Book - Uterus And Embryo - Man (1889)

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Minot CS. Uterus And Embryo - I. Rabbit II. Man. (1889) J Morphol. 2:

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This historic paper by Minot published in 1889 shows the development of mammalian animal models to study embryonic development.
Also bu this author:

Minot Links: Harvard Collection | 1889 Uterus And Embryo - Rabbit | 1905 Harvard Embryological Collection |1897 Human Embryology | 1903 A Laboratory Text-Book of Embryology | 1905 Normal Plates of Rabbit Embryo Development | Category:Charles Minot

See also his later volume in the series on normal rabbit plates in this series:

Normal Plates Series: 1897 Pig | 1900 Chicken | 1901 Lungfish | 1904 Sand Lizard | 1905 Rabbit | 1906 Deer | 1907 Tarsiers | 1908 Human | 1909 Northern Lapwing | 1909 South American and African Lungfish | 1910 Salamander | Franz Keibel | Embryology History

Modern Notes:

Rabbit Links: 2009 Student Project | Category:Rabbit | Animal Development
Historic Embryology - Rabbit 
1889 Uterus and Embryo | 1905 Normal Plates | 1905 limb veins | 1908 Pancreas | 1908 Pharyngeal Pouches | 1908 intestinal diverticula | 1909 Lymph glands | 1918 Pituitary | 1929 ovulation | 1931 prochordal plate | 1935 Oocyte | 1935 Somites | 1964 Placentation
Historic Disclaimer - information about historic embryology pages 
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Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

Man Uterus And Embryo

--Mark Hill (talk) 12:40, 4 April 2014 (EST) The term "Man" designates "human".

The following observations are of a fragmentary character, but may serve to round out our information in certain respects. Some of the facts have already been recorded in the series of numerous embryological articles contributed to Dr. Buck's Reference Handbook of the Medical Sciences ; but as that work is for consultation rather than the publication of original observations, it will hardly seem a mistaken repetition if I include here some things already published there.

Allantois and Umbilical cord

Prof. W. His has shown that the entodermal cavity of the allantois is the terminal stretch of the entodermal canal ; the posterior end of the body is prolonged into a mass to which he gives the name of "Bauchstiel" {Anatomic tnenschlicher Embryonen, III., 222— 226), and which develops in the same general manner as the body proper, having a rudimentary medullary groove, a somatopleure and splanchnopleure, Cut 2. It is morphologically the hind portion of the body. After its closure and separation from the amnion it appears as the umbilical cord. Its development requires that the umbilical cord should be covered, not by the amnion, as it is almost universally stated, but by an extension of the foetal epidermis. Histological examination shows that this is the case. The amnion is characterized by the ectoderm remaining a single layer of cuboidal or low cylinder cells, and by the matrix of mesoderm being distinct, owing to its high refrangibility. The foetal skin is characterized by the ectoderm becoming many layered, while the cutis remains for a long time undifferentiated from the mesoderm below, and the matrix is of low refrangibility. In comparing the ectoderm of the umbilical cord with the skin, therefore, we do not expect to find any differentiated cutis. The epithelium of the cord is at first, of course, single layered, the condition which is permanent over the amnion. In the cord of a three-months embryo. Cut 3, I find the two-layer stage. The outer layer is granular, and in some parts each cell protrudes like a dome. Dome cells also appear on the young epidermis, and as I learn from Dr. J. T. Bowen, who has been investigating the subject in my laboratory, are probably the precursors of the epitrichium. The cells of the inner layer are larger and clearer than those ^outside. By the fifth month the epithelium is distinctly stratified, and the superficia.1 layers consist of flattened cells similar to those of the horny layer of the skin at an early stage. The ectoderm of the cord agrees therefore entirely with that of the embryo proper in its general development, but the differentiation proceeds more slowly, so that at any given age the ectoderm of the cord is at a less advanced stage than that of the embryo.

Minot1889 fig02.jpg

Cut 2. — Diagrammatic section of the Bauchstiel of a human embryo, modified from W. His. Am, amnion; md, medullary groove; v, v, veins; A, A, umbilical arteries; All, allantois; coe, coelom.

Minot1889 fig03.jpg

Cut 3. — Epithelial covering of the umbilical cord of an embryo of three months. X 545 diams.

The appearance of the cord in cross-sections is instructive. Cut 4, A, is a section through a cord of sixty days ; the right umbilical vein is already aborted ; the coelom, coe, is a large cavity, and contains the yolk stalk, Y.S, with its two vessels, and its entodermic cavity entirely obliterated. Near the embryo the coelom may become much enlarged, and is often found during the second month and even later to contain a few coils of the intestine, as has been long known. Above the body cavity is the duct of the allantois, All, lined by entodermal epithelium ; and in this region are situated the two arteries and single vein ; the section is bounded by ectoderm. ^ The further development of the cord depends upon three factors : 1°, the growth of the connective tissue and blood-vessels ; 2°, the abortion of the coelom, yolk stalk, and allantoic duct in the order named ; 3°, differentiation of the connective tissue and of the ectoderm.

Minot1889 fig04.jpg

Cut 4. — Two sections of umbilical cord. A, at sixty days; B, at three months; V, vein; Ar, artery; All, allantois cavity; Coe, coelom; Y, yolk sack ; X 22 diams.

Cut 5. — Connective tissue of the umbilical cord of an embryo of 21 X 540 diams., stained with alum-cochineal, and eosine.

Cut 6. — Connective tissue of the umbilical cord of a human embryo of about three months, X 511 diams. Stained with alum-cochineal, and eosine.

  • 1 The ectoderm is often wanting, owing to its frequent destruction post mortem.

The growth and differentiation of the mesoderm proceeds rapidly, encroaching upon the coelom, which is obliterated (early in the fourth month). At first the connective tissue, Cut 5, is composed merely of numerous cells embedded in a clear substance ; the cells form a complex network, of which the filaments and meshes are extremely variable in size ; the nuclei are oval, granular, and do not have always accumulations of protoplasm aboutthem, forming main cell bodies. I notice, also, a few cells which I suppose to be leucocytes, but see no other structures. By the end of the third month the cells have assumed nearly their definite form ; the protoplasm has increased in amount, and forms a large cell body around each nucleus. Cut 6. The network has become simpler and coarser, the meshes bigger, and the filaments fewer and thicker ; in the matrix are numerous connective tissue fibrils, not yet disposed in bundles, except here and there ; as they curl in all directions many of them are cut transversely, and therefore appear as dots. In older cords there is an obvious increase in the number of fibrils, and they form many bundles. In the cord at term the matrix contains mucin, and may be stained by alum haematoxylin ; at what period this re

Cut 7. — Section of the allantois from the umbilical cord of an embryo of three months; .?«/', entoderm; w^5, mesoderm. X 340 diams.

action is first developed I have not ascertained. I have observed nothing to indicate the presence of special lymph channels in the cord at any period, but I have not investigated the point. Tait's lymph channels are merely the intercellular spaces. The tube of allantoic entoderm increases very little in diameter after the second month ; compare A and B, Cut 4. It is very persistent, appearing usually even in the cord at full term, at least in the proximal end, according to Kolliker {Entivickhmgsges. 2te Aufl., p. 34). After the second month it is a small group of epithelioid cells, with distinct walls, irregularly granular contents, and round nuclei ; around the cells, ent, which may or may not show a remnant of the central cavity, there is a slight condensation, ines, of the connective tissue to form, as it were, an envelope. This structure has been regarded by Ahlfeld and others as the persistent yolk sack. I think the correct interpretation was first suggested by Kolliker.


