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Foetal Membranes

Extra-Embryonic Phenomena of Development

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Sharpey W. Thomson A. and Schafer E.A. Quain's Elements of Anatomy. (1878) William Wood and Co., New York.

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1878 Elements of Anatomy: The Ovum | The Blastoderm | Fetal Membranes | Placenta | Musculoskeletal | Neural | Gastrointesinal | Respiratory | Cardiovascular | Urogenital
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While the changes before described in the central part of the blastoderm lead to the formation of the rudiments of the embryo, there are simultaneously developed in its peripheral parts, or extended into them from within, certain membranes which lie external to the body of the embryo, but are for a time more or less organically connected with it by the original continuity of the blastodermic elements in which both sets of parts originate.

Of these membranes, the yolk-sac exists in all vertebrate animals ; the amnion and allantois are common to birds and mammals, bub are absent in amphibia and fishes ; and the chorion, in the sense in which the name will be employed here, may be considered as peculiar to mammals.

The Yolk-sac

This name is given to an organised and vascular covering formed by the extension of the layers of the blastoderm over the surface of the yolk within the original vitelline membrane. In human embryology it has also received the name of umbilical vesicle. It consists originally of all the layers of the blastoderm, and in fishes and amphibia retains these throughout the whole term of development ;

Fig. 512. Diagrammatic Section's of the Ovum in different stages of development to show the progress of formation of the membranes (from Kolliker).

1. Ovum in which the choriou has begun to be formed, with the blastoderm and rudiment of the embryo within. 2. Ovum in which the cephalic and caudal folds have contracted the itmbilical ui)erture towards the yolk-sac, and the amniotic folds are turning towards the dorsal aspect. 3. The amniotic folds being completed have met in the dorsal region ; the umbilical opening is more contracted, and the allantois has begun to sprout. 4. The true amnion is detached from the reflected or false amnion which has disappeared or combined with the chorion ; the cavity of the amnion is more distended ; the yolk-sac is now pediculated, the allantois projects into the space between amnion, choriou, and yolk-sac, and the villi of the chorion begin to ramify. 5. The ovum when it has become embedded in the uterine decidua ; the yolk-sac (umbilical vesicle) is now connected to the fcetus by a long duct, the amnion is increased in volume ; the allantois remains ouly as a pediculated vesicle towards the attachment of the short umbilical cord to the part of the chorion where the placenta is about to be formed. The vascular layer of the allantois has now combined with the chorion, the villi of which have undergone further development.

d, vitelline membrane or ijrimitive chorion ; d', commencing villi of the chorion ; sp, epiblast ; sz, villi of the chorion more advanced ; ch, permanent chorion with which the vascular layer of the allantois is combined : cJi, z, true vascular villi of the chorion ; am, amnion ; uh, its cavity ; Ics, cephalic fold ; ss, caudal fold of the amnion ; a, the ejubryonal rudiment in the epiblast ; m, that in the hypoblast or mesoblast ; st, margin of the vascular area in its early stages : dd, hypoblast ; kli, hollow of the vesicular blastoderm, becoming afterwards d», the hollow of the yolk-sac ; dg, ductus vitellomtestinalis ; al, allantois ; e, embryo ; v, original space between amnion and chorion ; -vl, wall of the thorax in the region of the heart ; M, pericardial cavity.

The batrachia seem to be an exception to this statement, but only in consequence of the yolk sac being so extremely limited that it merges in the intestine itself. The yolksac and primitive intestine are in fact combined togethei", there being no umbilical constriction between them.

But in the higher animals, as the greater part of the epiblast and Bomatop eure laver of the mesoblast comes to be detached from the surface of the yolk by the expansion of the amnion and allantois, the hypoblast layer of the mesoblast are alone the Xanent constituents of the wall of the yolk-sac ;. -d hiwgh here last the membrane of the yolk-sac is continuous with the wall ot the intestine in the vitello-intestmal aperture.

The yolk-sac is the seat of the first circulation of the blood m the omphalo-mesenteric vessels of its e animals especially these yessels spread at a later period oyer the whose surface of the yolk in the membrane which forms the sac. Ihe food material of the yolk is probably absorbed by these vessels and conveyed by them as nourishment into the system of the embryo. In many of these animals however a quantity of the yolk substance also remains at the end of incubation, and by actual transference into the intestine of the embryo serves for a time as its digestible food.

