Book - Text-Book of the Embryology of Man and Mammals 13
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Hertwig O. Text-book of the embryology of man and mammals. (1892) Translated 1901 by Mark EL. from 3rd German Edition. S. Sonnenschein, London.
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The Foetal Membranes of Man
THE investigation of the first stages in the development of man, which are accomplished during the first four weeks of pregnancy, is coupled with extraordinary difficulties. Only very exceptionally does the embryologist come into possession of young human ova, whether found in the uterus at the time of dissection, or coming into the hands of a physician as the result of miscarriage. In the latter case the ova have often been dead for a long time in the uterus, and consequently are in process of decomposition. Finally, a good preservation and an accurate investigation of such small and delicate objects demand no slight degree of -skill.
This accounts for the fact that we do not possess in the case of Man a single observation upon the process of fertilisation or that of cleavage, upon the formation of the germ-layers, or upon the first establishment of the form of the body, i the foetal membranes, and a large number of other organs. Concerning this whole period we are dependent upon the conclusions which are furnished by the development of other Mammals. Thus we assume that fertilisation normally takes place in the enlarged beginning of the oviduct (Fallopian tube) ; that the seminal elements, which remain alive in the female sexual organs perhaps for days or weeks, here await the ovum as it emerges from the ovary ; that the ovum already segmented enters into the cavity of the uterus, attaches itself in the mucous membrane, and during the first weeks of pregnancy gives rise to the germ-layers, the outer form of the body, and the foetal membranes, according to the well-known rules for other Mammals.
A little, although very scanty, information has been acquired, but this concerns only the second and subsequent week. A small number of ova have been described in the literature, which for the most part come from miscarriages, and the age of which has been estimated at from twelve to fifteen days. The blastodermic vesicles measured 5 to 6 mm. in diameter. Here belong two ova described by ALLEN THOMSON, and those by SCHRODER v. D. KOLK, HENNIG, REICHERT, BREUSS, BEIGEL UND LOWE, as well as the cases published by AHLFELD, KOLLMANN, FOL, and GRAF SPEE.
Upon critical comparison of the discoveries, there are two facts which we can regard as established.
First. At the end of the second week the blastodermic vesicle (blastula) no longer lies free in the cavity of the uterus, but is enclosed in a special capsule produced by the growth of the mucous membrane. Hitherto no one has had the opportunity to make observations concerning the formation of this capsule. Following an hypothesis of SHARPEY, which has been somewhat modified by
Fig. 139. Diagrammatic section through the gravid human uterus, from WIEDERSHEIM.
U, Uterus ; UH, cavity of the same ; Tb, Fallopian tube ; Dv, decidua vera ; Dr, decidua reflexa ; Pv, placenta uterina (decidua serotina) ; Pf, placenta fcetalis or chorion frondosum (Ckf) ; Chi, chorion Iseve ; A (on black background), cavity of the amnion filled with amniotic fluid ; D, yolk- (umbilical) vesicle ; in the embryo one sees the umbilical vessels (Al) ', t the liver traversed by the vena umbilicalis; H, the heart; A, the aorta; ci and cs, the vena cava inferior and superior ; p, vena portarum.
REICHERT, it is now generally assumed that the ovum upon its entrance into the uterus imbeds itself in a depression of the mucous membrane, which is thrown into ridges and is in process of being metamorphosed into the decidua. The margins of the depression soon grow around the blastula on all sides, and fuse together to form a closed foatal capsule. The fusion takes place at a point diametrically opposite the attachment, and is described as resembling a cicatrix. It is destitute of blood-vessels, whereas these, as well as uteirne glands, are present in the remaining portion of the overgrowing mucous membrane. The blastula lies in this receptacle now, and even into the beginning of the second month, loosely enclosed ; after opening the capsule the blastula can be removed easily and without injury.
Whereas in other Mammals only that part of the uterine mucous membrane which contributes to the formation of the placenta is cast off, in the case of Man there occurs a much more extensive ecdysis of the most superficial layer, namely, over the whole inner surface of the uterine cavity. Here, too, the part which is cast off is designated as deciduous membrane or decidua, and three regions are distinguishable (fig. 139) the part which is thrown around the blastula as decidua reflexa (Dr), the part which forms the floor of the depression in which the ovum has established itself as decidua serotina (Pu\ and the remaining portion as decidua vera (Dv).
In the reflexa we become acquainted with a structure which in this complete form occurs only in the case of Man and the Apes, whereas beginnings of such a structure are also found in other groups, as, e.g., in the Carnivores. Since the foetal capsule does not at first completely fill the uterus, there remains between reflexa and vera a space filled with mucus.
A second and in many respects astonishing result is, that even in very young and small blastodermic vesicles, as all discoveries agree in showing, a well-developed chorion with abundant villi is begun.
The villi are either distributed over the whole surface of the ovum, or, as in REICHERT'S case (fig. 140 A and ), they leave two opposite poles of the blastula free. They attain a length of one millimetre, and in part have the form of simple cylindrical elevations ; in part they already possess lateral branches. At no place have they fused with the decidua. Like the chorion itself, they consist of two layers -of a superficial epithelial layer, derived from the serosa, concerning which AHLFELD and KOLLMANN have made very definite and reliable statements, and of a layer of embryonic gelatinous tissue, which extends into the axis of the villi and already appears to bear here and there blood-vessels.
Unfortunately we have learned nothing from investigations of these youngest of all human embryos concerning the structures within the chorion, the remaining foetal membranes and the fundament of the embryo itself. Either the ova were already more or less pathologically altered, or the contents were considerably damaged in consequence of the method of preservation and by the preparation. At all events with other investigators one, I think, may conclude from the condition of the chorion that the embryo must have been in an advanced stage, in which germ-layers, yolksac, and amnion were already formed.
This assumption is all the more reasonable, since well-developed embryos from blastodermic '.vesicles which were only a few millimetres larger have been described by COSTE, ALLEN THOMSON, His, and others. In these cases the head-end of the embryo only is rather sharply differentiated from the yolk-sac, which is continuous with the fundament of the intestine throughout nearly its entire length. The neural canal is not yet closed, but the amnion nevertheless is completely developed, and in fact lies almost in contact with the embryonal body ; at its posterior end it is connected with the chorion by means of a short cord, which is connected with the fundament of the allantois and has been named the belly-stalk (Bauchstiel) by His.
Fig. 140. The human ovum at an early stage of development.
A and B, Front and side views of a human ovum of 12 to 13 days, figured by REICHERT. e, The part designated by REICHERT as embryonic spot. From QUAIN'S " Anatomy." C, An ovum of 4 to 5 weeks, showing the general character of the villous membrane before the formation of the placenta. A part of the wall of the ovum is removed in order to show the embryo in situ. After ALLEN THOMSON, from KULLIKER'S " Entwicklungsgeschichte des Menschen, etc."
Also in the only slightly older embryo of COSTE (fig. 141) in which the neural tube is closed, the body distinctly segmented (us), the head provided with visceral arches (vb), behind the latter the heart (h) recognisable, and the yolk-sac (ds) further constricted offa short belly-stalk (bst) is present. It is composed of the amnion (am 1 ) drawn out to a point and of a connective-tissue cord, which arises from the ventral surface of the embryo out of the intestinal cavity of the pelvic region, encloses at its attached end a small cavity (the allantois), and conducts the allantoic blood-vessels from the pelvic portion of the intestine to the chorion.
This cord is a characteristic structure for the human embryo, the significance of which is still in dispute. KOLLIKER and His have given somewhat different explanations of it. KOLLIKER brings the cord into relation with the development of the allantois. He makes the fundament of this important embryonic appendage arise, as in other Mammals, from the hind gut of the embryo, and approach the serosa as a thick vascular connective-tissue growth lined with a narrow, short epithelial tube, without previously developing inside itself a large epithelial sac. He also maintains that the c on nee t i vetissue part of the short allantoic cord, or bell y-s talk, grows around on the whole inner side of the serosa, arid into the epithelial villi.
