in the second month, after which it goes on successively increasing. Prior to the full term, however, it is said to be less heavy, more dense, and less vascular, owing to several of the vessels, that formed it, having become obliterated and converted into hard, fibrous filaments; a change which has been regarded as a sign of maturity in the foetus, and a prelude to its birth.

At the full period, its extent has been estimated at about onefourth of that of the ovum; its diameter from six to eight inches; its circumference, twenty-four inches; its thickness from an inch to an inch and a quarter, at the centre, but less than this at the circumference; and its weight, with the umbilical cord and membranes, from twelve to twenty ounces. All this is subject, however, to much