In mammals the yolk-sac grows for a time with the embryo and other parts of the developing ovum, and the yolk substance within it must undergo a corresponding increase. There are however great cliflFerences among the difierent tribes of mammals in the extent of the development of the yolk-sac during uterogestation. In some it remains large and vascular, while in others it becomes atrophied and its vessels are obliterated at a comparatively early period. In rodentia it attains its highest degree of development, and comes in contact for a space with the interior of the chorion. In ruminants it is very soon elongated into two attenuated tubes which extend towards the ends of the ovum. In carnivora it is of considerable size, stretching through the ovum towards its opposite poles.

Fig. 513. Magnified View of the Human Embryo of four 1/2 weeks with the membranes opened (from Leishman after Coste). umbilical vesicle with the omphalo-mesenteric vessels, v, and its long tubular fie ..P^rl n ^ f testiue ; c, the villi of the chorion ; m, the amnion opened ; u, cul oL on each side of this the umbilical vessels passing out to the- vesicle ; h, the heart ; I, the itver ; o, the yiMestine rwdy, in front of which the mesentery arid fold ot intestine. The Wolffian duct and tubes are not represented.

In the human species it retains its vascularity and continues to grow up to the fifth or sixth week, at which time it has assumed a pyriform shape, and is connected by a tubular vitelline duct with the intestine.

But notwithstanding all these varieties of form and development of the yolk-sac in the more advanced stages, we recognise the same fundamental structure and relations to other parts as in oviparous animals. Thus in human embryos of from two up to four weeks there have been observed the same progressive changes from the wide communication of the yolk-sac with the open primitive intestine, to the narrower vitello-intestinal aperture, and the subsequent elongation of this into a tubular vitello-intestinal duct (figs. 511 and 51a.).

The human yolk-sac or umbilical vesicle, which expands proportionally with the early increase of the ovum, never, however, surpasses the size of a small pea ; in an ovum of from five to six weeks it lies loosely in the space between the amnion and chorion. At a later period, the duct elongating with the umbilical cord, the vesicle remains in the same relation to these membranes : it now also becomes flattened and more closely attached in the narrower space remaining between them. In the third month it is found connected with a coil of intestine which in the form of a hernia occupies the umbilical cord outside the abdomen of the embryo. At a later period the much elongated and attenuated duct with the omphalo-mesenteric vessels, now impervious and shrunk, may still be traced through the umbilical cord, while the flattened vesicle may be found, even up to the end of the term of nterogestation, somewhat further removed from the place of implantation of the umbilical cord on the internal surface of the placenta, but still between the amnion and chorion.

The Amnion

This vesicular covering of the embryo does not exist in amphibia and fishes, but in reptiles, birds, and mammals, it is formed at an early stage of development, and contains a fluid in which the foetus is suspended by the attachment of its umbilical cord or an equivalent uniting medium.

The structure of the amnion is essentially similar in the three classes of animals in which it exists and its mode of formation nearly the same. It is destitute of blood-vessels, and consists of two layers, derived respectively, the inner from the epiblast, and the outer from the somatopleure layer of the mesoblast ; the first consisting of distinct nucleated cells, the second presenting a fibrous structure. To its external or fibrous layer it also owes the property of muscular contractility, which it possesses in a considerable degree.

The formation of the amnion takes place by the gradual backward inflection from the flat part of the blastoderm adjoining the embryo of the two layers before mentioned, first at the cephalic, and a little later at the caudal extremity and at the sides (see fig. 512, 2, 3, and 4, Jcs, ss, am), so that the two layers of which the amnion is composed are lifted up and separated from the remaining two lower layers (splanchnopleure" and hypoblast) of the blastoderm, by a space which is the same as, or rather a peripheral extension of the pleuro-peritoncal cavity. The embryo thus comes to sink down as it were (the cephalic part before the rest) into the hollow produced by the rising of the amniotic folds round it.