His regards as unwarranted " the assumption, in opposition to the actual state of affairs, that the human embryo at first separates itself from the part of the blastodermic vesicle which is employed for the chorion, and subsequently unites with it again by means of the fundament of the allantois." He does not admit that the fundament of the embryo in Man is ever wholly constricted off from the chorion, as in the remaining Mammals, and he recognises in the belly-stalk " the bridge of connection between the fundament of the embryo and the chorionic part of the original blastodermic vesicle, which has never been severed." According to him the allantois in the human embryo has nothing to do with the development of the belly-stalk.
Fig. 141. Human embryo with yolk-sac, amnion, and belly-stalk of 15 to 18 days, after COSTE, from His ("Menschliclie Embryunen").
His has untwisted somewhat the posterior end of the body in comparison with the original figure, in order to bring into view the right side of the end of the body, the left side being represented in COSTE'S fig. 4. The chorion is detached at ci//<- 1 . u >>i, Amnion ; am 1 , the point of attachment of the amnion to the chorion drawn out to a tip ; bst, belly-stalk ; Sch, tail-end ; us, primitive segment ; dg, vitelline blood-vessels ; ds, 'yolk-sac ; h, heart; cb, visceral' arch.
Neither of these two explanations seems to me entirely satisfactory. According to my view, the structure under consideration may be explained in a manner which is not only in complete harmony with the facts of the case, but also reconciles the views of KOLLIKER and His.
As COSTE'S embryo appears to show, the origin of the belly-stalk is connected in the first place with a somewhat irregular formation of the amnion. It follows from the fact that the latter is drawn out posteriorly to a point (fig. 141 aw 1 ), the apex of which reaches to the chorion, that its closure in the human embryo takes place at the extreme posterior end of the body, and that at the same time a union with the chorion is retained at the place of closure. The fundament of the embryo therefore remains in connection with the chorion, not directly, as His maintains, but only indirectly by means of the amnion.
In the second place, the allantois, the somewhat eccentric development of which in the case of Man is perhaps intimately connected with the above-mentioned peculiarity in the formation of the amnion, takes part in the formation of the belly-stalk. It is therefore proper in this connection to enter somewhat more fully into the allantoisquestion in Man, so actively discussed during the last decade.
Since in other Mammals the allantois (fig. 142 al) has the form of a large stalked sac. which grows out from the navel till it comes in contact with the serosa (sz), and carries to it, along with connective tissue, the umbilical vessels, attempts have been made ever and anon to discover such a structure in the case of human embryos also. The proof of its existence in Man appeared to be furnished by a premature embryo, on which KRAUSE described a spherical, sac-like allantois.
The embryo of KRAUSE presented, however, in many respects such deviations from other known human embryos of the corresponding stage as to cause the statements to be accepted on the part of many persons with great reservation, and to permit the suggestion of His, that in this case it was not after all a human embryo.
Upon critical examination of the facts relating to the question, I am likewise of the opinion that in the case of Man a stage of development with a free allantoicsac protruding out of the body-cavity is not reached.
As results from the fine investigations of human embryos by His, the belly-stalk is found upon cross section to be composed of :
- The pennant-like prolongation of the amnion ;
- Beneath this, abundantly developed embryonic connective tissue ;
- The fundament of the allantois, which has the form of a very narrow passage with epithelial lining ;
- The umbilical blood-vessels, of which the arteries lie close upon the allantoic duct, while the veins run nearer to the amnion.
To the question, How have these parts arisen ? that appears to me the most natural answer which permits of being harmonised with the known conditions in other Mammals. Now, such an agreement is possible upon the following assumption.
Fig. 142. Diagram of the foetal membranes of a Mammal, after TURNER.
2>c, Zona pellucida with villi (prochorion) ; *.:, serous membrane; am, amnioii AC, amuiotic cavity ; E, outer germ-layer ; M, middle germ-layer ; //, inner germ-layer ; UV, yolk-sac (vesica umbilicalis) ; al, aJlantois ; ALC, allautoic cavity.
Very early, when the hind gut begins to be formed, there arises on its ventral side as a fundament of the allantois a knob composed of many cells, and containing only a small evagination of the entodermic layer. The allantoic knob does not, however, grow free into the body-cavity, as in. the remaining Mammals (fig. 142 al), but extends along the ventral wall of the embryo, and, from the place where this is reflected off to form the amnion, along the ventral wall of the latter (fig. 141 am 1 ) up to its place of attachment to the chorion. The evagination of the entodermic layer meantime becomes elongated into the narrow allantoic duct ; the more voluminous connective tissue growth carries with it the umbilical blood-vessels to the chorion, then spreads itself out on the inner surface of the latter in the well-known manner, and penetrates into the villi of the serosa.
The allantois, therefore, in its development, instead of growing out free to the serosa, makes use of the already existing connection between the latter and the embryo established by the pennant-like elongation of the amnion (am 1 ). But this mode of development perhaps results from the fact that the posterior end of the embryo in Man, as fig. 141 shows, is closely attached to the serosa at the place of the amniotic suture, whereby the allantois has only a short distance to grow in order to reach the serosa.
Finally, the early appearance of the allantois will become intelligible to us, if we remind ourselves that organs of great physiological importance have in general the tendency to an accelerated development, and that in the series of Mammals the provisions for the nutrition of the embryo by means of a placenta have become more and more complete.
While there is still much obscurity about the first stages of Man's development, we possess more satisfactory insight into the changes which the embryonic membranes in Man undergo from the third week onward.
From this point forward we shall examine each separate embryonic membrane by itself : first the structures that are developed from the blastodermic vesicle (1) the chorion, (2) the amnion, (3) the yolk-sac ; then (4) the deciduee which are produced by the mucous membrane of the uterus ; and finally (5) the after-birth (placenta) and (6) the umbilical cord.
During the first weeks of pregnancy the whole surface of the chorion is covered with villi (fig. 132 5 , p. 226, and fig. 140), and provided with terminal branches of the umbilical blood-vessels. After its growth has proceeded for a time uniformly, there begin to appear from the beginning of the third month onward differences between the part which lies directly against the wall of the uterus that is destined to become the decidua serotina and the remaining greater part, which has become overgrown by the decidua reflexa (fig. 143). While on the latter the villi (z') cease to grow, on the former they increase enormously in size and take the form of long, and at the base thick, tree-like, branching structures (~), which, united into tufts, project far beyond the surface of the membrane that bears them, and grow into pits of the maternal mucous membrane (ds). This part, to which we shall give more particular attention at the time of investigating the mature chorion frondosuin from the remaining larger part, the chorion Iceve or the smooth chorion.
Fig. 143. Diagrammatic section through the gravid human uterus with contained embryo, after LONGET, from BALFOUK.
enta > ] al, Stalk of the allantois ; nb, umbilical vesicle ; a,n, amnion ; ch.
fore d i S t i n- chorion ; ds, decidua serotina ; </c, decidua vera ; dr, decidua , , reflexa ; I, Fallopian tube ; c, cervix uteri ; u, uterus ; z, villi of g U 1 S n 6 d the feetal 1)1;u . ellt;l . 2 ] vini ,, f the chorion Iseve.
The expression " smooth chorion ' : is, strictly speaking, not quite applicable. Of the villi which are at first everywhere developed, some afterwards remain preserved on the chorion Iseve, especially in the vicinity of the placenta. They grow into the decidua reflexa, effecting a firm union with it (fig. 143 z').