The tissues of the amnion do not progress beyond an early embryonic stage ; the ectoderm remaining at the one-layered stage, the mesoderm preserving much of the primitive matrix. Emery {Arch. Ital. Biol., III., 37) has directed attention to the primitive homogeneous matrix of the vertebrate mesoderm, and especially to the separate sub-epidermal layer of the embryo, which contains no cells at first. In the human amnion there is a non-cellular layer under the epithelium, as is well shown in Cut 8, A and B. Sometimes this layer is invaded to a certain extent by connective tissue cells, B ; in other cases the portion of the matrix towards the chorion acquires a fibrillar character. A, as if partially resorbed, but in no case have I seen the matrix entirely altered from its primitive character. The cells of the mesoderm lie in lacunae ; they are flattened in the plane parallel to the surface, and hence in vertical sections, Cut 8, appear more or less fusiform. They present no special features, so far as I have observed, to distinguish them from other embryonic connective tissue cells. Their bodies have little affinity for coloring-matters, hence it is difficult to follow the processes by which the cells are united. Their nuclei are at first round or oval. After the third month they often show a great variety of alterations in shape and size, Cuts 9, 10 ; some of the nuclei are then very large, with a distinct net-work, d ; others are smaller and differ but slightly from the normal ; some are very irregular, b, and others again strangely elongated, a; many other forms beside those represented in Cut 9 are to be found. The changes indicated I consider of a degenerative character, and in fact many of the nuclei are breaking down, for one finds in some specimens every stage between a nucleus and scattered granules, — nuclei, nuclei with indistinct membranes, nuclei without membranes, masses of granular matter, clusters of granules crowded together, and finally other clusters more or less scattered. This degenerative process may be compared with that described by Phisalix {Arch. Zool. Expt., Ser. II., T. III., 382) as occurring in the blood cells of the spleen of teleosts. Compare also the chromatine degeneration observed by Flemming to occur in ova of the vertebrate ovary {His and Braime s Archiv. 1885, 221-244). In the human amnion the nuclear degeneration described is not always to be recognized so clearly, although the nuclei in all amnia older than three months, which I have observed, are more or less irregular and distorted. Finally it is to be added that not infrequently the cells form a distinct epithelioid layer upon the surface of the amnion next the chorion, as represented in Cut 8, B, a.

Cut 8. — Two sections of the placental amnion : A, from an embryo of the eighth month; B, at term; ect, ectoderm; 7?ies, mesoderm; a, layer of mesodermic cells. X 340.

Cut 9. — A natural group of nuclei from the mesoderm of the amnion of a foetus of the fifth month. X 1225 diams.

Cut 10. — Mesodermic nuclei of the amnion of an embryo of about four months. X 713 diam.

Cut 11. — Surface view of the amniotic epithelium of an embryo of 144 days; stained with alum — haematoxyline, and eosine. //, protoplasm; pr, intercellu-lar processes; nu, nucleus. X 1225 diams.

The epithelium of the amnion varies in appearance, as seen in transverse sections. Usually the cells are cuboidal or low cylinders, Cut 8, A, each one with a rounded top, in which is situated the more or less nearly spherical nucleus ; sometimes, however, the nuclei lie deeper down. Less frequently the epithelium is thin, Cut 8, B, and its nuclei, which are transversely elongated, lie further apart. It is probable that these differences are not structural, but conditional upon the greater or less degree to which the amnion is stretched. I have observed no constant differences between the placental and the remaining amnion. The most interesting peculiarity of the epithelium is best seen in surface views ; namely, the intercellular bridges. They display themselves with a clearness which I have never seen in other epithelia; see Cut ii.

The nuclei, nu, are relatively large, rounded with distinct outlines ; they have a more or less well marked intra-nuclear network, with thickened nodes, and a small number of deeply stained granules, which are probably chromatine. Each nucleus is surrounded by a cell body,//; and the adjacent cell bodies are separated from one another by clear spaces. With high powers, as represented in the cut, one sees that these spaces are separated from one another by threads of material, /r, stretching across as bridges, connecting neighboring cells. Examined attentively, the protoplasm of the cells exhibits a vacuolated appearance. One is thus led to view the epithelium as a sponge work of protoplasm somewhat condensed around each nucleus ; according to this interpretation the intercellular spaces are large meshes of the sponge work, and the intercellular bridges are protoplasmic. A recent paper ^ by M. Manille Ide, which I owe to the kindness of the author, brings a series of interesting observations to show that the intercellular bridges of the Rete Malpighi of the mammalian epidermis are not protoplasmic, but processes of the cell membranes. This paper has led me to reexamine my preparations of the amniotic epithelium, but I have been unable to find in them any indications of membranes around the cells or reasons for considering the intercellular bridges as other than protoplasmatic in constitution. Whether this result is due to the imperfection of my preparations, or is in accordance with the truth, must be decided by further investigation.

1 Manilla Ide, La membrane des cellules du corps muqueux de Malpighi. La Cellule, IV., 2me fasc, 1888,

Meola, 59, ascribes a much more complex structure to the amnion than his predecessors, in which he is followed by Viti, 21. Both of these authors subdivide the mesodermic stratum into three layers : a lamina comietivale, next the ectoderm, a sostanza intermedia, and a me77ibrana limitante. As to the histological details, Viti differs somewhat from Meola, but agrees with him in finding a histological distinction between the three layers enumerated. The extent to which I can distinguish three layers is indicated by the description of the mesoderm given above : I have been unable to find the marked structural differences affirmed by Viti. Viti's paper is to be commended for its excellent historical reviews, particularly for his summary of the various theories as to the origin of the amniotic fluid.


The human chorion has been the object of greater misconception than perhaps any other organ of the body. Even at the present time there prevail numerous false conceptions concerning it, nor do I know of any text -book which gives a satisfactory or even tolerably correct account of its structure. This singular confusion is not due to deficiency of observations, for from the vast literature of the subject (by trusting the accurate observers, such as Coste, Farre, Kdlliker, Turner, Langhans, Waldeyer, etc.), we may cull a fairly complete and exact history of the development of the chorion. But the literature of the chorion consists chiefly of papers of little value, and often remarkable for the gross crudeness of the observations they record, and for the proofs they are of their author's ignorance of other and better investigations. It appears that the anatomists and physiologists, by a species of tacit understanding much to be regretted, have regarded the uterus and placenta outside of their province, and have left the investigation of the anatomy and functions of these organs to gynaecologists and others, whose capacities have lain rather in the direction of medical practice than of original research, although among them are some notable exceptions. The majority of the practitioners who have written on the uterus and foetal appendages have done at least as much harm as good. It would be a sheer waste of time to subject this mass of literature to a critical revision in order to extract from it what little there may be of value. I have, however, read a large number of the articles, and studied those which seemed worthy of it. Upon this course of reading and a study of my own extensive material I have based the following history of the chorion, which passes briefly over what is known, and dwells upon what is founded on my own observations.

The human chorion as I have defined it ^ is " the whole of that portion of the extra-embryonic somatopleure, which is not concerned in the formation of the amnion." The human chorion is remarkable for its very early complete separation from the yolk sack, and for its precocious development of villi. Both of these developments had already taken place in His' youngest embryo, and in Reichert's ovum, which is supposed to be normal and the youngest known, there were chorionic villi, though no embryo was distinguished. Reichert's description is not satisfactory, his long memoir ^ being principally concerned with speculations.

The villi of the chorion, as shown long ago by the observations of Coste, are formed at first only by the ectoderm. I reproduce here Fig. 6, of PL II., referring to the human species from Coste's great work. The hollowness of the villi and their clumsy shape are to be especially noted. The mesoderm grows into the villi subsequently. The branches of the villi grow out in a similar manner, the process being led, as it were, by the ectoderm. Orth, in a special paper, 118, has used these facts to argue against Boll's Princip des Wachsthums. Kollman's observations^ on the growth of villi during the fourth week are particularly instructive. The outgrowth of the branches is very rapid and occurs with every degree of participation of the connective tissue. The two extremes are : i°, a bud consisting wholly of epithelium, which may become a process with a long, thin pedicle, and a thickened free end remaining entirely without mesoderm ; 2°, a thick bud with a well-developed core of connective tissue, and having a nearly cylindrical form. Between these extremes every intermediate state can be found. Other observers have noted this peculiar manner of growth, which I have found still going on in the placental chorion during the fourth month. Robin, 125, appears also to have crudely observed both the young hollow villi, and the solid epithelial buds. The blood-vessels he traces to the division of the cavity of the villi into an artery and a vein ; from the nature of things he offers no observations in support of this assertion.