The backward folds deepening more and more, gradually conrerge on the dorsum of the embryo, and at last come together (fig. 512, 3), the margins of the reflection narrowing rapidly and being finally completely obliterated or lost by their convergence and by the subsequent dissociation of the inner from the outer divisions of the folds (fig. 512, 4). The separated inner division now becomes the entire closed sac of the amnion, connected only with the rest of the parts at the umbilical constriction where it is continuous with the integument of the embryo. The outer dissociated division is the false amnion of Pander and Von Baer, passing out into the remaining peripheral part of the blastoderm, and constituting for a time an external covering of the ovum, -which in birds and reptiles appears afterwards to be lost by thinning or absorption ; but which in mammals may be connected with the development of the permanent chorion in a manner to be referred to hereafter.

Fig. 514. Human Embryo of between the Third and Fourth Week, Magnified about five diameters (from Koiliker after Allen Thomson).

a, amnion adherent (unusually) to the interior of the chorion in the dorsal region ; h, umbilical vesicle or yolk-sac with a wide communication with the intestine ; c, lower jaw ; d, superior maxillary process ; c, second j)Ostoral plate, and behind it other two, with the pharyngeal clefts behind each ; f, commencement of the anterior limb ;g, primitive auditory vesicle ; h, eye ; i, heart.

In the human ovum, as in most mammals, the amnion is formed at a very early period. The membrane lies at first so close to the embryo that it is with difficulty distinguished from the surface of the body : but after the dorsal closure is completed, it is soon separated by the fluid which accumulates in its cavity.

The muscular contractility possessed by the amnion doubtless resides in its outer layer derived from the somatopleure. The contractions appear to be rhythmic, as they may be seen in the opened incubated egg of the fowl, or even in the entire egg. by means of a bright light in a dark chamber, from the sixth or seventh day of incubation ; and it is probable that they are of a similar nature in mammals.

The amniotic fluid contains about 1 per cent, of solid matter, consisting chiefly of albumen, but also traces of urea, which is probably derived from the urinary secretion of the foetus.

It would appear that there is a, difference in the structure of the reflected or false amnion in birds and in mammals. In the former it is composed of the same two layers as the amnion itself, but in mammals the development of the jnesoblast appears to cease at the place of reflection of the true into the false amnion, so that the latter consists only of the coi-neous layer or epiblast.

The Allantois - Urinary Vesicle

Although this membrane becomes in the more advanced stage of development widely distributed iu the periphery of the ovum, yet in its origin it differs from the other membranes now under consideration in its close connection with one of the internal organs of the embryo. As ah-eady stated, this membrane does not exist as a foetal structure in fishes or amphibia.

In reptiles, birds and mammals, it is formed in connection with the hinder part of the primitive intestine, is the bearer of an extended capillary distribution of the umbilical or hypogastric vessels, and in combination with them performs important functions connected with the nutrition of the foetus and the aeration of the foetal blood.

The recent observations of His and of Dobrynin have shown that it springs from splanchnopleure elements of the mesoblast and hypoblast, below and in front of the caudal extremity of the embryo close to the place of division of the mesoblast into its somatopleural and splanchnopleurallaminfe. The former of these is reflected in the caudal fold of the amnion already described ; the latter buds out from the end of the primitive intestine into the pleuro-peritoneal space, and receives within it an evolution or outfolded process of the hypoblastic lining of the alimentary canal. It is placed at first rather behind the part which later becomes the cloaca, the orifice of which is still closed : but very soon it is doubled forwards upon the cloaca, so as to lie below it, and when this orifice is afterwards opened it forms the common outlet of the intestine and the allantois (fig. 510, and fig. 512, 3 and 4).

The blood-vessels, which are developed with great rapidity in the outer layer of the allantois, are formed in connection with those which become the two umbilical arteries and the corresponding umbilical veins, which last, however, do not run entirely in the same course as the arteries, but join the omphalo-mesenteric and pass towards the liver; one of the original veins very frequently becoming obliterated, as occurs in the human subject. The capillary network spread over the surface of the allantois appears almost as soon as the first prominence of the membrane begins to bud out from the wall of the primitive intestine, and the vessels appear at first to be in direct connection with the terminations of the two primitive aortas ; but subsequently, when the two aortge coalesce, the umbilical arteries appear as branches of the iliac arteries (see the Development of the Vascular System).