At the same time a second distinction between chorion froiidosum and chorion Ia3ve is developing. In the territory of the latter the blood-vessels arising from the umbilical arteries begin to dwindle, whereas the former becomes more and more abundantly supplied with blood-vessels, and finally alone receives the terminal distribution of the umbilical arteries. Thus the one region becomes destitute of vessels, while the other becomes extraordinarily vascular, and the nutritive organ for the embryo.
Histologically the chorion laeve, which upon examination from the surface appears thin and translucent, consists of (1) a connective-tissue membrane, and (2) an epithelial covering, which is identical with the original serosa.
The connective-tissue membrane possesses at first the character of embryonic mucous tissue, and exhibits therefore branched stellate cells in a homogeneous matrix. Subsequently the mucous tissue is converted, as at other places in the body, into fibrous connective tissue.
The epithelium of the chorion consists in the first months, according to the statements of KASTSCHENKO and SEDGWICK MINOT, of two layers a superficial one, in which no cell-boundaries are visible (protoplasmic layer), and a deeper one, in which the individual cells are distinctly separated. Additional particulars are given in the description of the placenta.
The embryonic adjuncts enclosed within the chorion the amnion and yolk-sac undergo in Man during pregnancy the following changes.
The amnion (am] immediately after its origin lies close on the surface of the embryo (fig. 144), but soon becomes distended by the accumulation of fluid, the liquor amnii, in its cavity (fig. 132 5 ). It increases to a much greater extent than in other Mammals, in which it is often found to be smaller than the allantoic sac (compare the foetal membranes of the Rabbit, fig. 133). Finally, in Man it Jills out the entire blastodermic vesicle, since it everyiohere applies itself (fig. 143 am) closely to the inner -wall of the chorion (ch).
Its wall is rather thin and translucent, and also consists, like the chorion, of an epithelial and a connective-tissue layer.
The epithelium, derived from the outer germ-layer of the embryonic fundament, lines the amniotic cavity within, and is continuous with the epidermis of the embryo at the dermal navel ; at the place of transition it is composed of layers ; but elsewhere it is a single sheet of pavement cells. The connective-tissue layer is thin and at the navel continuous with the corium.
The amniotic orfcztal water is slightly alkaline, and contains about I/ solid constituents, among which are found albumen, urea, and grape-sugar. Its volume is greatest in the sixth month of pregnancy, and it often attains a weight of not less than a kilo [2'2 Ibs. avoirdupois] ; then it diminishes to about one-half that amount at the time of birth, and in the same ratio as the embryo by its increased growth demands for itself more room. Under abnormal circumstances the secretion of amniotic water can become nmch r greater, and can, by a considerable distension of the amnion, lead to conditions which have been called dropsy of the amnion, or hydranmion.
The yolk-sac or the umbilical vesicle (vesicula umbilicalis) in Man pursues the opposite course of development from that of the everincr easing amnion, and shrivels to a structure that easily escapes observation.
In human foetuses of the second and third week (fig. 144) the yolksac (ds) tills somewhat more than half of the blastedermic vesicle and is not constricted off from the intestine, which still has the form of groove.
Fig. 144. Human embryo with yolk-sac, amnion, and belly-stalk of 15 to 18 days, after COSTE, from His (" Menschliche Embryonen ").
His hits untwisted somewhat the posterior end of the body in comparison with the original figure, in order to bring into view the right side of the end of the body, the left side being represented in COSTE'S fig. 4. The churion is detached at am 1 , am, Amnion ; am 1 , the point of attachment of the amnion to the chorion drawn out to a tip ; bst, belly-stalk ; Sch, tail-end ; us, primitive segment ; dg, \itulline blood-vessels; (/,<, v.ilk-.sic'; It, heart; vb, visceral arch.
In somewhat older embryos it is seen to be connected by means of a thick stall: or vitelline duct with the middle of the rudimentary intestine, now converted into a tube. It is supplied with blood by the vasa omphalomesenteriea.
During the sixth week the vitelline duct or duct us omphalomesentericus has grown out into a long, narrow tube, which sooner or later loses its cavity and is converted into a solid epithelial cord. It terminates in the small egg-shaped umbilical vesicle (figs. 139 D and 143 nb). Since the amnion, in consequence of a greater accumulation of fluid, now fills the whole blastodermic vesicle (fig. 143), it has enveloped both the vitelline duct and the neck of the allantois (al), and, as it were, surrounded them with a sheath (amniotic sheath). The structure thus produced, the umbilical cord, funiculus umbilicalis, is now the only means of connection between the embryo, which floats free in the amniotic fluid, and the wall of the blastodermic vesicle. Its attachment to the latter always coincides with the place where the placenta is developed.
By the enlargement of the amnion the umbilical vesicle is crowded out to the surface of the blastodermic vesicle, where it is enclosed between amnion (am) and chorion (ch\ at some distance from the place where the umbilical cord is attached. It continues to exist here up to the time of birth, although in a very rudimentary condition. It is only by painstaking examination that it is to be found, usually several inches away from the Gi.u.^^f\nr\nnnt^T^t3 margin of the placenta. Its longest diameter measures only from 3 to 10 millimetres. It was on this account that the older text-books of anatomy, physiology, and embryology contained the statement that in Man the vesicula umbilicalis disappeared as a useless structure ; this idea prevailed until the constancy of its presence was demonstrated by B. SCHULTZE.
Fig. 145. Cross section through the mucous membrane of the uterus, after KUNDKAT VXD EXCELMANN.
Gl.v,, Uterine glands ; M, muscular layer of the uterus.
The deciduce or caducous fetal membranes take their origin from the mucous membrane of the uterus, the structure of which is greatly altered during pregnancy.
In the unmodified condition the mucous membrane is a soft layer about a niillimetre thick, which reposes directly and immovably upon the musculature ( M) of the uterus, which does not possess a submucosa in this region (fig. 1-45). It is traversed by numerous tubular uterine glands (glandulae utriculares, Gl.it,), which begin at the surface with small orifices and pass directly downward in a sinuous course close to one another until they reach the musculature (M), where they terminate, often after dichotomous division.
Mucous membrane and glands are lined with ciliate cylindrical cells. The connective tissue that separates the glands embraces an extraordinary abundance of cells, some of which are spindle-shaped, others roundish.
From the beginning of pregnancy the mucous membrane undergoes very profound changes, which affect all- parts. Concerning these we possess accurate observations, which relate to every month of pregnancy, by KUXDRAT UND ExGELMAXN. as well as by LEOPOLD and SEDGWICK MINOT.
We take up in succession (1) the decidua vera, (2) the decidua reflexa, and (3) the decidua serotina or placentalis, the part which enters into the formation of the placenta.
(1) Decidua vera. As LEOPOLD remarks, with the beginning of pregnancy the mucous membrane constantly increases in thickness, until it becomes 1 cm. or more thick, up to the time, indeed, when the growing ovum attaches itself completely to the walls of the uterus, therefore approximately up to the end of the fifth month. From that time forward there begins, as it were, a second stage, in which, under the pressure of the growing foetus, it again becomes thin and finally is only 1 to 2 mm. thick. Meanwhile both the glands and the tissue between them undergo changes.
During the first stage the uterine glands, which at the beginning are tubes of uniform calibre, increase in size, especially in their middle and deeper parts (fig. 146) ; whereas at their open ends they are rectilinear and drawn out lengthwise, deeper down they take a spiral course and are covered with evaginations and pocketings.
Upon sections therefore one can now distinguish two layers in the decidua vera : (1) An outer more compact layer ((7), possessing more abundant cells, and (2) A deeper ampullar or spongy layer (Sp).
In the former one sees the glands as elongated, parallel canals. In consequence of a great growth of the inter-tubular tissue they are separated from one another farther than at first ; they begin at the surface with enlarged funnel-sharped pits (tr\ The surface of a mucous membrane stripped off from the musculature has, as KOLLIKER states, a sieve-like appearance, due to the enlarged orifices of the glands.
