We shall defer the minute description of the amnion and its relations, during the very early periods of utero-gestation, until we describe the embryo. The amnion is reflected upon the umbilical cord at its insertion into the placenta, envelopes the umbilical vessels, the external covering of which it forms, and is continued to the anterior surface of the child's abdomen, pa.s.sing into that projecting portion of the skin which forms the future navel.

Blood-vessels and nerves have not as yet been discovered in the structure of the amnion, but Meckel considers it extremely probable that the fine layer of cellular tissue by which it is connected with the chorion contains vessels for its nutrition.

_Liquor amnii._ The amnion contains a fluid known by the name of liquor amnii. In the earlier months of pregnancy it is nearly, if not quite transparent; as pregnancy advances it becomes turbid, containing more or less of what appears to resemble mucus: it has a distinctly saline taste; its specific gravity is rather more than that of water. Its relative and absolute quant.i.ty vary considerably at different periods of pregnancy: thus the relative weight of liquor amnii to that of the foetus is very considerable at the beginning of pregnancy, at the middle they are nearly equal, but towards the end, the weight of fluid to that of the child, diminishes considerably, so that during the last weeks of pregnancy it scarcely equals a pound, and seldom more than eight ounces, whereas the medium weight of the child is usually between six and seven pounds: the quant.i.ty, however, varies considerably, sometimes amounting to several quarts. In the early months the absolute quant.i.ty increases, so that between the third and fourth months it sometimes equals as much as thirty-six ounces. Chemically it consists chiefly of water, a small quant.i.ty of alb.u.men and gelatine, a peculiar acid called amniotic, with a little muriate of soda and ammonia, and a trace of phosphate of lime.

The source of the liquor amnii is still unknown. Dr. Burns a.s.serts that "it is secreted from the inner surface of the membrane by pellucid vessels," but as he confesses that "these have never been injected or traced to their source (_Principles of Midwifery_, by J. Burns, M. D. p.

222.,) little weight can be attached to such a view." Meckel considers (_Handbuch der Menschlichen Anatomie_, vol. iv. p. 707,) that the greater part of it, especially in the early months, is a secretion from the maternal vessels, but that afterwards, as pregnancy advances, it becomes mingled with the excretions of the foetus. It appears to be a means of nourishment to the foetus during the first part of pregnancy, from the fact that it contains more nutritious matter in the early than in the latter months, since at that time a considerable coagulation is produced by alcohol, &c. The disappearance of this coagulable matter of the liquor amnii, towards the end of pregnancy, may be attributed to its having been absorbed at an earlier period, and to the process of nutrition being now carried on by other means. Besides being a source of nourishment to the foetus, it serves many useful purposes; it secures the foetus against external pressure or violence, and supports the regular distension of the uterus; on the other hand it diminishes and equalises the pressure of the foetus upon the uterus; during labour by distending the membranes into an elastic cone, it materially a.s.sists to dilate the os uteri; it also serves to lubricate and moisten the external pa.s.sages.

_Placenta._ The placenta is formed essentially by the chorion and decidua; it is a flat, circular, or more or less oval ma.s.s, soft, but becoming firmer towards its edge. It is the most vascular part of the ovum, and by which it is connected most intimately with the uterus. Its longest diameter is generally about eight, its shortest about six inches; its greatest thickness is at that spot where the umbilical cord is inserted, which is usually about the middle of the placenta, although it occasionally varies considerably in this respect, the cord coming off sometimes at the edge. The placenta, as ordinarily seen after labour, is barely an inch in its thickest part, but when filled with blood or injection it swells very considerably, and is then little short of two inches. It is generally attached to the upper part of the uterus in the neighbourhood of one of the Fallopian tubes, and more frequently on the left side than on the right; its inner or foetal surface is smooth, being covered by the chorion, which at this part is much thicker.

The placenta cannot be distinguished from the other parts of the ovum until the end of the second month, at which period it covers nearly half the surface of the ovum, gradually diminishing in relative size, but increasing in thickness and absolute bulk up to the full period of utero-gestation. It forms a spongy vascular ma.s.s, its uterine surface being divided unequally into irregular lobes called _cotyledons_.