1 Buck's Reference Handbook, Medical Sciences, — Art. Chorion.

^ Reichert, Berlin Akad. Abhandlungen, 1873.

3 Kollman, Arch. Anal. Physiol. Anal. Ablh., 1879, 297.

Cut 12. — Portions of the chorion of an embryo supposed to be about eighteen days; mes, mesoderm; ec, ectoderm; vi, hollow villi. After Coste.

Only the tips of the villi touch the surface of the decidua, either at first or subsequently, except of course, over the chorion laeve during the abortion of the villi. The tips of the villi are attached to the uterine surface ; they penetrate the decidua for a short distance, but even in the placental area at the close of gestation, the penetration is slight, and the villi make their way only into the surface stratum of the decidua serotina. There is no evidence of any sort that the villi penetrate the glands at any period. The relation of the villi to the decidua has now been so accurately ascertained, that there can be, I think, no longer any question whatsoever on this point. The best discussion is by Langhans, 110, p. 231 £f.

Cut 13. — Isolated terminal branch of a villus from the chorion of an embryo of twelve weeks.

The shape of the villi varies according to the part of the chorion and the age of the embryo. They gradually abort over the chorion laeve, and gradually grow over the chorion frondosum. Let us begin with the placental villi : At first they are short, thick-set bodies of irregular shape, as shown in Cut 1 2 ; at twelve weeks their form is extremely characteristic, Cut 13 ; the main stem gives off numerous branches at more or less acute angles, and these again, other branches, until at last the terminal twigs are reached ; the whole of the space between the chorion and decidua is occupied by these ramifications ; the

branches and twigs, as the illustration shows, are extremely irregular and variable, although in general they may be described as club-shaped, being more or less constricted at their bases. The branches may be bigger than the trunk which bears them, or of any less size ; some of the smallest are merely slender outgrowths of the epithelial covering of the villus, such as have already been alluded to. Gradually there is a change. During the fifth month we find the irregularity, though still very marked, decidedly less exaggerated, Cut 14; the branches tend to go off at more nearly right angles ; placenta of the fifth month. X 9 one finds very numerous free ends, diams. as of course only a small proportion of the branches touch the decidual surface ; the branches, too, are less out of proportion to the stems, less constricted at their bases, or, in other words, less remote from the cylindrical form ; the awkward cucumber shapes of the twelfth week are no longer found except here and there. The change continues in the same direction ; that is, is towards greater regularity of configuration. It is hardly necessary to describe the intermediate phases that have been examined, but it will suffice to describe the form at full term, Cut 15, when the branches are long, slender, and less closely set, as well as less subdivided, than at earlier stages ; they have nodular projections, like branches arrested at the beginning of their development ; there are numerous spots upon the surfaces of the villi ; microscopical examination shows that these spots are proliferation islands, as we may call them, or little thickenings of the ectoderm with crowded nuclei. It appears that not all the villi change to the slender form ; for some villi, having still the earlier, thicker form, are found even in the mature placenta, a fact already noticed by Jassinsky, 105, 346. These thick villi usually show also a distinct " cellular layer " in their ectoderm, a peculiarity to be considered below again. Seller, 131a, has given figures of the villi at various ages, but fails to show the characteristic forms. Langhans has observed the alteration in the villi, 110, 199, and even justly remarks that many of the villi in so-called " moulds " are not pathological, as they have been frequently considered, but normal young villi. The differences in the villi, according to age, are very conspicuous in sections. The sections should of course be made so that the fragments of the villi will remain in situ ; imbedding in celloidine is convenient for this purpose ; if this end be attained, one finds below the chorionic membrane numerous sections of villi ; if the specimen be a young chorion, — first to third month, — the villi are large, with a good deal of room between them ; their outlines are very irregular, and there are relatively few small branches (Cut i6). The older the specimen, the larger the proportion of small branches. In an old chorion — seventh to ninth month — the number of small villi of nearly uniform size is very striking (see the figure of a section through a placenta in situ, given in Cut 35).

Cut 14. — Villous stem from a

Cut 15. — Terminal villi of a placenta at full term. The little spots represent the proliferation islands of the covering epithelium.

Cut 16. — Section of the chorion at three weeks, a, layer of coagulum; b, mesoderm of chorion; Ep, epithelium, also extending over the villi; Vi and Vi '; the mesoderm, b, contains a number of blood-vessels, nearly all in transverse section. X 65.

The abortion of the villi of the chorion lasve takes place by an arrest of development and a subsequent slow degeneration of the tissues, which lose all recognizable organization in the protoplasm, and to a large extent of the nuclei ; at the same time they alter their shape (Cut 17), becoming more and more filamentous ; by the fourth month only a few tapering threads, with very few branches, remain. The villi disappear almost completely from the Iseve, except near the edge of the placenta, where they are to be found, even in the after-birth, imbedded in the degenerated epithelium of the chorion and the upper layers of the decidua, as shown in Cut 25, vi, the epithelium and decidua being so fused at this point that it is impossible to determine any line of demarcation between them.

Cut 17. — Aborting villus chorion of the second month.

Cut 18. — Section of the chorionic membrane of an embryo of three weeks; stained with osmic acid; mes, mesoderm; ect, ectoderm; a, outer, b, inner layer of ectoderm. From a section prepared by Prof. Theodor Langhans. X 445 diams.

The chorion, being a portion of the somatopleure, consists, of course, of two primary layers, the mesoderm and ectoderm. During the second half of the first month, the earliest period concerning which we have any accurate knowledge, the mesoderm is already a vascular layer of considerable thickness (Cuts 16 and 18, mes), and the epithelium (ectoderm) has two layers of cells (Cut \%, a and h) ; of which the outer is the darker in specimens stained with osmic acid, carmine, cochineal, or hsematoxyline, and has also smaller and more granular nuclei. The same distinction exists in the two-layered stage of the ectoderm of the umbilical cord (Cut 3), and of the foetal skin. Hitherto most authors have entirely overlooked the inner layer at early stages. It was first clearly recognized by Langhans, who directed attention to it in a special memoir. 111, he having already described its later history, 110. In some earlier writers are allusions to the layer. Kastschenko, in his paper on the chorionic epithelium, has also described it, although he has not followed its history very far. The interpretation to be offered seems to me clearly to be, that the chorionic epithelium advances in its differentiation to a stage equivalent to the two-layered stage of the epidermis and there stops ; whatever further change occurs is degenerative.

The two primitive layers of the chorionic epithelium have been more or less clearly observed at later stages by several anatomists, and have been variously interpreted. Ercolani and Turner regard them as absolutely distinct, assigning the deep layer to the chorion as its true and only epithelium, and the outer layer to the uterus, thus enabling themselves to conceive the villi as covered by a maternal as well as a foetal epithelium, so that maternal blood found between the villi is still within the maternal tissue. After accepting the outer layer as maternal, the question as to its origin still remained. Some authors affirmed it to be the uterine epithelium, others to be the lining of expanded uterine blood sinuses. So far as I am aware, no one has made observations to show by the developmental history of the layer, that one or the other of the last mentioned hypotheses is correct. When we consider the precision and exactitude of Kastschenko's observations, which actual specimens enable one to verify, there is in my judgment no reason left for differing from the conclusion that both layers are parts of the foetal ectoderm.

Governed by the difficulty of accounting for the presence of maternal blood in the intervillous spaces, and therefore apparently outside the maternal tissues, several investigators have been led to seek for at least an endothelium outside the chorionic epithelium. Some authors, as for instance Winkler, have asserted the existence of such an endothelium, but after a prolonged and careful search, I fail to find anything of the kind, and in this result it seems to me the best observers are agreed.