In the spongy layer (Sp) one encounters irregular, lobed cavities (dh) one above another, the capacity of which continually increases up to the middle of pregnancy, and which are finally separated from one another by thin septa and cords of the matrixtissue only. The appearance is explained by the fact that in the middle of their course the glands are highly tortuous and have enlarged and become pocketed.
The ciliate cylindrical epithelium at the surface of the mucous membrane of the uterus gradually disappears entirely; it is destroyed as early as the end of the first month of pregnancy (MINOT). In the glands it undergoes fundamental changes. In the first months all the cavities are still lined with it, a condition which, on account of the increase in the size of the cavities, presupposes an active cell-growth. Meanwhile the originally elongate cylindrical cells are in part converted into small cubical, in part into small flat structures, except in the portions of the glands which adjoin the muscular membrane. The cells here preserve more or less their normal form up to the end of pregnancy, and subsequently serve for the regeneration of the epithelial lining of the mucous membrane of the uterus.
Fig. 146. Cross section through the mucous membrane of a uterus at the beginning of pregnancy, after KUNDRAT UND ENGELMANN.
C, Compact layer ; Sp, spongy layer ; M, musculature of the uterus; tr, funnel-shaped mouths of the uterine glands ; e, enlarged region ; dh, ampullae produced by the windings and e\ aginations of the growing glands.
In the fourth and fifth months one still finds all cavities up to the mouth of the glands lined with a thin layer of cubical or flat epithelial cells.
Likewise in the first stage there occurs in the inter-glandular tissue an active process of growth, especially in the upper compact layer. In this there are formed spheroidal structures, 30 to 40 x in diameter, which have been called decidtcal cells by FRIEDLANDER. In many places they lie so close together that, as a consequence and because of their form, they appear very similar to an epithelium.
They are also found in the spongy layer, but in the cords and septa they are more elongated and spindle-shaped.
In the second stage, from the sixth month forward, in lohich the decidua vera becomes much thinner, and under the pressure of the growing foetus gradually diminishes from 1 cm. to 2 mm. in thickness, nidiiy regressive processes take place in the individual parts that have / /r st been described (fig. 147).
The mouths of the glands, which caused the sieve-like condition of the inner surface of the decidua, become more and more difficult to see and finally disappear altogether.
The inner compact layer (0) assumes a uniform, compact, lamellar condition, since by the pressure the cavities of the glands occupying it become wholly obliterated, and then by disappearance of the epithelium their walls become fused.
In the spongy layer (Sp) the cavities of the glands (dh] persist, but, in consequence of the pressure, are converted into fissures, which are parallel to the wall of the uterus, and are separated by partitions which in comparison to earlier months of pregnancy have become very much thinner. The glandular cavities which are adjacent to the compact layer have lost their epithelium or exhibit cellular debris (de), swollen bodies, and a slimy mass permeated with fine granules ; toward the uterine musculature, on the contrary, they possess a wellpreserved epithelium of short cylindrical or cubical cells.
(2) The decidua rejlexa (fig. 148 Dr) exhibits close agreement in its structure with the decidua vera. That it has arisen from the latter by a process of folding may be inferred, as KUNDRAT has rightly maintained, especially from the circumstance that during the first months of pregnancy the mouths of uterine glands (glu), at least at the place of transition to the vera, are found upon both its surfaces. The mouths lead into fissures (glu) which are parallel to the surface of the reflexa and are lined with cuboidal epithelium. In the inter-glandular tissue there appear the same large, round decidual cells as in the vera.
From the fifth month forward the space between vera and reflexa begins to disappear ; both membranes now, after loss of their epithelium, become firmly pressed together, and finally completely fused with each other (fig. 147). By this process the reflexa, from which the glandular spaces disappear except in the transitional region, becomes so extraordinarily thinned that it constitutes [in sections] only a narrow band, occasionally | mm. broad.
A separation of the two membranes at the close of pregnancy is very difficult, but occasionally it may still be accomplished to some extent.
Moreover in later months the inside of the decidua reflexa is firmly fused with the chorion, and since the chorion in its turn is in contact with the amnion (tig. 147 ch and w), one now comes, by cutting through the muscular wall of the uterus, and then opening the foetal membranes, which are thus pressed together, directly into the arnniotic cavity, in which the embryo lies bathed in the amniotic fluid.
Fig. 148. Section through decidua serotina (Dse) at the transition into decidua vera (Z)r) and reflexa (Dr), after KUNDEAT USD EXGELMANN.
M, Musculature of the uterus ; Sj>, spongy layer of the decidua vera and serotina ; C, compact layer of the same ; glv,, uterine glands ; sp, fissures in the serotina resulting from growth of the glands ; dh, ampullarial spaces in the spongy layer produced by growth of the glands.
(3) The third region of the uterine mucous membrane, or the decidua serotina (fig. 148 Dse), is that part which joins with the chorion frondosum to form a nutritive organ for the embryo, the after-birth, or placenta.
According to the statements of KUNDRAT and LEOPOLD it undergoes changes similar to those of the decidua vera. Here also the uterine glands grow rapidly in its deeper portions (fig, 148) and are converted into irregular spaces (d/i), which are from the beginning, however, most extended in breadth. Subsequently they are crowded together still more by the pressure and the growth of the placenta until they become narrow fissures which lie parallel to the surface of the uterus.
The glandular epithelia disintegrate to a still greater extent than in the vera, and by disintegrating and swelling up become detached from the connective-tissue walls ; only those regions of the glands which are adjacent to the muscular layer (J/) retain their cylindrical cells.
In this presentation KUNDRAT and LEOPOLD disagree with KOLLIKER and with TURNER, who likewise, it is true, find great spaces in the deeper layer of the serotina, but interpret them for the most part as greatly enlarged blood-vessels, an assumption according to which there would exist an important difference between the serotina and the vera.
In the superficial layer the outlets of the glands must disappear early, since they become pressed together. Besides, more active cellproliferation takes place in the inter-glandular tissue.
Therefore the decidua serotina (fig. 148 Dse) is also converted into two readily distinguishable layers :
- A deeper spongy layer (/$)?), in which the detachment of the placenta subsequently takes place, and
- A superficial, more compact layer ((7). The latter alone shares in the formation of the placenta, and is accordingly called the placenta uterina (or materiia). It undergoes from the second month forward more profound alterations. We shall become acquainted with these in the description of the placenta, to which we now pass.
The placenta is a very vascular, and when filled a spongy or doughy, disc-shaped structure, which at the height of its development measures 15 to 20 cm. in diameter and is 3 to 4 cm. thick. Its weight reaches somewhat more than a pound (500 grammes). The surface which is turned toward the embryo is concave (figs. 139 and 143) and altogether smooth, since it possesses a covering of the amnion (am) ; the surface which reposes on the wall of the uterus is convex, after its detachment at birth feels uneven, and is divided by deep furrows into separate lobes or cotyledons.
The normal position of the placenta is, in the majority of cases, at the fundus uteri, where it is sometimes developed more to the left side, sometimes more to the right. Consequently the opening of one or the other of the Fallopian tubes may be covered and sealed by it.
In rare cases the placenta, instead of being attached to the fundus, is united to the wall of the uterus nearer its mouth [os uteri]. This results from the fact that the fertilised egg, when it passes from the Fallopian tube into the cavity of the uterus, sinks down farther owing to abnormal conditions, instead of attaching itself at once to the mucous membrane.
Occasionally the attachment takes place quite low, in the immediate vicinity of the inner mouth of the uterus. In this case, as the placenta with the growth of the foetus extends itself, it grows either partly or wholly over the mouth of the uterus, and closes it more or less completely. This anomaly is known as placenta prcevia (lateralis or centralis) and presents a dangerous condition, because the regular progress of birth is disturbed.