The allantois in expanding takes the shape of a pediculated flask-like vesicle, extends into the pleuro-peritoneal space, and is filled with fluid like the other membranes of the ovum. It is usually directed towards the right side of the embryo, or the opposite from that on which the yolk-sac is laid. In its subsequent great expansion in the egg of birds the allantois spreads out in a flattened form over the whole internal surface of the membrane of the shell, thus coming to occupy more and more of the space previously held by the albumen, the rapid liquefaction and disappearance of which are coincident with the greatest expansion of the allantois and other membranes.

The allantois, though greatly flattened out in its most advanced state, still consists of an outer and an inner wall, separated by the fluid, and both bearing the finely ramified blood-vessels, which, however, are most richly distributed on the outer division ; and in these last it is easy to see, on opening an egg during incubation from the eighth day onwards, the marked difference of colour of the blood in the outgoing and returning' vessels fi'om the action of the surrounding air on the blood which has passed through the capillaries.

It is also worthy of notice that from the time when the allantois has attained some size, it, like the amnion, is possessed of contractility, ■which probably resides in its external layer ; and accordingly, on opening an incubated egg, from the effect of change of temperature or other stimuli, active motions may be perceived, caused by the alternate contraction and relaxation of different parts.

In mammalia the origin and early development of the allantois are nearly the same as in birds, but in a more advanced stage of development, the important connection which the outer layer of this membrane has with the formation of the vascular part of the chorion and foetal placenta, modifies considerably the relations of the membrane to the other parts of the ovum. In all of them, however, the two layers of the allantois (splanchnopleure and hypoblast) are easily distinguished from each other, the internal being entirely devoid of blood-vessels, of a simple cellular structure, and containing the fluid with which the inner sac of the allantois is filled. The external layer, on the other hand, is highly vascular, and is composed of fibro-cellular and contractile fibrous elements.

In the ruminants, pachydermata and the cetacea, the allantois attains to very large dimensions, extending widely into the greatly elongated ovum. In the carnivora it passes round the middle of the ovum externally in accordance with the zonal form of their placenta. while in the rodentia and in man its vesicular or deeper membrane at least, containing the fluid, has a much more limited expansion, and stops apparently in its growth as soon as it has assumed the flasklike form and has reached the interior of the chorion. This appears to be the most probable explanation of the appearance, described by several embryologists, and observed also more than once by the writer, of a pyriform space extending in early human ova from the umbilicus to the inside of the chorion at the place where the placenta is beginning to appear or ndll afterwards be formed. (.See a recently described case by W. Krause in Reichert and Dubois, Archiv, 187;").) But in this and all other fonns the umbilical vessels which pass out of the embryo are placed externally to the vesicle of the allantois or its continuation by the ui-achus towards the iirinary bladder : and these vessels undergoing an extremely rapid development, pass off into the chorion and placenta, which thus owe their- vascular structures to the outer layer of the allantois.

In the human subject the allantois is both of very early formation, and its non-vascular or internal part ceases to extend itself at a very early period, that is. before the end of the fouith week. But already by this time the blood-vessels of the outer layer, by themselves or more probably in association with a connective tissue layer in which they were originally situated, have overrun the whole interior of the chorion, and very soon furnish to the developing villi of that structure, the fibrous element with vessels, of which they secondarily become possessed. The manner of the completion of this process will be apparent from what follows, as to the formation of the chorion (Von Baer, Reichei-t, Remak, Ivolliker).

The Chorion

The ovum of the mammifer when it enters the cavity of the uterus is covered only by the vitelline membrane, or zona pellucida, which is of ovarian origin, and as a rule (notwithstanding the apparent exception of the rabbit to be afterwards referred to) it does not appear that it acquires any other covering for some days after its arrival in the uterus. By the time, however, that it becomes fixed in that part of the uterus which it is to occupy during the subsequent period of its intrauterine life, a great change takes place in the nature of the external covering of the ovum, by its conversion into a new membrane, which acquires more or less of a composite villous structure, becomes vascular throughout the whole or a part of its extent, and which, by its farther development, comes to form the principal means on the side of the ovum of establishing an organic connection between the embryo and the uterus. While the name of prochormi, or primitive chorion, might without impropriety be given to the altered and expanded zona pellucida as the sole early covering of the ovum in mammals, the term chorion is most suitably reserved for the newly formed membrane here referred to.