The conclusion, I think, may now be safely formulated that the chorion is covered externally by the foetal ectoderm, and has no other covering in any part except, of course, where the chorion laeve rests upon the surface of the decidua, and where the tips of the villi touch the serotina ; but the morphological distinction holds, and the decidua is no more the covering of the chorion, than are clothes morphologically the covering of the body. I believe further, on grounds stated below, that the conclusion just formulated holds true of the chorion at all periods.

The further history of the chorionic mesoderm is so fully given by Langhans in his invaluable memoir, 110, and Kastschenko, 107, that there is little to be added. In the earliest stage I have been able to examine, an ovum of the third week, the matrix of the chorionic connective tissue, in a preparation stained with cochineal or haematoxyline, and imbedded in paraffine for cutting, appeared hyaline and glistening, owing to its refrangibility (Cut 19) ; it has lacunae in which the cells lie ; the cell bodies are either shrunken or colorless, so that lacunae, except for the staining of their contained nuclei, are clear and light. This appearance I find again in specimens a little older. The image is entirely distinct from that of the same layer later, for then the cells are stained darker than the matrix, which at the same time has lost its homogeneous character, and acquired a fibrillated look. Very different from my own sections are several which I owe to the kindness of Professor Langhans of Bern, and which that distinguished investigator informs me are from a three-weeks ovum, which had been preserved in osmic acid (see ante, Cut 18). In Professor Langhans' preparations the cells are all stained much deeper than the matrix ; they have an elongated form, and run in various directions more or less parallel to the epithelium ecto ; hence many of them are cut transversely or obliquely. Whether the differences noted are due to the methods of preparation must be decided by preserving the same chorion in part with osmic acid, in part with Muller's fluid or picrosulphuric acid, the latter being the reagents I have used. In specimens of the tenth week, the matrix of the chorionic mesoderm has quite altered in character, being no longer homogeneous, and at the same time it has increased in thickness. For the most part the matrix stains lightly, and where it is lighter it contains fibrils of extreme fineness, and running curly courses ; there are also streaks of lightly stained matrix, giving the impression of fibres resulting from portions of the primitive colorable matrix being left. In other parts of the layer the primitive matrix is still present, and we find a homogeneous well-colored basal substance, the cell lacuna; of which appear light by contrast, as in Cut 19. One can distinguish also the commencement of the perivascular coats, at least of the larger vessels, the matrix being quite dense around them, and the cells elongated almost into fibres, and possessing a slightly increased affinity for coloring-matters. The larger blood-vessels and unmetamorphosed part of the layer occupy a middle portion between the two surfaces, but the smaller blood-vessels lie near the ectoderm (compare Cut 19, i^, thus presaging the formation of Langhans' vascular layer {GefdssschicJit). The development of the mesoderm of the chorion IcBve stops at about this stage, or at the stage when the matrix has completely changed from its first state ; in the region of the frondosum, however, development proceeds much further by the production of fibres throug out the whole of the layer ; usually, but not invariably, the fibres become much more numerous near the ectoderm than in the inner part of the mesoderm, thus differentiating a well-marked sub-epithelial fibrillar layer, Cut 20, fib, from the deeper and wider stroma, Str. The fibrillar layer is that commonly spoken of as the connective tissue layer of the chorion : for details of its structure, including the Gefdssschicht," see Langhans and Kastschenko. The inner layer, Sir, is called the GallertscJiicht by many German writers, and seems to be what Kolliker {Entwickelungsgeschichte, 2te Aufl., p. 322) designates as " Gallertgewebe zwischen Chorion und Amnion " ; it usually contains a considerable number of large granular wandering cells. Jungbluth, 106a, describes a network of capillaries, which exist during the first half of pregnancy, apparently in the upper part of the stroma, — i.e. next the amnion — but I fail to find any. Where the amnion comes into contact with the chorion the adjacent parts of the two membranes are more or less loosened, forming a network of strands by which the membranes are united : most of the uniting strands appear to belong rather to the chorion than to the amnion. This loose tissue is perhaps that which Kolliker designates as a Gallertgewebe distinct from the chorion.

Cut 19. — Sectiun of tnc cliurionic membrane of an ovum supposed to belong to the third week; ect, ectoderm; mes, mesoderm; a, outer, b, inner layer of ectoderm; stained with alum-cochineal. X 445 diams.

Cut 20. — Section of the amnion and placental chorion of the fifth month. Ep, amniotic epithelium; Am, amnion; Str, stroma; Fib, fibrillse layer; fir, fibrine layer; c, cellular layer; Vi, villi. (From a section cut in celloidine, and stained with Weigert's Hsematoxyline. The drawing is only approximately correct as to details. X 71 diams.).

Although the chorion bounds the coelom, I have observed no mesothelium upon its mesodermic surface ; but I have not made search for it by any special methods. In the rabbit, it will be remembered, the mesothelium is very evident over the placenta, but the rabbit differs from man by the absence of union between the amnion and chorion. Nor have I been able to find any basement membrane, properly so called, under the chorionic ectoderm. As to the appearance which suggests it, I accept Kastschenko's explanation, 107, 455.

The mesoderm in the villi is differentiated otherwise than that of the membrane of the chorion. In the youngest stage I have examined there is some of the primitive matrix present in the villi ; and I presume that earlier the whole mesoderm has the same character. In my specimen (three weeks) the change is progressing. I have not succeeded in satisfying myself as to the process of change which takes place, but I think it probably essentially as follows : The cells gradually develop large bodies and acquire a more decided affinity for coloring-matters ; meanwhile vacuoles appear in the matrix, presumably by its modification into a new substance; the vacuoles increase in size and number, transforming the matrix into a network and ultimately causing its total disappearance, leaving the intercellular spaces filled entirely with the new substance, which has come from a metamorphosis of the original matrix; probably this new substance is more or less fluid, since wandering cells are scattered freely through it. Leaving this half-hypothetical history, let us pass on to direct observations. In the placental villi of embryos of four months and older, the mesoderm exists in two principal forms, — adenoid tissue and fibre-cell tissue around the bloodvessels. The adenoid tissue, Cut 21, is that of which the supposed development has just been sketched ; it may be considered as the proper tissue of the villus. It consists of a network of protoplasmic threads, which start from nucleated masses (cells). There are many large meshes, which are partly occupied by the coarsely granular wandering cells, /, /, which are scattered about, and are usually present in large numbers. About the capillaries the network is much more finely spun. Kastschenko, 107, 454, found the wandering cells most abundant near the epithelium, but I have noticed no such peculiarity, except that they do not often enter the dense perivascular tissue ; and as the bloodvessels are centrally situated, the adenoid tissue and the wandering cells in it are of course more peripheral. It seems to me that the leucocytes are distributed more or less evenly throughout the adenoid tissue. I fail to recognize any intercellular substance. The abundance of nuclei deserves special mention. Around all the non-capillary vessels the mesoderm is very different, for it exhibits distinct intercellular substance, with a tendency to fibrillar differentiation in quite a wide zone around the blood-vessels ; in this zone the cells become elongated and irregularly fusiform ; around the larger vessels the cells are grouped in lamina, making the structure similar to that already described in the walls of the vessels of the umbilical cord ; after the perivascular coats acquire a certain thickness, the cells of the inner layers are more elongated, more regularly fusiform, and more closely packed than those of the outer layer ; the transition from the denser to the looser tissue is gradual. We are perhaps entitled to recognize in the denser inner layer the media, in the outer looser layer the adventitia, although neither of the layers has by any means the full histological differentiation characteristic of the like-named layers of the blood-vessels of the adult.

Cut 21. — Adenoid tissue of a villus from a placenta of four months. /, /, /, wandering cells; v, v, capillary blood-vessels; d, finer meshwork from near a capillary. X 352 diams.