In consequence of the low position of the placenta perilous bleeding is produced, either during pregnancy, or at least at the beginning of labor pains, because the placenta detaches itself from the wall of the uterus prematurely, whereby large blood-vessels are ruptured and laid open.
In the investigation of the finer structure of the placenta serious obstacles are encountered, since it is a very soft organ traversed by numerous capacious blood-vessels. Therefore very contradictory views still prevail concerning many points which are of the greatest importance in judging of the structure. It does not appear to me possible to give at present a final opinion upon these points.
In the description it is best for us to start with the fact that the placenta, as was previously stated, is composed of two parts, of one part which is furnished by the embryo, and another part which is produced by the mother, the placenta fcetalis and the placenta uterina (Plate II.).
The placenta fo&talis is the part of the chorion (chorion frondosum) which is thickly covered with much- branched villi. The villi (z), united into great tufts or cotyledons, elevate themselves from a firm membrane, the membrana chorii (m), in which the chief branches of the umbilical arteries and veins take their course. They consist of (1) large main stems (z), which grow straight out from the membrana chorii, and the ends of which (A 1 ) sink into and firmly unite with the placenta uterina, which faces them, and (2) numerous lateral branches (/) which arise on all sides at right angles or obliquely, and which are in turn covered with fine twigs. A small part of these (7i 2 ) also fuse, by means of their tips, with the tissue of the placenta uterina (LANGHANS), so that a separation of the foetal and the maternal portions can be accomplished only by forcible detachment. KOLLIKER has therefore appropriately divided the branches of the chorionic villi into roots of attachment (h l , A 2 ) and free processes (f).
To each arborescent chorionic villus there goes a large branch of an umbilical artery, which, corresponding to the ramifications of the former, is divided up into branches ; the capillary networks which arise from this are situated quite superficially immediately under the epithelium of the villi. From this network the blood is collected into vessels, leading from the villi, which are again united into a single chief stem that emerges from the chorionic tuft.
Consequently the vascular system of the placenta fcetalis is entirely closed. A direct mingling of the foetal and maternal blood cannot take place in any manner ; on the other hand the prerequisite for an easy exchange of fluid and gaseous components of the blood is furnished by the very superficial position of the thin-walled capillaries.
Diagrammatic section through the human placenta at the middle of the Jiftk month, after LEOPOLD.
The musculature of the uterus is followed by the spongy layer of the clecidua serotina (sp), in which the separation of the placenta takes place at birth along the line of separation indicated by two heavy marks ; this is followed by the compact layer ( CS), which is thrown off at birth as the placenta uterina, and which consists of the (WINKLER'S) basal plate (BP), closing plate (Schlussplatte) (P), cavernous blood-spaces (>), the arteria advehentes (), and the marginal sinus. The placenta foetalis has grown into the placenta uterina; it consists of the membrana chorii (m~) and the villi (z) arising- from it ; on the latter are to be distinguished the roots of attachment (h\ /j, 2 ) and the free processes (/). [ejj, Foetal epithelium derived from the serosa.] The chorion is still covered internally by the amnion. (The foetal part of the placenta is reproduced in blue, the maternal part in black and brown ; pink indicates the blood-spaces.)
The connective substance of the chorionic villi is gelatinous tissue with stellate and spindle-shaped cells in the finer branches ; in the larger stems it takes on a more fibrillar condition.
The views of investigators are still at variance upon the important point whether the epithelium of the membrana chorii and the villi is of frptal or maternal origin. KOLLIKER, LANGHANS, LEOPOLD, and others derive it from the cells of the serosa, whereas ERCOLANI and TURNER, whom BALFOUR has followed in his text-book, state more or less explicitly that, although originally the cells of the serosa cover the villi as an epithelium, during the mutual intergrowth of the placenta fetalis and the placenta uterina they perish, and are replaced by proliferating cells of the decidua serotina.
The recent investigations of KASTSCHENKO and SEDGWICK MINOT, as well as the observations of WALDEYER, KUPFFER, GRAF SPEE, and KEIBEL, afford much enlightenment on this controversial subject.
KASTSCHENKO, who has most carefully investigated the epithelium of the chorion frondosum in the different months of pregnancy, and with whom recently S. MINOT essentially agrees, can readily distinguish two layers : (1) a cell-layer (LANGHANS), which lies immediately upon the gelatinous substance of the villi and the connective-tissue membrana chorii, and in which the limits of some of the cell-territories may be made out, and (2) a multinuclear protoplasmic layer, in which separate cells cannot be demonstrated in any manner. These layers are rather sharply contrasted from each other.
The double-layered chorionic epithelium is already distinctly present in eggs four weeks old, as is confirmed by KUPFFER, GRAF SPEE, and KEIBEL. The deeper layer consists of a single sheet of well-marked cubical cells ; the outer layer discloses at the free surface a striated border, the significance of which is obscure.
In the following months the chorionic epithelium undergoes noteworthy alterations. The deeper layer becomes thickened in many places into special cell-patches, in which the elements are much superposed. The outer, protoplasmic layer changes still more ; it is converted into a hyaline, peculiarly lustrous substance, which is traversed by numerous fissures and spaces, and has therefore received from LANGHANS the name " canalised fibrin."
There is one conclusion that in my opinion results from these investigations : the view of TURNER, according to which the chorionic epithelium is replaced in the course of pregnancy by uterine epithelium, must be abandoned. The chorionic epithelium, which is derived from the serosa, is preserved ; it constitutes in any event the deeper layer, composed of epithelial cells, which lies immediately on the membrana chorii or the gelatinous tissue of the villi. Perhaps there belongs to it in addition the so-called protoplasmic layer and the canalised fibrin. However, the source and significance of these structures, especially the latter substance, appear to me to be les^ satisfactorily explained, and to be in need of still further investigations, in which the question of its origin from the maternal mucosa is not to be overlooked. For even if TURNER has erred in regard to the degeneration of the chorionic epithelium, he is probably in the right in the second point, that the whole surface of the chorion frondosum is directly invested by a layer of maternal tissue.
The connective-tissue framework of the chorion frondosum, then, is provided, as I think must be assumed, with a double investment : (1) with a foetal epithelium, derived from the serosa, and (2) with a layer, however thin it may be, of maternal tissue.
I shall endeavor to establish this view in now turning to the discussion of the placenta uterina, the structure of which likewise presents great difficulties, and is therefore interpreted in very different ways.
The placenta uterina is developed out of the part of the uterine mucosa designated as decidua serotina (fig. 148 Dse). At birth it detaches itself, like the corresponding part of the decidua vera, from the inner surface of the womb at the line of separation shown on Plate II., by the breaking down of the thin connective-tissue septa of the underlying spongy layer. It then forms a thin membrane of only 0-5 to 1 mm. thickness, the basal plate of WINKLER (Plate II. BP], and forms a complete investment over the placenta fcetalis, which it covers up at the time of the detachment of the foetal membranes. At the margin it is directly continuous with the vera and reflexa (fig. 148).
The surface turned toward the wall of the uterus is divided by deep furrows into separate divisions. Larger and smaller partitions, the septa placenta (figs. 139 and 143), corresponding in position to the furrows, arise from the opposite surface of the membrane and penetrate in between the chorionic villi (fig. 143 z) ; they always unite a small number of these into a tuft or a cotyledon. If we imagine the cotyledons wholly removed, there would be formed in the placenta uterina a corresponding number of irregular compartments. These are in turn subdivided into smaller and more shallow compartments by finer connective-tissue outgrowths from the membrane and the septa.