By some authors. indeed, the name of chorion has been applied to the external covering of the ovum of all animals without regard to its source or its relations to other parts. Thus by some the vitelline membrane has been regarded as a chorion when it appeared that no other membrane existed external to it ; and by others the name has been given to such adventitious parts as the albumen, shell, or shell membrane of the ovipara : but such a use of the term chorion is liable to create confusion, and it seems more expedient that it should be restricted to the peculiar external covering of the mammiferous ovum, which, as will be shown hereafter, is not an original constituent of the ovum like the vitelline membrane, but a structure of new formation in the course of development.

Fig. 515. View op the Chorion of the Human Ovum of about Four or Five Weeks, opened (from Kolliker after Allen Thomson). Natural size.

This figure gives a general view of the villous structure of the chorion previous to the formation of a placenta, and shows the large space which frequently intervenes at an early period between the amnion and chorion.

At a very early period in the majority of mammals, and especially in the human species, the chorion acquires numerous villous processes over the whole or a part of its outer surface. These soon undergo a great development, and constitute a peculiar feature in the human ovum, whence the membrane has been known in human embryology as the chorion frondosum, or shaggy chorion.

the blood-vessels borne by the developed villi of the chorion, and named umbilical m human anatomy, are originally derived from those of the allantoid membrane, and are the seat of an extended circulation of the foetal blood in a system of outgoing arteries and returning veins witli their intervening widely diffused capillary vessels. It is by this system of vascular chorionic villi being brought into contact or close proximity with the blood-vessels of the uterus, that the essential conditions of nterogestation, as regards the continued supply of nourishment to the foetus and the aeration of its blood, are secured in the whole class of mammiferous animals. There is, however, very great difference among these animals in the extent and form of the development of the villous structure of the chorion now referred to, as well as of the concomitant changes which occur in the uterus itself, by which a more or less intimate organic nnion is established between the maternal parent and the offspring. The history of these differences belongs to the account of the structure and formation of the placenta, which will be given hereafter. At this place it will be sufficient to state that, while in some animals, as the pachydermata and cetacea, the connection between the ovum and uterus is reduced to its simplest form, and consists in little more than the implantation of comparatively simple and diffused chorionic villi in minute recesses of the vascular lining membrane of the uterus ; in others there is a greater or less degree of deeper interpenetration of the more highly developed and complex villi with a vascular structure formed from the uterine lining membrane, and which, from its being in whole or in part separated along with the ovum from the uterus in certain animals at the period ol^ birth, has received the name of decidua.

Fig. 516. Surface and Profile Views OP THE Ovum of the Rabbit at the time of the formation of the Chorion (Kolliker after Bischoff).

A and B, an ovum of 3 lines in diameter ; C, one of 4 lines. «, the chorion, with commencing villi ; h, the vesicular blastoderm ; c, the thickened part forming the embryonic area ; d, the increasing extent in which the blastoderm was found to consist of two layers.