The uterine surface of a full-grown placenta is covered by a pulpy membrane, resembling in structure the decidua which covers the chorion, and of which it seems to be a continuation. This is always found present at the end of pregnancy: it covers the lobes of the uterine surface of the placenta, descending into the sulci which runs between them: in some parts it is thicker than in others, especially where it is connected with, or in fact becomes, the decidua of the chorion or decidua reflexa. This membrane, which has been called the _placenta decidua_, is pretty firmly attached to the vessels of the placenta, so as not to be separated without rupture; but by maceration, its texture is more or less destroyed, so that we may easily distinguish the extremities of these vessels. "This decidua, or uterine portion of the placenta," says Dr. Hunter, "is not a simple thin membrane expanded over the surface of the part: it produces a thousand irregular processes, which pervade the substance of the placenta as deep as the chorion or inner surface; and are every where so blended and entangled with the ramifications of the umbilical system, that no anatomist will perhaps be able to discover the nature of their union.

While these two parts are combined, the placenta makes a pretty firm ma.s.s, no part of it is loose or floating; but when they are carefully separated, the umbilical system is evidently nothing but loose floating ramifications of the umbilical vessels, like that vascular portion of the chorion, which makes part of the placentula in a calf; and the uterine part is seen shooting out into innumerable floating processes and rugae, with the most irregular and minutely subdivided cavities between them that can be conceived. This part answers to the uterine fungus in the quadrupeds: it receives no vessels demonstrable by the finest injection from those of the navel string; yet it is full of both large and small arteries and veins: these are all branches of the uterine vessels, and are readily filled by injecting the arteries and veins of the uterus, and they all break through in separating the placenta from the uterus, leaving corresponding orifices on the two parted surfaces." (Hunter, _op. cit._ p. 42.)

According to Lobstein's observations, although this membrane appears to be a continuation of the decidua which covers the chorion, it nevertheless does not exist during the earlier months. During the first months of pregnancy the placenta does not present a solid ma.s.s, with its uterine surface covered with projecting lobuli, as it does at the full term of pregnancy; but the vessels of which it is composed (foetal) are loose and floating, as if it had been subjected to maceration. It has been supposed, that this irregular lobulated appearance of the uterine surface of the placenta was produced at the moment of its separation from the uterus during labour; this, however, is not the case, for Lobstein having opened the uterus of a woman who died in the fifth month of pregnancy, and separated the placenta with great care, found these lobular prominences, although not yet covered by the membrane of which we have just spoken.

Wrisberg, professor of anatomy at Gottingen, considered that this membrane was distinct from the decidua reflexa, since with care the two membranes can be easily separated.

[Ill.u.s.tration: _Uterine surface of the Placenta._]

In examining the uterine surface of a full grown placenta it is necessary to place it upon something convex, in order that it may resemble, as nearly as possible, the form which it had when attached to the concave surface of the uterus; the cotyledons are thus rendered prominent and separated from each other; the sulci, which run between them, are wide and gaping: whereas, when the placenta is laid upon a flat surface, its cotyledons are closely pressed together, and the sulci more or less completely concealed. On minute examination of these sulci a number of openings may be observed, varying in size and shape, but usually more or less oval, their edges distinct, smooth, and thin; on directing a strong light into some of the larger ones a number of smaller apertures may be observed opening into them, in much the same way as is observed when looking down a large vein. Some of these ca.n.a.ls do not immediately lead to smaller orifices as above described, but open at once into an irregular-shaped cell or cavity, in the parietes of which numerous small apertures may be observed, through which blood oozes when the adjacent parts of the placenta are slightly pressed upon. Besides these openings at the bottom of the interlobular sulci, others may be seen here and there upon the cotyledons; these are generally smaller, their edges thicker, and in most instances they are round; but they are not so invariably met with as the openings between the cotyledons, these lobular projections being sometimes very thickly covered with placental decidua. The openings observed on the uterine surface of the placenta correspond to the mouths of the uterine veins and arteries, which, in the unimpregnated state, open into the cavity of the uterus, but which now, by means of the decidua, convey maternal blood to and from the placenta. "Any anatomist," says Dr.

W. Hunter, "who has once seen and understood them, can readily discover them upon the surface of any fresh placenta; the veins, indeed, he will find have an indistinct appearance from their tenderness and frequent anastomoses, so as to look a good deal like irregular interst.i.tial void s.p.a.ces: the arteries which generally make a snake-like convolution or two, on the surface of the placenta, and give off no anastomosing branches, are more distinct." (Hunter, _op. cit._ p. 46.) From the observations of Messrs. Mayo and Stanley, and from their examination of the original preparations in the Hunterian museum at the College of Surgeons, London, ill.u.s.trating this subject, it appears that, in all probability, most of the large thin-edged apertures at the bottom of the interlobular sulci are connected with the uterine veins; whereas, the smaller orifices, the margins of which are thicker, and which are chiefly observed upon the cotyledons, are continuations of the uterine arteries.