The edges of the septa do not reach to the roots of the villi in the middle of the placenta, but only in a narrow peripheral region, where they come into immediate contact with the membrana chorri (Plate II. in), and are joined together underneath it into a thin, closely applied membrane, which is pierced by the roots of the villi. This has been called by WINKLER closing plate (Schlussplatte, SP), by KOLLIKER decidua placeiitalis subchorialis. Still more appropriate is the term employed by WALDEYER, subchorial terminal ring (Schlussring), because it is thereby stated that the membrane in question is present only at the margin of the placenta, leaving the middle area of the chorion free.
The connective-tissue framework of the placenta uterina possesses in general the properties of the compact, abundantly cellular layer of the decidua vera and reflexa, but exhibits one peculiarity in the presence of a very special form of cells, the so-called giant cells. These are large masses of protoplasm appearing yellowish grey, and with from ten to forty nuclei ; they begin to develop in the fifth month, and are found in the after-birth in great numbers ; they lie partly in the basal plate, partly in the septa, ordinarily in the immediate vicinity of large blood-vessels; but they are also found isolated in the spongy layer of the decidua serotina and even between the adjacent muscle-bundles of the uterus.
The greatest difficulties in the investigation of the placenta uterina are caused by its blood-courses. Numerous spirally twisted arterial stems (Plate II. a) penetrate through the muscular layer of the womb, and, passing through the spongy layer, reach the basal plate of the placenta uterina, where their structure undergoes important changes. For they here lose their muscular layer, and now appear as large tubes, lined with endothelium only. From the basal plate they penetrate in part into the septa placentae. From here they are not to be followed further as closed vessels ; a transition to capillaries does not take place anywhere. On the contrary, it can be proved that through openings in the basal plate and the septa they pour their blood into a system of cavities between the chorionic villi, i.e., into the intervillous or intraplacental spaces (c). The latter are bounded on the one side by the membrana chorii (m) with its villi (z), on the the other side by the basal plate (BP] with its septa.
The blood is collected from this system of cavernous spaces into large veins, which are likewise simply tubes lined with endothelium.
These are distributed as a network in the septa, as well as in the basal and closing plates of WINKLER, and they begin with narrow openings, which connect with the intervillous spaces. At the margin of the placenta they are joined together, and thereby produce the marginal sinus (Plate II.), or the ring-like sinus of the placenta. This, however, is not to be regarded as a vessel of uniform calibre, but as a system of irregular spaces joined together.
In virtue of the conditions described, the chorionic villi are directly bathed by the maternal blood. At the same time, from what has already been said, it is to be seen that the motion of the blood is retarded, owing to the great enlargement of the blood-courses, and that it is irregular, corresponding to the form of the intervillous spaces. In general the motion of the blood is from the middle and from the convex side of the placenta, where the arteries chiefly enter, toward its concave surface and its margin.
The question as to the significance and the origin of the intervillous blood-spaces constitutes the key to the comprehension of the structure of the placenta.
According to one view, which for a long time was the dominant one in Germany, and is defended by KOLLIKER, LANGHANS, and others, the intervillous spaces originally have no connection with the maternal blood-system. Developmentally they are nothing but spaces between chorion and uterine rnucosa, and owe their existence to the fact that the two structures have not everywhere come in contact, but have acquired firm connection only by means of the tips of the villi. The spaces in the earliest stage would be bounded by the epithelium of the villi and the maternal mucosa. LANGHANS therefore designates them as placental sj)aces. According to this view they would acquire their blood-contents later only, and in this way, as KOLLIKER expresses it : " The proliferating chorionic villi everywhere corrode, and in part destroy the maternal placental tissue, and thus produce an opening of their vessels, which must naturally lead to a gradual penetration of the maternal blood into the intervillous spaces."
This view has been modified by other observers (BRAXTOX HICKS. AHLFELD, HUGE, and others) to this extent, that the intervillous spaces, even in the mature placenta, do not normally contain blood nor have connection with the maternal blood-vessels. The almost universally received views concerning placental nutrition are thus called in question. The denial of a regulated blood-circulation has induced the further hypothesis, that a uterine milk, as in the Ruminants, is secreted by the cells of the decidua serotina into the intervillous spaces, and is taken up by the foetal villi.
According to the second diametrically opposite view, which finds its defenders in VIRCHOW, TURNER, ERCOLANI, LEOPOLD, WALDEYER, and others, the intennllous spaces are not/line/ else than the enormously enlarged capillary blood-vessels of the maternal mucosa. Chorioii and decidua serotina early unite very intimately by means of their surfaces, so that no fissures are left between them. The villi grow into the mucous tissue, the superficial capillaries of which enlarge to capacious spaces.
If this view is correct, the chorionic villi will necessarily be surrounded on all sides by thin coverings of maternal tissue, 01-, since a partial degeneration of the covering would certainly be possible, there will of necessity be at least a stage in the development in which such a covering will be demonstrable.
ERCOLANI, ROMITI, and TURNER have in fact, as has been previously stated, expressed themselves to the effect that probably the epithelial layer resting upon the connectivetissue axis of the villi is not the original chorionic epithelium derived from the serosa, but a covering which arises from the decidua placentalis a view the untenableness of which has already been shown.
In the diagram which TURNER has sketched to illustrate his view of the structure of the human placenta (fig. 149) the real original villous epithelium is degenerated.
The cell-layer e' is the epithelium of the uterine mucosa, into which the villons tufts (F) have grown, and with which the most intimate contact everywhere prevails. Outside the epithelium TURNER describes in addition a thin membrane (:>), which he interprets as an exceedingly thin connective-tissue layer, upon which is probably to be found an endothelial covering which lines the blood-spaces. The cords indicated by /, are connective-tissue strands of the maternal inucosa, which join the tips of certain f<i>tal villi with the septa placenta* (ds), b} T which the origin of the so-called attachmentroots (Haftwurzeln) is explained. The great blood-spaces <!' are simply enormously enlarged, superficially located capillaries of the mucosa.
Fig. 149. Diagrammatic representation of the finer structure of the human placenta, after TURNER.
F, Placenta fcetalis ; M, placenta uterina ; ca, tortuous artery ; tip, vein which conducts the blood away from the intervillous maternal blood-sinus ((/') ; x, a continuation of the maternal tissue over the villi : this lies outside the layer e' (the metamorphosed epithelium of the uterine mucosa), and is probably a connect! veti^ue membrane with vascular endothelium ; t, cords of the placenta uterina, which unite with the tips of some of the foetal villi (Haftwurzeln) ; ds, decidua serotina of the placenta.
The exact determination of the true state of affairs is coupled with great difficulties.
However, it seems to me that the second of the two hypotheses cited, according to which the intervillous spaces are the enlarged maternal capillaries, is the more probable because the more natural, and the following facts especially appear to me to favor it :
- From a comparative-anatomical point of view it can be maintained that in all Mammals where a special adaptation to intra-uterine nutrition is developed, the epithelial surfaces of the chorion and the mucous membrane of the uterus lie directly on each other, and with the increase of surface produced by the formation of folds effect mutual ingrowth. An intra-placental fissure, such as LANGHANS and KOLLIKER assume for Man, is found nowhere else among Mammals. We also see in some instances how the capillaries of the uterine mucosa become enlarged and acquire attenuated walls (Rodents, Carnivora, etc.), so that the foetal villi are almost directly bathed in maternal blood. The enlargement of the blood-courses in Man may therefore be regarded as a further elaboration of an already existing arrangement.
- That capillaries become metamorphosed into a cavernous system is also realised in other parts of the human body (corpora cavernosa of the sexual organs), whereas the employment of spaces lying outside the blood-courses as component parts of the vascular system ivould be a phenomenon without analogy.
- In the placenta uterina the capillaries originally present are wanting between the arteries and veins, whereas they ought to be demonstrable, if they have not been converted into the intervillous spaces.