The epithelium of the chorion becomes differentiated in three different ways: 1°, upon the chorion frondosum ; 2°, upon the chorion lasve ; 3°, upon the villi. For a correct knowledge of the remarkable changes which the epithelium undergoes, particularly in the placenta, we are indebted to the remarkably exact investigations of Langhans, llO and 111. This author left two points of importance unsettled ; namely, the origin of his " Zellschicht" and of the '^ canalisirtes Fibrin.'^ Kastschenko has traced the cellular layer {Zelhchicht) to the epithelium, as already stated : compare pp. A,6y\6<^ of his memoir, 107. My own observations show, I think conclusively, that the canalized fibrine arises through a degenerative metamorphosis of the epithelium, which begins in the outer layer and may invade the inner layer (Langhans' Zelhchicht). Let us consider separately the three series of modifications of the chorionic ectoderm.

In the region of the chorion frondosum the inner layer of the ectoderm (the cellular layer of Langhans) becomes irregularly thickened in patches, which present every possible degree of variation as to number and as to their breadth and thickness. Although at first the cellular layer is more or less continuous and composed of uniform cells, this is not the case in later stages. We must assume that with the growth of the membrane the epithelium increases in area, but remains in many places single layered, developing no '^ Zellschicht." The patches of cells have been well described by Langhans, llO, and Kastschenko, 107, 466, and are represented with lower power in Cut 20, c, and with a higher power in Cut 22, c. They vary much in appearance : the cells are more distinct in the small patches, but are less individual in the large patches, owing to the spread of the process of degeneration into the layer. Cut 22, c. The cell bodies are lightly stained, and the granular nuclei are not very sharply defined and vary in size and shape. The cellular layer is always sharply defined against the stroma, although there is no true basement membrane, but towards the outer layer of the ectoderm its boundary is sometimes distinct, sometimes lost in a gradual transition.

Cut 22. — Placental chorion of an embryo of seven months; vertical section through the ectoderm and portion of the adjacent stroma. 77ies, mesodermic stroma; c, cell layer; fb, fibrine layer; ep, remnant of epithelium. X 445 diams.

The outer layer of the ectoderm of the frondosum is even more variable. As stated by Kastschenko, it is primitively a dense protoplasmic reticulum, with nuclei in a single layer and without any cell boundaries. In the chorion frondosum at four months and after I find spots where this structure still prevails, either with or without an underlying cellular layer ; in other spots the layer is thickened and contains an increased number of nuclei, which are sometimes crowded in a bunch ; elsewhere the layer is thinned out and has no nuclei ; in still other spots the thickening has gone on much further, and usually, but not always, where the outer layer is much thickened the cellular layer under it is also thickened ; wherever it is thickened, and occasionally where it is thin, the outer layer of the ectoderm shows a marked tendency to degenerate into canalized fibrine, Cut 20, fbr, and Cut 22, fb. It is not difficult to assure one's self that the fibrine arises by direct metamorphosis of the ectoderm. I now think that its formation begins in the outer layer and thence spreads into the cellular layer ; for, in fact, when both layers are distinguishable, as in Cut 22, the fibrine layer, fb, is always external, and the external layer of nucleated protoplasm has either totally disappeared or is represented by mere remnants, as in Cut 22, ep. The fibrine layer consists of a hyaline, very refringent substance permeated by numerous channels. Cut 22, fb\ the substance has a violent affinity for carmine and haematoxyline, and is always the most deeply colored part of a stained section ; the channels tend to run more or less parallel to the surface of the chorion and are connected by numerous cross-channels ; some of the channels contain cells or nuclei. This complex system of canals is by no means of uniform appearance in all parts of the placenta, both the spaces and dissepiments varying in size and shape. The fibrine often sends, as shown in Cut 22, long outshoots into the cellular layers upon which it seems to encroach. The frequency of these images in my preparations led me to the opinion ^ that the fibrine arises from the cellular layer only, and I concluded that the ectoderm was first transformed into the so-called cellular layer, which was then transformed into fibrine. It still appears to me that much of the degeneration goes by these stages ; but, on the other hand, it seems clear that the degeneration begins, as above stated, in the outer layer. Another appearance is presented by the ectoderm where it is thickened and wholly transformed into the cellular layer. In brief : the ectoderm of the placental chorionic mesoderm undergoes patchwise manifold changes ; it exists in three chief forms : i°, the nucleated protoplasm ; 2°, the cellular layer ; 3°, canalized fibrine. A patch of the ectoderm may consist of any one of these modifications, of any two or of all three, but they have fixed relative positions, for when the nucleated protoplasm is present, it always covers the free surface of the chorion; when the cellular layer is present, it always lies next the mesoderm ; and when all three forms are present over the same part, the fibrine is always the middle stratum. In general terms it may be said that the amount of canalized fibrine increases with the age of the placenta, but it is very variable in its degree of development. The peculiar layer into which the ectoderm is transformed has long puzzled anatomists. E. H. Weber recognized the fibrine layer and described its appearance correctly ; it has probably been often seen, but generally regarded as either pathological or a blood coagulum. Robin, for instance, may be cited, 125, 70-71, as one who saw, without observing correctly and understandingly, the tissue in question. An important gain was made when Winkler recognized the modified ectoderm as a constant layer, and in 1872 directed especial attention to it under the name of " Schltissplatte," 152. Kolliker {Entwickehmgsgeschichte, 2te Aufl, 337) added essentially to our knowledge of its structure, but it is to Langhans that we owe the first clear light. Meanwhile, other writers, following the lead of Ercolani and Turner, 146, 551-553, have been influenced chiefly by the presence of the cellular layer, in the large size of the elements of which they found a resemblance to the decidual cells, which has guided them to the conclusion that the cellular layer is derived from the wall of the uterus. This error has been definitely corrected by Kastschenko, as already stated. In further support of the conclusion that the chorionic cellular layer is not decidual, may be brought forward the fact that there is a certain immigration of decidual cells into the placenta at its margin ; but they remain entirely distinct from the cells of the cellular layer. This is readily seen in radial sections through the margin of a placenta from a normal after-birth — compare below, the account of the ectoderm of the chorion laeve. The origin of the canalized fibrine from blood, which Langhans left in his first paper as an open possibility, and which even so recent a writer as Ruge, 129a, 123 and 130, has advocated, cannot be maintained. Of course, there may be a deposit of blood fibrine (coagulum), but it would be pathological, and therefore to be distinguished from the normal fibrine of ectodermal origin. Moreover, the microscopic appearance of a blood clot or thrombus is so extremely characteristic that one can readily distinguish it from the placental canalized fibrine.