These openings were also pointed out by the late Dr. Hugh Ley, in describing the _post mortem_ examination of a woman who had died at the full term undelivered (_Med. Gaz._ June 1, 1833:) "The uterine surface (of the placenta) thus detached from the uterus, exhibited its lobules with their intersecting sulci, even more distinctly than they are seen in the uninjected placenta; and in several parts there could be perceived, with the naked eye, small apertures of an oval form, with edges perfectly smooth, regularly defined, and thicker, as well as more opaque, than the contiguous parts which they penetrated." The communication between the openings of the placental cells, and the mouths of the uterine veins and arteries, which convey their blood to the placenta, as before observed, is effected by means of the placental decidua. The connecting portion of ca.n.a.l is of a flattened shape, runs obliquely between the uterus and placenta, and appears to be formed entirely of decidua. The manner in which the arteries pa.s.s to the placenta is very different to that of the veins: "the arteries," as Dr. W. Hunter observes, "are all much convoluted and serpentine; the larger, when injected, are almost of the size of crow-quills: the veins have frequent anastomoses." Mr. J. Hunter has described this point more minutely, and gives still more precise notions of the manner in which the arteries pa.s.s to the placenta. "The arteries of the uterus which are not immediately employed in conveying nourishment to it, go on towards the placenta, and, proceeding obliquely between it and the uterus, pa.s.s through the decidua without ramifying: just before they enter the placenta, making two or three close spiral turns upon themselves, they open at once into its spongy substance, without any diminution of size, and without pa.s.sing beyond the surface as above described.

The intention of these spiral turns would appear to be that of diminishing the force of the circulation as it approaches the spongy substance of the placenta, and is a structure which must lessen the quick motion of the blood in a part where a quick motion of this fluid was not wanted. The size of these curling arteries at this termination is about that of a crow's quill. The veins of the uterus appropriated to bring back the blood from the placenta, commence from this spongy substance by such wide beginnings as are more than equal to the size of the veins themselves.

These veins pa.s.s obliquely through the decidua to the uterus, enter its substance obliquely, and immediately communicate with the proper veins of the uterus; the area of those veins bear no proportion to their circ.u.mference, the veins being very much flattened."[21]

On examining these vessels in an injected uterus to which the placenta is attached, we shall therefore find that all traces of a regular ca.n.a.l or tube are suddenly lost upon their entering the placenta; each vessel (whether artery or vein) abruptly terminating in a spongy cellular tissue.

If a blow-pipe be introduced into a piece of sponge, we shall have a very simple but correct ill.u.s.tration of the manner in which the uterine blood circulates through the placenta. The cell into which each vessel immediately opens is usually much larger than the rest, so that when the cellular structure of the placenta is filled with wax, a number of irregular nodules[22] are found continuous with these vessels and pa.s.sing into an infinity of minute granules, which are merely so many casts of smaller cells. That this cellular tissue pervades the whole ma.s.s of the placenta, and communicates freely with the uterine vessels by which it is filled with blood, is proved by repeating a very simple experiment of Dr.

Hunter, viz. "if a blow-pipe be thrust into the substance of the placenta any where, the air which is blown into the cellular part opens, and rushes out readily by, the open mouths both of the arteries and veins." (Hunter, _op. cit._ p. 46.) That it also envelopes the umbilical vessels of the cord is shown by the fact, that if a pipe be inserted beneath the outer covering of the cord near to its insertion into the placenta, we shall be able to "fill the whole placenta uniformly in its cellular part, and likewise all the venous system of the uterus and decidua, as readily and fully as if we had fixed the pipe in the spermatic or hypogastric vein; so ready a pa.s.sage is there reciprocally between the cells of the placenta and the uterine vessels." (_Ibid._ p. 47.)

The maternal portion of the placenta therefore consists of a spongy cellular tissue, which is filled by the uterine vessels, and also of those trunks which pa.s.s through the decidua, and which form the communication between these vessels and the placental cells.

[Ill.u.s.tration: _Foetal surface of the placenta._]

The foetal surface of the placenta is smooth and glossy, being covered by the amnion and chorion; it is much harder than the uterine surface, and is streaked over by the larger branches of the umbilical vein and arteries, which radiate irregularly from the point where the cord is inserted; and which pa.s.s beneath the amnion, and between the two layers of which the chorion is composed, to which they are intimately connected. These vessels supply the various lobuli of which the placenta is composed, so that each lobulus receives at least one of these branches; for, although the umbilical cord consists of two arteries and one vein, this arrangement does not continue into the body of the placenta. "Every branch of an artery," as Dr. Hunter observes, "is attended with a branch of a vein: these cling to one another, and frequently in the substance of the placenta entwine round one another, as in the navel string." (_Ibid._ p.