Origin of the Chorion

The manner in which the permanent chorion is fr'st formed has not yet Ijeen fully ascertained. The deposit of an albuminous layer on the external surface of the zona pellucida of the rabbit, which takes place in the course of the descent of the ovum through the Fallopian tube, naturally led to the supposition that the chorion might be derived from some external deposit or uterine secretion of this nature ; Ijut the fact that a similar deposit from without has not been obseiwed to occur in other animals, and that the albuminous coat in the rabbit very soon thins away like the zona itself, and gives place to other structures, has caused this view to he abandoned. For is it probable that the chorion proceeds mainly, as held by some, from a development of the vitelline membrane. For when the rapid expansion of the o\-um occm-s shortly after its arrival in the cavity of the uterus, the zona pellucida becomes proportionally dilated, and is reduced to an extreme degree of thinness, so that at this period it is liable to be ruptured -^'itti the slightest force, and there is thus caused great difficulty in the examination of the o\Tim. After a few days the external covering of the ovum, which was previously smooth on its siu'face. becomes covered with slight projections, which gradually rise in the form of simple villi, and these, according to Bischoff, have at first the same homogeneous stiiicture as the zona originally presented. But according to Kolliker it may be doubted whether these villi are at first entu-ely homogeneous, and. at all events, he has ascertained that in a veiy early stage of their formation in the human ovum, as in the ovum of from fifteen to eighteen days, described by Coste, and which Kolliker had an opportunity of examining microscopically, the simple villi consist of hollow tubular processes, which are entirely composed of nucleated cells, similar to those of the upper layer of the blastodenn. It is, therefore, most i^robable that according to the view first suggested by Reichert. the villous chorion of the mammal's ovum is a product of the development of the blastodenn, and is formed in fact by the extension of its outer layer, now termed epiblast.

Fig. 517. Front and Side Views of an Early Human Ovum Four Types the natural size (from Kolliker).

This ovum is supposed to be of thirteen days after impregnation. The surface bare of villi is that next the wall of the uterus, showing at e, the opacity produced by the thickened embryonic disc. The villi covered chiefly the marginal parts of the surface.

Villi of the Chorion

A large part of the external surface of the ovum is ill the earlier stage beset with villi, and these villi acquire vascularity by the extension into them of the blood-vessels of the allantoid membrane from within. In subsequent stages, however, the form and extent of the development of the villi are subject to great variety in different animals, according to the peculiar form which is assumed" in each tribe by the organic connection established in uterogestation between the uterus and the ovum.

In the human species the villi appear to become vascular at a very early period, as ascertained by Kolliker in an ovum of between three and four weeks, in which he found that, while a delicate loop of bloodvessels penetrated into each of the villi, the internal part of the villus, which, as before stated, was previously a hollow cellular tube, was now filled with a fibrous connective tissue bearing the simple blood-vessels, and of a structure precisely similar to that of the outer layer of the allantois. It is therefore extremely probable that the primitive zona of homogeneous structure, after being thinned out to great tenuity by the continued expansion of the ovum, disappears entirely, and is replaced by a cellular membranous structure derived from the upper layer of the blastoderm, while the deeper fibro-vascular part proceeds from the outer layer of the allantois ; and from this it necessarily follows that the chorion is no original component of the ovum, but an acquired or newly-formed structure developed from a union of epiblastic and mesoblastic elements.

Endochorion or Vascular layer of the Allantois

The separation of the outer vascular layer of the allantois from the deeper layer (hypoblast) M'liicli contains the fluid, is sufficiently obvious in many animals, as. for example, in the sheep or pig. But in the human subject, assuming that the vascular elements of the chorion are derived from the allantois as in animals, which there is no reason to doubt, it has been found difficult to determine the exact manner in which they first pass into the villi, in consequence of the very early time and extreme rapidity of the development of the allantois. But notwithstanding the observations previously mentioned of a nonvascular liediculated vesicle in relation with the allantois, passing from the umbilicus of the embryo into the space between the amnion and chorion : yet, in the great majority of instances, so rapid is the expansion of the membrane, that even in ova of from three to four weeks old it has been found impossible to trace more than the connection of the pedicle of the allantois through the urachus with the genito-urinary sinus ; and in all the cases which have been observed, already the umbilical vessels are found detached from the deeper membrane, and passing widely over the whole interior of the chorion to penetrate everywhere into its villi. We are led thus to suppose that by the early and rapid expansion of the outer layer, or by some other naode of development of its fibrous and vascular elements, the blood-vessels of the human allantois have been brought into combination with the cellular layer of the chorion, and have penetrated everywhere into its villi, into the whole of which blood-vessels and fibrous elements may at first be traced. According to this view it is to be understood that while the vascular layer of the allantois may thus become widely diffused, the vesicular or deeper layer may have only a comparatively restricted range of development.

1878 Elements of Anatomy: The Ovum | The Blastoderm | Fetal Membranes | Placenta | Musculoskeletal | Neural | Gastrointesinal | Respiratory | Cardiovascular | Urogenital

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