1 Anatom. Anzeiger, ii. 23.

The ectoderm of the villi of the placenta differs from that of the chorionic membrane in several respects : 1°, the cellular layer after the first month becomes less a,nd less conspicuous, and after the fourth month is present only in a few isolated patches, known as the Zellknoten, and carefully described by Langhans and Kastschenko ; both of these authors were impressed by the resemblance of the cells to those of the decidua serotina ; Langhans concludes that the Zellknoten arise from the serotina, but Kastschenko, having traced their development from the chorionic epithelium, denies his predecessor's conclusion, but still clinging to the idea of a genetic connection between the Zellknoten and the decidua, reverses the reasoning, and concludes that the decidual cells arise in part at least from the Knoten. Neither of these authors have found the intermediate forms between the two types of cells, and when we examine their descriptions critically we find that they have really no evidence except the likeness of the cells to offer in favor of their genetic relationship, and accordingly Langhans expresses himself with characteristic caution. To me the resemblance appears altogether superficial ; hence my conclusion that the Zellknoten are remnants of the cellular layer. 2°, For the most part the villi remain covered by the nucleated protoplasm, which in many places is thickened. In the later stages these thickenings are small and numerous, constituting the so-called " Proliferatiojis-insehi" : compare Cut 15. Many of the little thickenings appear in sections of the villi, Cut 23, a, a, and here and there are converted into fibrine, /. I have interpreted them (Wood's Reference Handbook of the Medical Sciences, V., 695) as commencing buds, and consider that in earlier stages they grow into branches, but in later stages are in part at least arrested in their development. 3°, The proliferation islands are converted into canalized fibrine, and at the same time grow and fuse, forming larger patches, particularly on the larger stems : in this manner are produced the large areas and columns of fibrine found in the placenta at four months and after ; they have been well described by Langhans, and form a striking feature in sections of placentae. Some of the columns, as stated by Langhans, stretch along the villi from the chorionic membrane to the surface of the serotina as if to act as supports. Ercolani appears, if I understand his account, to have seen the fibrine columns, without, however, ascertaining either their structure or their origin. 4°, Over the tips of the villi, which are bent considerably where they are imbedded in the decidua serotina, the relations are not clear ; the epithelium is certainly not present in its original form over the imbedded ends of the villi, which are, however, surrounded by a hyaline tissue of the character of the canalized fibrine, except that the canals are often indistinct or even wanting ; the hyaline tissue forms an almost continuous coat over the decidual surface ; in earlier stages the ectoderm of the terminal villi is often considerably expanded. The natural interpretation of these facts is that the ectoderm of the villi expands over the decidua serotina and degenerates. In this manner we account for both the absence of any cellular ectoderm over the ends of the villi and the presence of canalized fibrine upon the serotinal surface — but the hypothesis must await the final test by observation.

Cut 23. — Cross-section of a villus from a placenta of seven months; three blood-vessels are shown; a, a, thickenings of the ectoderm; f, a thickening transformed into canalized fibrine. X 222 diams.

The ectoderm of the chorion Iseve loses by the seventh month all traces of the protoplasmic layer, and is without any canalized fibrine, except near the placenta ; cf. infra. It is transformed into a ZellscJiicht. In a section of the laeve in situ at seven months. Cut 33, the chorionic ectoderm, c, rests directly upon the decidua, which has no epithelium of its own. The ectodermal cells lie two or three deep ; they are described by Kolliker and Langhans, the former designating them as the chorionic epithelium, while the latter doubtfully traces their origin to the uterus. That Kolliker {EntwickelnngsgescJiichte, 2te Aufl., p. 322) is right, I am confident. It is easy to follow the layer of cells in question at the edge of the placenta, and see that it is directly continuous with the cellular layer of the frondosum, which it resembles in character. On the other hand, the ectodermal cells of the lasve are distinct in character from the decidual cells next to them, Cut 34, having smaller and more darkly stained nuclei, and much more coarsely granular protoplasm ; the ectodermal cells are much smaller than the decidual. The ectoderm is sharply marked off from the decidua, but its surface is often corrugated, and then the line of separation between the tissues is irregular, and in sections it may even appear that there is a true interpenetration and mingling of the decidual and ectodermal cells ; but it is only apparent, and the demarcation is always preserved.

Cut 24. — Placenta at full term. A, vertical radial section through the margin; D, decidua; vi, aborted villi outside the placenta; Cho, chorion; Si, circular sinus; J^i, placental viUi; Fib, canalized fibrine. B, portion of A more magnified to show the decidual tissue near b; v, blood-vessel; d.d', decidual cells; d, with one, d', with several nuclei.

At the edge of the placenta, as shown by examination of afterbirths, the relations of the layers are somewhat different. I reproduce with a few additions the descriptions given in my article on the Placenta'^ of a radial section through the margin of a normal placenta discharged at full term. Cut 24, A, from which the amnion had been removed. The chorion, Cho, and decidua, D, are in immediate contact at the left of the figure; that is, outside of the placenta, though remnants of the aborted villi, vi, are still plainly recognizable ; but, as stated previously, they occur only in the immediate neighborhood of the placenta. These villi are surrounded by hyaline matter which resembles and can be followed into continuity with the canalized fibrine layer. Fib, covering the surface of the decidua serotina and the fibrine layer of the chorion frondosum. Below the aborted villi, vi, of the chorion laeve, the fibrine layer is broken down and penetrated by the decidual tissue, so that the demarcation between the foetal and maternal tissues is here lost, and in fact, at the edge of the placenta the decidual cells make their way into the chorionic tissue, and for a certain distance towards the centre of the placenta they are found lying chiefly in the ectoderm. In other placentae the fibrine layer and the decidual tissue around the margin of the placenta have not only intergrown, but also undergone a common degeneration, Cut 25, in consequence of which all distinct structure is obliterated, and we find the villi, vi, imbedded in a stratum, f, of more or less colored substance, without definite organization except irregularly scattered nuclei. Attentive examination shows that this layer,/", has unmistakable remains, c, of the cellular layer next the mesoderm of the chorion, and that it passes into an outer layer, D\ in which the traces of decidual structure are unmistakable ; the dark line at the lower edge of the decidua, D\ is merely detritus and coagulum, as is often found on after-births. If we follow the layers in this, or a similar specimen, in the direction away from the placenta, the layers gradually alter, losing their degenerated character, until we reach a point where the chorionic ectoderm and the uterine decidua both exhibit their normal features. Returning now to the placenta we were previously considering. Cut 24 : The placental chorion begins to exhibit its characteristic stratification a short distance within the margin. I have found, however, that the distinctness of that stratification varies considerably, not only in different placentae, but also in different parts of the same placenta. The decidua, D, outside the placenta is very thick, but at the edge of the placenta it begins to thin out, and as it passes over the under side of the placenta, rapidly becomes so much reduced as to be even less in thickness than the chorion, cJio. The decidua is everywhere crowded with an immense number of decidual cells, but in some other specimens they are less crowded. The surface of the decidua serotina is covered by a layer of fibrine, easily recognized by its deep staining ; this coat of degenerated material has not yet received the attention it deserves, as a feature of the human placenta, which is quite constant, so far as my observations go ; as stated previously, I consider its origin to be the epithelium of the ends of the villi imbedded in mucosa. Up to the edge of the placenta the chorion laeve and decidua are united ; at the edge they separate, to make room for the villi, Vi, Vi, of the frondosum. In the angle, Si, where the two membranes first separate there are very few villi, so that there is a comparatively clear space left, which is known as the circular sinus. It is not, as some of the older writers have believed, a distinct vessel, nor does it extend as a clear space completely around the placenta ; but, on the contrary, it is interrupted here and there by an ingrowth of villi. In the cut, the spaces occupied of maternal blood are left white ; the foetal blood-vessels are drawn black.

Cut 25. — After-birth at full term; vertical section of the amnion, chorion, and decidua in their natural relations near the placenta, am, amnion; cho, chorion; c, cellular layer or ectoderm; f, fibrine and decidual tissue, degenerated; D', decidual tissue. X 125 diams.

  • Wood's Reference Handbook Medical Science, V., 694, 695.

Minot1889 fig26.jpg

Cut 26. — Placenta at full term, doubly injected by Df. H. P. Quincy, to show the distribution of blood-vessels upon the surface; the arteries are drawn light; the veins dark. X 0.7 diams.

The chorionic circulation is complete in itself. The single vein and the two arteries of the umbilical cord spread out over the surface of the chorion, marking their course by projecting ridges. The insertion of the cord is always, so far as I have observed, obviously eccentric ; the degree of eccentricity varies from a nearly central position to the so-called velamentous insertion, — compare B. S. Schultze, 159; the degree of eccentricity is easily seen to be related to the distribution of the vessels, — a point not mentioned in current text-books. The arteries come down together from the cord, and are usually connected, but not invariably, by a short transverse vessel, situated about half an inch above the surface of the placenta, and which has been noted by many observers. I have never noticed any arterial or venous anastomoses on the surface of the placenta. The two kinds of vessels do not run together ; the arteries lie nearer the surface, the veins deeper. Cut 26 ; the arteries fork separately until they are represented only by small branches and fine vessels ; some of the small branches disappear by dipping down suddenly into the villi below ; the veins are considerably larger than the arteries, and some of the larger branches disappear from the surface in the same abrupt manner as do the smaller arteries. There is the greatest possible variability in the vessels of the placenta ; I have never seen two placentas with the vessels alike. The more eccentric the insertion of the cord, the more do the vessels tend to distribute themselves symmetrically ; the more central the position of the cord, the less can any vascular symmetry be made out.