40.) Each cotyledon receives its own vessels, so that the vessels of one cotyledon have no direct communication with those of the adjacent ones, as proved by Wrisberg's examinations; for if we inject the vessel or vessels of one of these lobuli, the injection will not pa.s.s into those of the others. When the vessels have reached the cotyledons, they are divided and subdivided _ad infinitum_; they are connected together by a fine cellular membrane, which may be very easily removed by maceration, and then they may be seen ramifying in the most beautiful and delicate manner possible; the main branches having no communication or anastomosis with each other.

The umbilical arteries anastomose freely with each other upon the foetal surface of the placenta, before dividing into the branches above-mentioned; hence, if an injection be thrown into one umbilical artery it will return almost immediately by the other; but if this be tied also, the injection, after a time, will return by the umbilical vein, but not until all the vessels of the placenta have been filled, proving that there is a free pa.s.sage of blood from the arteries into the veins.

From these remarks, founded chiefly on the admirable observations of the Hunters, and repeated examinations of the placenta, which we have made with the greatest care and impartiality, it may be stated with confidence, that the placenta consists of two portions--a maternal and a foetal. The maternal portion consists, as we have before observed, of a spongy cellular tissue; and also of those trunks which pa.s.s through the decidua, and which form the communication between the uterine vessels and the placental cells. The foetal part is formed by the ramifications of the umbilical vessels: "that each of those parts has its peculiar system of arteries and veins, and its peculiar circulation, receiving blood by its arteries, and returning it by its veins; that the circulation through these parts of the placenta differs in the following manner: in the umbilical portion the arteries terminate in the veins by a continuity of ca.n.a.l; whereas, in the uterine portion there are intermediate cells into which the arteries terminate, and from which the veins begin." (Hunter, _op. cit._ p. 48.)

Although various observations and anatomical injections show that to a certain degree, there is a communication between the uterus and the placenta, inasmuch as the blood of the former is received into the sinuses or cells of the latter, we possess no proof that the blood can pa.s.s from these sinuses into the umbilical vessels: on the contrary, every thing combines to prove that the circulation of the foetus is altogether independent of that of the mother. We know from daily experience that in labour at the full term of pregnancy, the placenta is easily expelled from the uterus: that, upon examining the surface which had been attached to the uterus we find no laceration, and that a discharge of more or less blood takes place for some days afterwards. We know, also, that when the placenta becomes detached from the uterus during the progress of gestation, it is followed by a considerable hemorrhage, which greatly endangers the life of the mother. These facts prove that there is a circulation of uterine blood in the placenta, which is destroyed upon its being separated from the uterus. That this uterine circulation in the placenta is unconnected with the circulation of foetal vessels in the placenta is proved by the fact first pointed out by Wrisberg, viz. that, where the mother has died from loss of blood, and the maternal vessels therefore drained of their contents, those of the foetus have been full of blood. Still farther to ill.u.s.trate this fact, he killed several cows big with calf, by a large wound through the heart or great vessels, so as to ensure the most profuse and sudden loss of blood possible, and never found that the vessels of the calf were deprived of blood, although those of the mother were perfectly empty; moreover, no anatomist has ever yet succeeded in making injections pa.s.s from the foetal into the uterine vessels, or _vice versa_. Lobstein has mentioned a mode of ill.u.s.trating this fact (_Essai sur la Nutrition du Foetus_,) which is both simple and striking.

Upon examining the uterine surface of a placenta which has been expelled at the full term, it presents the appearance of a spongy ma.s.s gorged with blood, which may be removed by washing or maceration, and if a placenta thus prepared be injected, the fluids will pa.s.s with the greatest facility from the umbilical arteries into the umbilical vein, but not one drop into its cellular structure; it is evident, therefore, that the blood which had filled the intervals between the vessels, and which had been removed by washing and maceration, could not have belonged to the foetus, but must have come from the mother; for if any of the vessels had been ruptured the injection would not have succeeded.