- The exposition which LEOPOLD has given of the development of the placenta in the second month of pregnancy favors the second of the hypotheses cited. " Yilli and the tissue of the decidua," he says, " become shoved into each other, as one can interlock the outspread fingers of the two hands. If now the blood-vessels of the serotina be followed, one will recognise here the greatly enlarged capillary network of the surface, upon which the egg comes to lie when it lodges. But its innumerable vessels apparently continue with the sprouts of the decidua to grow toward the villi, and become distended and more voluminous ; on the other hand the villi increase rapidly in size, and thus it is intelligible that the new branches of the villi, whose stems have, as it were, sucked themselves fast in the decidua by means of their tips, at once encounter the enlarged capillaries of the surface, and press forward against these and break into them."
The weightiest objection that can be brought against this interpretation is the assertion of many investigators that the chorionic villi are not covered with a mantle of maternal tissue, and that the inter villous spaces are not lined with vascular endothelium. However, it is precisely upon this point that more exhaustive and especially ontological investigations are desirable. For one is not at liberty to draw conclusions from the conditions of delivered' placentae, since degeneration may have taken place. Moreover TURNER and LEOPOLD claim to have demonstrated endothelia at certain places of the intervillous spaces. But especially decisive here appear to me to be, first, the important investigations which WALDEYER has recently published upon the placental circulation in Man, and, secondly, KEIBEL'S very noteworthy preliminary communication upon the embryology of the human placenta.
WALDEYER has injected the maternal blood-vessels of placentae which still possessed their normal attachment to the uterus, and has prepared sections through the hardened organ. He finds that the intervillous spaces are nothing else than the enormously enlarged maternal blood-vessels, and that at many places there is still present outside the villous epithelium a layer of flat cells, which he is inclined to interpret as vascular endothelium. He appropriately compares the intrusion of the chorionic villi into the intervillous blood-spaces with the ingrowth of the arachnoideal villi into the blood-sinus of the dura mater, carrying before them imaginations of the endothelia 1 covering of the latter.
KEIBEL has investigated by means of sections a well preserved and prepared human embryo, which was in about the middle of the fourth week. He saw the villi (fig. 150 /?), which were provided with numerous secondary sprouts and were clothed in a two-layered chorionic epithelium, already attached by their tips in the maternal tissue (attachment villi), and also the, intervillous spaces filled with maternal Hood. But this was distinctly separated from the chorionic epithelium by a special thin cellular membrane (A 1 ). This membrane consisted of very thin endothelial cells, and was frequently elevated more or less from the chorionic villi, probably owing to the method of preparation. KEIBEL justly concludes from the establishment of the existence of an endothelial membrane that the intervillous spaces are the enormously dilated maternal capillaries.
Between the chorionic epithelium and the walls of the maternal chorionic blood-vessels capillaries KEIBEL finds no further remnant of maternal tissue in the very young ovum. This would indicate an early and complete disappearance of the uterine epithelium, and would make it probable that the protoplasmic layer and the canalised fibrin described at p. 261 are to be derived from the cell-layers of the chorion, a mooted point concerning which I have been unable to form a definite opinion. Thus the observations are increasing which favor a special limitation of the intervillous spaces and the existence of a thin layer of maternal tissue, a vascular endothelium, upon the villi.
Fig. 150. Diagram of the structure of the human placenta from an embryo four weeks old, after KEIBEL. Z, Chorionic villi ; $/;>, attachment of the tips of the same in the maternal decidua (Z>) ; C, enlarged maternal blood-capillaries.
The Umbilical Cord
The umbilical cord (funiculus umbilicalis) constitutes the union between the placenta and the embryonic body (fig. 143). It is a cord about as thick as the little finger (11-13 mm, or 0.5 inch), and attains the considerable length of 50 to 60 cm. (20-24 inches). It almost always exhibits a very pronounced spiral twist, which, regarded from the embryo, runs usually from left to right.
There are often knot-like thickenings of the umbilical cord, which may be due to either of two causes. For the most part they are due to an increased growth here and there in the connective -tissue matrix of the cord (false knots). More rarely they are formed by a knotting of the cord, which results from the fact that the embryo, in the motions which it executes in the amniotic fluid, accidentally slips through a loop of the cord and then gradually tightens it into a knot. The thickening then presents, in distinction from the other, a true knot.
The attachment of the umbilical cord to the placenta ordinarily takes place in or near its middle (insertio centralis). However, exceptions to the rule are not rare. Thus one distinguishes in addition an insertio marginalis and an insertio velamentosa. In the first case the umbilical cord unites with the margin of the placenta ; in the second place it does not reach the placenta at all, but attaches itself at a lesser or greater distance from the margin of the latter, to the foatal membranes themselves, and sends out from that point the outspreading large branches of its vessels to the placenta.
Man is distinguished from almost all of the remaining Mammals by the possession of a long slender umbilical cord. Its condition in Man results from the great distension of the amniotic sac. Whereas this at first lies close upon the body of the embryo, it subsequently becomes so distended (compare fig. 144 with fig. 143) that it fills the whole cavity of the blastodermic vesicle and everywhere clings closely to the inner surface of the chorion. Owing to this, the remaining structures the yolk-sac with its blood-vessels, the slender canal of the allantois with its connective-tissue envelope, and the umbilical blood-vessels -which emerge through the dermal navel of the embryo into the extra-embryonic body-cavity and betake themselves to the chorion, become more and more hemmed in by the amnion, and finally are crowded together into a small cord.
At first the umbilical cord is short, since it pursues a straight course in uniting the navel of the embryo to the foetal membranes ; afterwards it becomes greatly elongated and folded in the amniotic fluid.
Its structure varies at different times during pregnancy corresponding to the changes which the yolk-sac and the allantois with their blood-vessels undergo.
I shall give a detailed description of its finer structure for the end of pregnancy only, and shall consider especially the following parts : (1) the gelatin of WHARTON, (2) the umbilical vessels, (3) the remnant of the allantois, of the vitelline duct, and of the vasa omphalomesenterica, (4) the amniotic sheath.
(1) The gelatin of WHARTON forms the common matrix in which the remaining parts are imbedded. It is a gelatinous or mucous tissue. In this soft gelatinous substance there run strands of connectivetissue fibrillse and elastic fibres, which are the scantier the younger the umbilical cord. They are joined together into a network, the meshes of which are narrower at some places than at others. In this way there are formed in the gelatin numerous firm peculiarly differentiated strands. The cells of the gelatinous connective tissue are partly spindle-shaped, partly stellate elements, the latter with widely branching processes.
(2) The umbilical blood-vessels consist of two large arteries (art. umbilicales), which conduct the blood from the embryo to the placenta, and a capacious vena umbilicalis, in which the blood flows back to the embryo after having traversed the placental circulation. The two arteries are wound spirally, like the umbilical cord itself, and are joined to each other by an anastomosis near their entrance into the placenta. They are very contractile, and exhibit a thick muscular membrane (tunica muscularis), consisting of circular and longitudinal fibres.
(3) The canal of the allantois and the vitelline duct, which are essential components of the umbilical cord during the first months of pregnancy, subsequently undergo reduction, and are present at the end of embryonic life only in the form of insignificant remnants, as has been shown by KOLLIKER, AHLFELD, and HUGE. The canals lose their lumens; there then exist in the gelatin of WHARTON solid cords of epithelial cells ; finally, these also disappear in part, so that only here and there strands and nests of epithelial cells have been preserved. The vitelline blood-vessels (vasa omphalomesenterica), which have a role to perform at the beginning of development, soon become inconsiderable, and diminish more and more in comparison with the enlarging umbilical blood-vessels. In the mature umbilical cord they are very rarely to be demonstrated (AHLFELD); usually they have wholly degenerated.