The two following paragraphs are copied without change from my article on the placenta (Buck's Reference Handbook of the Medical Sciences, V., 696, 697) : —

" To follow the course of the foetal blood-vessels within the placenta, the best method is by corrosion injections. These may be made either with fusible metal, wax, or celloidine. The first is specially suited for the study of the large trunks ; the latter, for that of the smaller vessels also. I have a very beautiful celloidine injection by Dr. S. J. Mixter, which, with others of wax and metals, has served as the basis of the following description : The veins leave the surface somewhat more abruptly than do the arteries, which gives off more small branches to the surface than do the veins. Cut 26. Both kinds of vessels leave the surface by curving downward for a short distance into the trunk of a villus ; the vessels then divide, and their branches again take a more horizontal course ; the branches then curl over downward, and, after a second short descent toward the decidua, again send out horizontal branches. The result of this arrangement is a terrace-like appearance in the course of the vessels ; they approach the uterine side of the placenta in this very characteristic manner. The number of terraces is variable ; usually there are two or three, but sometimes there is only one, or they may number four or even five. Arrived at the end of its terraces, the main vessel takes a more nearly perpendicular

course, and rapidly subdivides. Immediately after entering the villi, the arteries and veins give off but few capillaries, but after a short course in the main stalk of the villus, the vessels give rise to many branchlets, and gradually the character of the circulation changes, until in the smallest villous twigs there are capillaries only, Cut 27. The vascular trunks have a marked tendency to dichotomous division, which is maintained within the villi to a certain extent ; the arterioles and veinlets in the mature placenta go from their trunks at wide angles for the most part, and subdivide in the same manner, so that they spread out through the whole substance of the placenta. The vessels next the decidua take a more horizontal trend, like the top branches of a wind-swept tree. As the vessels run in the villi, of course the way in which the latter branch out determines the paths of the former ; hence by following the distribution of the vessels we inform ourselves as to the ramifications of the villi. Thus the horizontal course of the vessels on the uterine side of the placenta corresponds to the wellknown fact that the ends of the villi attached to the uterus become bent and adhere by their sides to the decidual surface."

Cut 27. — Portion of an injected villus from a placenta of about five months; magnified 210 diams.

"The capillaries of the villi are remarkable for their large size, and on this account have been described as arteries or veins by E. H. Weber, Goodsir, and other writers. Their calibre is often sufficient for from four to six blood-disks abreast. They are very variable in diameter, and also peculiar in exhibiting sudden restrictions and dilatations, Cut 27, In the short bud-like branches there is often only a single capillary loop, but as the branch becomes larger, the number of loops increases, and they form anastomoses. In branches large enough to serve as a stem, some one or two of the vessels may be enlarged, as may be seen in Cut 27 ; in the branches large enough to admit of it, there are two (or sometimes only one) longitudinal central vessels, an artery and vein, and a superficial network of capillaries. Cut 27^. Goodsir and other early writers laid great stress on the formation of the capillary loops, but this feature is a common one in the development of the foetal vascular system, as is also the width of the capillaries. In my opinion these peculiarities are characteristic rather of the foetus than specifically of the placenta. In some of the older writers (Goodsir, Farre, et al.) villus, to show the central vessels and superficial capillaries.

it is asserted that the true capillary System disappears toward the end of gestation. I am unable to confirm this, but find instead that in the slender terminal villi of the placenta at term there is often only a single, sometimes long, capillary loop ; the capillary is very wide, and its width is probably the reason of its having been held formerly to be a vein or an artery."

Uterus during menstruation

I have little to add to the descriptions of previous authors, particularly those of Leopold, 36, and Kolliker.[1] It is, however, worth while to present the accompanying illustration, Cut 28, since there is a lack of figures. The cut represents a transverse section of tlie corpus uteri of a fine specimen, for which I am indebted to Dr. W. W. Gannett. The woman died from acute miliary tuberculosis ; the autopsy was made almost immediately after death, and within four hours from death the complete genitalia were placed in Miiller's fluid, the uterus having been first carefully opened by a single median ventral incision. Death is said to have occurred on the day of the regular period. The hymen was intact. There was no sign of pathological change in any of the genitalia. In one ovary, the right, there was a fresh corpus hcemorrhagicum. These data afford a sufficient basis for the belief that the uterus was well preserved in a perfectly normal condition.

Cut 28 a. Placenta of about five months; portion of a small

The mucous membrane is from 1.1-1.3 mm. thick; its surface is irregularly tumefied ; the gland openings lie for the most part in the depressions. In the cavity of the uterus there was a small blood-clot. The mucosa is sharply limited against the muscularis. Cut 28. In transverse sections one sees that the upper fourth of the mucosa is very much broken down and disintegrated, Cut 28, d\ the cells stain less than those of the deep portions of the membrane ; as represented in the cut the tissue is divided into numerous more or less separate small masses ; some of the blood-vessels appear torn through, but it is difficult to make sure observation : Overlach, 39, considers it probable that the infiltration of blood takes place by diapedesin, not by rupture of the capillaries. The superficial epithelium, ep, is loosened everywhere ; in places fragments of it have fallen off, and in some parts it is gone altogether ; it stains readily with cochineal and its nuclei color well, the epithelium differing in this respect from the underlying connective tissue, which does not stain well ; the blood-vessels in the disintegrated layer are for the most part small.

The deeper layer of the mucosa is dense with crowded wellstained cells, which lie in groups separated by clearer lines ; in the cut this grouping shows less plainly than in the preparation ; the lighter channels are perhaps lymph-vessels, a suggestion which occurs to me, because in so-called "moulds" one sometimes finds similar channels crowded with leucocytes. The cells appear to be the proliferated interglandular tissue ; there are very few leucocytes, so far as I can distinguish ; the cells have small, oval, or elongated, darkly stained nuclei, with a very small granular protoplasmatic body each ; there is certainly no noticeable enlargement of the cells, but only a remarkable multiplication. The point is important ; I see nothing to suggest the presence of decidual cells, nothing even like definite enlargement of any of the cells. The image of the tissue is comparable to that of the connective tissue of the rabbit's placenta at six days, except that there the cells are widely separated, here closely crowded, but in each case the cells are small, with little protoplasm, and connected by their processes. In another specimen in my possession of a normal uterus at the close of menstruation, the condition of the mucous membrane agrees with that of the specimen we have considered, except, of course, that the disintegrated superficial layer is lost, and that the superficial layers stain poorly. In this second specimen, also, the interglandular cells are small and very crowded ; there are few leucocytes and no decidual cells. The two specimens further agree in having the glands distended and contorted ; each gland is surrounded by a distinct basement membrane or layer of connective tissue cells closely investing the epithelium, as has been observed by Leopold, 36. In my article on the decidua in the Reference Handbook, II., p. 390, is a summary of the changes occurring during menstruation, and stress is there laid upon two points emphasized by previous writers ; namely, the increase in the number of leucocytes and the presence of decidual cells. Since my own observations have failed to confirm these statements, I can no longer accept them. The proliferated connective tissue cells are those, probably, which become decidual cells when the decidua menstrualis is changed into the decidua graviditatis — compare the account of the one month's uterus in the next section.

Uterus one month pregnant

The specimen to be described came from a woman who committed suicide by violence, not by poison, and I was informed that she was known to be about one month pregnant. Further information was not obtained, and I was requested not to seek it. The specimen was received in very fresh condition, but it had been opened, the reflexa was torn and pretty much gone ; the embryo had been removed, and I was therefore unable to verify the age, or investigate the attachment of the villi of the chorion to the uterus. There was a beautiful corpus hitewn in one ovary, quite similar to that figured by Dalton in his Report on the corpus luteuni in the transactions of the American Gynaecological Society for 1877, Fig. 9. The surface of the uterus seemed uninjured. The specimen was hardened in Miiller's fluid, and found subsequently to be well preserved. It may be considered, I think, perfectly normal.