In concluding these observations upon the placenta, we may briefly state, that there is the same relation between the umbilical vessels and the maternal blood, which fills the placental cells, as there is between the branches of the pulmonary artery, and the air which fills the bronchial cell.[23]

_Umbilical cord._ The umbilical cord, funis, or navel string, is a vascular rope extending between the foetus and placenta, by which they are connected together. It usually arises, as we have before observed, from about the middle of the placenta, and terminates at the umbilical ring of the foetus; it consists of two umbilical arteries and one umbilical vein; the former conveying the blood from the common iliac arteries of the foetus to the cotyledons of the placenta; the latter formed by the union of the collected umbilical veins, on the inner surface of the placenta, and returning this blood to the foetus. In the early periods of pregnancy it also consists of the duct and vessels of the vesicula umbilicalis, the urachus, and more or less of the intestinal ca.n.a.l. The umbilical cord does not present the same form or appearance at every period of gestation; the younger the embryo, the shorter and thicker is the cord; in fact, there are no traces whatever of a cord at first, the embryo adhering, by its lower or caudal extremity, directly to the membranes. By the fifth or sixth week it becomes visible; at this early period the vessels of which it is composed pa.s.s from the foetus in a straight direction, but as pregnancy advances they become more or less spiral, winding round each other, and usually from left to right: according to Meckel, they take the opposite direction much less frequently, viz. in the proportion of one to nine.

The vessels of the umbilical cord are imbedded in a thick viscid substance; upon minute examination, it will be found to consist of a very fine cellular tissue, containing an alb.u.minous matter which slowly exudes, when pressed between the fingers. This cellular tissue itself may be demonstrated by the inflation of air or injection with mercury: it seems to accompany the umbilical vessels as far as the posterior surface of the peritoneum; and Lobstein is of opinion that it is a continuation of the cellular tissue, which covers this membrane. (Lobstein, _sur la Nutrition du Foetus_. -- 75.)

Externally, the umbilical cord is covered by a continuation of the amnion, which, although it be the inner membrane of the ovum, is the outer covering of the cord: in some places it is very thick and strong, and not easily ruptured. From repeated observations, the weakest part of the cord seems to be at about three or four inches distant from the umbilicus, this being the spot where it has invariably given way in every case we have seen, where the cord has been broken at the moment of the child's birth.

From the time of the commencement to the full time of utero-gestation, the cord becomes gradually longer, so that it attains an average length of from eighteen to twenty inches; this, however, varies remarkably. We have known the cord exceed forty inches; and a case is described by Baudelocque, where it was actually fifty-seven inches long: on the other hand, it is sometimes not more than four or five inches in length.

It is remarkable that the cord, which at the end of pregnancy is usually of about the same length as the foetus, is relatively much longer during the sixth month; hence we may conclude, that in those cases where knots have been found upon the cord, the knot must have been formed at this period when the foetus was small enough to pa.s.s through a coil of it.

Neither blood-vessels nor lymphatics have as yet been found in the structure of the cord itself. A filament of nerve from the solar plexus has been occasionally seen pa.s.sing through the umbilical ring, and extending to a distance down the cord.

The vesicula umbilicalis and allantois, being essentially connected with the earliest grades of foetal development, will be considered under that head.

_Embryo._ There is, perhaps, no department of physiology which has been so remarkably enriched by recent discoveries, as that which relates to the primitive development of the ovum and its embryo. The researches of Baer, Rathke, Purkinje, Valentin, &c. in Germany; of Dutrochet, Prevost, Dumas, and Coste, &c. in France; and of Owen, Sharpey, Allen Thomson, Jones, and Martin Barry in England, but more especially those of the celebrated Baer, have greatly advanced our knowledge of these subjects, and led us deeply into those mysterious processes of Nature which relate to our first origin and formation.

These researches have all tended to establish one great law, connected with the early development of the human embryo, and that of other mammiferous animals, viz, that it at first possesses a structure and arrangement a.n.a.logous to that of animals in a much lower scale of formation: this observation also applies of course to the ovum itself, since a variety of changes take place in it after impregnation, before a trace of the embryo can be detected.

At the earliest periods, the human ovum bears a perfect a.n.a.logy to the eggs of fishes, amphibia, and birds; and it is only by carefully examining the changes produced by impregnation in the ova of these lower cla.s.ses of animals, that we have been enabled to discover them in the mammalia and human subject.

As the bird's egg, from its size, best affords us the means of investigating these changes, and as in all essential respects they are the same in the human ovum, it will be necessary for us to lay before our readers a short account of its structure and contents, and also of the changes which they undergo, after impregnation. In doing this we shall merely confine ourselves to the description of what is applicable to the human ovum.