(4) At the beginning of development the amnion forms around the allantoic canal and the vitelline duct a sheath, which can be removed. Afterwards the sheath is firmly fused with the gelatin of WHARTON, except at the attachment at the navel, where for a short distance it may be peeled off as a special thin membrane.
Condition of the Footed Membranes during and after Birth
As a conclusion to the account of the foetal membranes some further remarks may be in place regarding their history at birth.
At the end of pregnancy, with the beginning of labor pains, the foetal membranes, which form a fluid-filled sac surrounding the embryo, are ruptured as soon as the contractions of the musculature of the uterus have reached a certain degree of intensity. The rupture ordinarily arises at the place where the wall of the sac is pressed out through the mouth of the uterus (rupture of the anmion). In consequence the amniotic water now flows away.
With the continuation and increase of the pains, the child is next forced out of the uterus through the rupture in the foatal membranes it is born, whereas the placenta and embryonic membranes usually still remain behind for a short time in the cavity of the uterus. Immediately after birth the union between child and foetal membranes has to be artificially interrupted, by the tying and cutting off of the umbilical cord at a little distance from the navel.
Finally, the foetal membranes with the placenta are detached from the inner surface of the uterus, and with renewed pains are discharged to the outside as the after-birth.
The separation takes place in the spongy layer of the decidua vera, approximately in the region which is designated as the line of separation in the diagram given by LEOPOLD (Plate II.). The after-birth is composed of both foetal and maternal membranes, which are quite firmly grown together: (1) the amnion, (2) the chorion, (3) the decidua reflexa, (4) the decidua vera, (5) the placenta (placenta uterina and placenta foetalis). Notwithstanding the growing together, a partial separation of the individual membranes from each other is still possible.
After birth the inner surface of the uterus is one great surfacewound, since by the detachment of the placenta and the deciduee numerous blood-vessels are ruptured. Also during the first days of childbed fragments of the spongy layer of the decidua vera and serotina, which remained behind at birth, continue to be detached from it. Only the deepest layer of the mucosa, that immediately in contact with the musculature of the uterus, is retained. This still contains remnants of the cylindrical epithelium of the uterine glands, as has been already stated. In the course of several weeks it is again converted, by an active process of growth, into a normal mucous membrane, whereby its superficial epithelium probably arises from the preserved remnants of the glandular epithelium.
- The human ovum establishes itself ordinarily at the base of the uterus (fundus uteri), between the mouths of the two Fallopian tubes, and becomes overgrown by folds of the mucosa and enclosed in a capsule.
- The mucous membrane of the uterus is developed into the maternal- 'envelopes of the ovum, the deciduse, which are distinguished as decidua serotina, reflexa, and vera.
- The decidua serotina is that part of the mucous membrane upon which the ovum immediately lies after its entrance into the uterus and on which the placenta is afterwards developed.
- The decidua reflexa is the part that grows around the ovum. (c) The decidua vera arises from the remaining portions of the mucous membrane lining the uterus.
- In the formation of the deciduse or deciduous foetal membranes the uterine mucosa undergoes profound alterations of structure, and, accompanied by a rapid growth of the uterine glands and a partial disappearance of its epithelium, becomes differentiated into an inner compact and an outer spongy layer.
- Out of the wall of the blastodermic vesicle, so far as it is not employed in the formation of the embryo itself, are developed the foetal envelopes of the offspring, which in the main agree with the foetal envelopes of the remaining Mammals in number and the method of their development, but which present in detail important modifications, which are essentially as follows :
- The anmioii is closed from before backward, remains united at the hinder end of the embryo with the serosa (subsequently the chorion) by means of a short pointed prolongation, and thus contributes to the formation of the so-called belly-stalk of human embryos.
- The allantois does not grow as a free sac into the extraembryonic part of the body-cavity, but, in the form of a narrow canal, shoves itself along the under surface of the pointed amniotic prolongation to the chorion, and thus furnishes the chief component of the belly stalk.
- The yolk-sac (umbilical vesicle) is reduced to an exceedingly small vesicle, and is connected with the embryonic intestine by means of a long thread-like stalk, the vitelline duct.
- By the enlargement of the amnion, which at length fills the entire blastodermic vesicle (increase of amniotic fluid), the canal of the allantois and the vitelline duct, together with the umbilical and vitelline blood-vessels, become completely enveloped by the amniotic sheath ; in this way is formed the umbilical cord (funiculus umbilicalis), a cord-like connection between the inner surface of the egg-membrane and the navel of the embryo.
- The serosa at a remarkably early period (second week) develops villi over its whole surface, and by the ingrowth of the connective tissue of the allantois into the latter it becomes the villous membrane (chorion).
- The villous membrane is differentiated into a chorion laeve and a chorion frondosum :
- The part which lies in contact with the decidua reflexa and is firmly united with it by means of villi which lag behind in growth becomes the chorion
- The region which abuts upon the decidua serotina, and in which the villi grow out into large, muchbranched tufts, is converted into the chorion frondosum.
- By the penetration of the villous tufts of the chorion frondosum into the decidua serotina and their firm union with it, there is formed an especial organ of nutrition for the embryo, the after-birth, or placenta.
- One distinguishes a foetal and a maternal part of the placenta : (1) the placenta fcetalis or the chorion frondosum, and (2) the placenta uterina or the original decidua serotina.
- The placenta fretalis consists First, of the membrana chorii, in wilich the chief branches of the umbilical blood-vessels spread themselves out, and to which the umbilical cord is attached, ordinarily in the middle (insertio centralis), rarely at the margin (insertio marginalis), still more rarely at a distance from the margin (insertio velamentosa)
- Secondly, of bundles of chorionic villi, the " attachment roots ' of which are firmly grown together with the uterine mucosa by means of their tips, whereas the free processes project into the cavernous blood-spaces of the placenta uterina.
- The placenta uterina, like the clecidua vera, is composed of a compact layer, which becomes detached at birth (pars caduca), and a spongy layer, in which the separation takes place, a part remaining behind on the musculature (pars iixa).
The compact layer (basal plate of WINKLER) sends partition- walls (septae placentae) between the chorionic tufts, and thereby divides them into separate bundles, the cotyledons. There are interpolated between the arteries and veins which run in the basal plate and the septa? enormously enlarged vascular spaces, in which the villi appear to hang free. The vascular spaces are probably extraordinarily distended maternal capillaries, in which case one may expect to find the chorionic villi invested by a very thin layer of maternal tissue (endothelial membrane), as is maintained by some investigators.
- At birth the decidtise or caducous membranes become detached from the uterus along the spongy layer, and together with the foetal envelopes and the placenta constitute the after-birth.
- In the first weeks after birth a normal mucosa is developed out of the remnants of the spongy layer left upon the musculature and the remnants of the uterine glands, from the epithelium of which the epithelium of the mucous membrane is probably regenerated.
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Text-Book of the Embryology of Man and Mammals: Description of the Sexual Products | The Phenomena of the Maturation of the Egg and the Process of Fertilisation | The Process of Cleavage | General Discussion of the Principles of Development | The Development of the Two Primary Germ-Layers | The Development of the Two Middle Germ-Layers | History of the Germ-Layer Theory | Development of the Primitive Segments | Development of Connective Substance and Blood | Establishment of the External Form of the Body | The Foetal Membranes of Reptiles and Birds | The Foetal Membranes of Mammals | The Foetal Membranes of Man | The Organs of the Inner Germ-Layer - The Alimentary Tube with its Appended Organs | The Organs of the Outer Germ-Layer | The Development of the Nervous System | The Development of the Sensory Organs | The Development of the Skin and its Accessory Organs | The Organs of the Intermediate Layer or Mesenchyme | The Development of the Blood-vessel System | The Development of the Skeleton
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