[Ill.u.s.tration: _Section of a hen's egg within the ovary._

_a_ The granulary membrane forming the periphery of the yelk. _b_ Vesicle of Purkinje imbedded in the c.u.mulus. _c_ Vitellary membrane. _d_ Inner and outer layers of the capsule of the ovum. _e_ Indusium of the ovary.]

The egg is known to consist of two distinct parts, the vitellus or yelk surrounded by its alb.u.men or white; to the former of these we now more particularly refer. The yelk is a granular alb.u.minous fluid, contained in a granular membranous sac (the _blastodermic membrane_) which is covered by an investing membrane called the _vitelline membrane_ or _yelk-bag_.

The impregnated vitellus is retained in its capsule in the ovary, precisely as the ovum of the mammifera is in the Graafian vesicle. The whole ovary in this case has a cl.u.s.tered appearance, like a bunch of grapes, each capsule being suspended by a short pedicle of indusium.

[Ill.u.s.tration: _a_ Vitelline membrane _b_ Blastoderma. _From T. W.

Jones._]

In those ova which are considerably developed before impregnation, the granular blastermodic membrane is observed to be thicker, and the granules more aggregated at that part which corresponds to the pedicle, forming a slight elevation with a depression in its centre, like the c.u.mulus in the proligerous disc of a Graafian vesicle. This little disc is the blastoderma, germinial membrane or cicatricula; in the central depression just mentioned is an exceedingly minute vesicle first noticed by Professor Purkinje of Breslau, and named after him: in more correct language it is the _germinal vesicle_.

According to Wagner, the germinal vesicle is not surrounded by a disc before impregnation; and it is only after this process that the above-mentioned disc of granules is formed. By the time the ovum is about to quit the ovary the vesicle itself has disappeared, so that an ovum has never been found in the oviduct containing a germinal vesicle, nothing remaining of it beyond the little depression in the c.u.mulus of the cicatricula.

The rupture of the Purkinjean or germinal vesicle has been supposed by Mr.

T. W. Jones to take place before impregnation; but the observations of Professor Valentin seem to lead to the inference that it is a result of that process, and must be therefore looked upon as one of the earliest changes which take place in the ovum or yelk-bag upon quitting the ovary.[24]

During its pa.s.sing through the oviduct (what in mammalia is called the Fallopian tube,) the ovum receives a thick covering of alb.u.men, and as it descends still farther along the ca.n.a.l the membrane of the sh.e.l.l is formed.

On examining the appearance of the ovum in mammiferous animals, and especially the human ovum, it will be found that it presents a form and structure very a.n.a.logous to the ova just described, more especially those of birds. It is a minute spherical sac, filled with an alb.u.minous fluid, lined with its blastodermic or germinal membrane, in which is seated the germinal vesicle or vesicle of Purkinje. When the ovum has quitted the ovary the germinal vesicle disappears, and on its entering the Fallopian tube it becomes covered with a gelatinous, or rather alb.u.minous covering.

This was inferred by Valentin, who considered that "the enormous swelling of the ova, and their pa.s.sage through the Fallopian tubes," tended to prove the circ.u.mstance. (_Edin. Med. and Surg. Journ._ April, 1836.) It has since been demonstrated by Mr. T. W. Jones in a rabbit seven days after impregnation. The vitellary membrane seems, at this time, to give way, leaving the vitellus of the ovum merely covered by its spherical blastoderma, and encased by the layer of alb.u.minous matter which surrounds it.

From what we have now stated, a close a.n.a.logy will appear between the ova of the mammalia and those of the lower cla.s.ses, more especially birds, which from their size afford us the best opportunities of investigating this difficult subject.

In birds, the covering of the vitellus is called _yelk-bag_; whereas, in mammalia and man it receives the name of _vesicula umbilicalis_. Its alb.u.minous covering, which corresponds to the white and membrane of the sh.e.l.l in birds, is called _chorion_: by the time that the ovum has reached the uterus, this outer membrane has undergone a considerable change; it becomes covered with a complete down of little absorbing fibrillae, which rapidly increase in size as development advances, until it presents that tufted vascular appearance, which we have already mentioned when describing this membrane.

The first or primitive trace of the embryo is in the cicatricula or germinal membrane, which contained the germinal vesicle before its disappearance. In the centre of this, upon its upper surface, may be discovered a small dark line;[25] "this line or primitive trace is swollen at one extremity, and is placed in the direction of the transverse axis of the egg."