Cut 29. — Uterus one month pregnant ; outlines of the glands from a vertical section : to show the division of the mucosa into an upper compact layer, D\ and a lower cavernous layer, D" ; gl',gl", glands; art, spiral artery; 7nitsc, muscularis.

My specimen enables me to confirm in most respects Turner's accurate description of two uteri of about the same age, 146, 546-548. The inner surface shows the hillocks {Insebi) described by Reichert in the uterus of two weeks, studied by him, which have been figured by Coste in slightly older specimens, and found by Turner also, 146, 540.

The three illustrations given herewith are all from sections through what I suppose to be the placental region.

There is an upper compact layer. Cut 29, D\ and a lower cavernous layer, Z?" ; the caverns, being gland cavities, which appear as rounded areolae lined with epithelium, are filled with broken-down epithelial cells. The drawing, reproduced in Cut 29, was obtained by drawing the outlines very carefully, stippling the areas occupied by the connective tissue, representing the blood-vessels by double outlines, and omitting the glandular epithelium altogether. It will be noticed that about threefourths of the diameter of the mucosa is occupied by the cavernous layer, D".

Cut 30. — Uterus one month pregnant; portion of the compact layer of the decidua seen in vertical section; coagl, coagulum upon the surface; d, d', decidual cells. X 445 diams.

The upper or compact layer is shown in Cut 30. The surface is without any trace of epithelium, but is covered only by a thin fibrous and granular coagulum, coagl ; the tissue itself consists almost exclusively of young decidual cells, d, d\ with a clear homogeneous matrix; here and there are leucocytes, but they are nowhere numerous ; the decidual cells are all quite large, with their bodies deeply stained by the eosine ; the nuclei are round, oval, or slightly irregular in shape, coarsely granular, and sharp in outline ; the cells themselves, though irregular and variable in shape, are all more or less rounded with processes running off in various directions ; scattered between the cells are many sections of their processes ; occasionally it can be seen that two cells are connected ; in fact, we have in this tissue evidently a modified embryonic or so-called anastomosing connective tissue. Now, as we know through the observations of Leopold, 36, which I have verified, the connective tissue of the uterine mucosa consists of anastomosing cells, and as stated in the previous section, the cells are found proliferated in the menstruating uterus ; we have therefore only to imagine the cells enlarged with certain accompanying modifications, to obtain the tissue figured in Cut 30. There is no special formation of cells around the blood-vessels, where, according to Ercolani, the decidual tissue arises by new formation. In Turner's specimens the upper part of the compact layer was imperfectly preserved, but according to his description there appears to have been a coagulum similar to that which I have found, but thicker. In the deep part of the layer the cells are less enlarged, and when the cavernous layer is reached, there occurs a rapid transition in the character of the cells, which become smaller and more fusiform, and their nuclei more elongate, smaller, and deeper stained by alum-cochineal. The gland openings upon the surface of the uterus lead into tubes, Cut 30,^/', which run slightly obliquely through the compact layer, taking a more or less nearly straight course and joining the contorted gland tubes, Cut 30, gl^\ of the cavernous layer. The gland ducts are completely devoid of lining epithelium, which has disappeared except for a very few loose cells, occasionally found lying free in the ducts ; the cells have not fallen out from the sections, but were lost before the tissue was imbedded. ^ The ducts then are wide tubes running nearly straight through the upper part of the decidua and bounded directly by the decidual tissue ; they communicate below with a contorted cavity. Similar tubes appear in later stages and have been described as blood-vessels — see the next section.

1 The blocks to be cut were stained hi toto with alum-cochineal and eosine, imbedded in paraffine, etc. The sections were fastened on the slide with celloidine, to keep the parts in place.

The cavernous layer contains numerous spaces, the areolae of Turner, 146, 547, who was uncertain as to their character, though he ascertained that many of them belonged to the glandular system. In my specimen it is perfectly clear that all the larger areolae belong to the glands, which must be extremely distorted and distended to give the shapes shown in Cut 29. The thin dissepiments between the areolae are composed of connective tissue, the long dark nuclei of which, Cut 31, are strikingly different from those of the cells of the compact layer, Cut 30. The areolae present two extreme modifications and all intermediate phases between these two. The smaller areolae are lined by a well-preserved cylinder epithelium, or by one in which the cells are separated by small fissures ; in other areolae the cells are a little larger. Cut 31, each for the most part cleft from its fellows, and some of them loosened from the wall and lying free in the cavity. The other extreme is represented in Cut 32 ; the size of the areolae is much increased, — compare Cuts 31 and 32, — both drawn on the same scale ; the epithelium is entirely loosened from the wall, and the cells lie separately in the cavity which they fill ; the cells are greatly enlarged, their bodies having three or four times the diameter of the cells in the small areolae ; they have not the cylinder shape, but are irregular in outline : their protoplasm is finely granular and stains rather lightly ; the nuclei are large, rounded, glandular, and with sharp outlines ; they are less darkly stained than the nuclei of the epithelium of Cut 31,

Cut 31. — Uterus one month pregnant; section of gland from cavernous layer, with the epithelium partly adherent to the walls. X 445 diams.

The obvious interpretation of the appearances described is, that the glandular epithelium is breaking down, that it is lost altogether from the ducts, but is still present in the deep portions of the glands ; in breaking down the cells separate from one another, and then from the wall, and falling into the gland cavity, there enlarge, the cavity enlarging also. Similar appearances are also found in "moulds" of the second month; very likely they have been often observed and mistaken for pathological changes.

Cut 32. — Uterus one month pregnant; section of gland from cavernous layer, with the epithelium loosened from the walls; X 445 diams.

The blood-vessels of course lie in the dissepiments between the glands. I observed nothing to correspond with the " colossal capillaries dilated into small sinuses," mentioned by Turner 146, 548. Were not these supposed capillaries gland cavities, from which the epithelium had fallen out } Occasionally the sections pass through a spiral artery. Cut 29, art, which is cut again and again as it twists around in its characteristic separate column of connective tissue.

Uterus seven months pregnant, with the foetal membranes in place

The specimen to be described was obtained for me through the kindness of Dr. W. W. Gannett. It is an apparently normal uterus, which contained a normal embryo weighing 11 50 grammes and having an umbilical cord 58 centimetres long, — probably about seven months old, or a little more : there were no data as to the duration of gestation. The uterus was opened, and preserved in Miiller's fluid without disturbing the membranes.

A section through the amnion, chorion Iseve, and uterine mucosa, stained with haematoxyUne, and viewed with a low power, is represented in Cut 33 ; the dark spots are maternal blood-vessels, which have been shaded for the sake of clearness. The amnion, a^n, and chorion, cko, present the characteristics previously described, §§ 14, 15 ; the chorion is bounded against the decidua by an epithelium c, which I interpret as the chorionic ectoderm ; there is no trace of a second layer of epithelium ; so that the uterine epithelium must be considered lost, a conclusion agreeing with the observations of Kolliker, Turner, and myself upon earlier stages, and the statements of Ercolani. The decidua has eight or nine times the thickness of the chorion ; it has an upper compact and a lower cavernous layer ; the former contains numerous decidual cells, most of which are a little larger than those nearer the muscularis ; the compact layer contains a few blood-vessels of moderate calibre, and occasionally a large vessel, v, surrounded by connective tissue containing no decidual cells. Examined with a higher power, the decidual cells — compare Cut 34, D' — are found to resemble quite closely those at one month. Cut 30, but they are much more numerous and closer together, and their processes are fewer ; they vary also ^nore in size ; some of the larger ones are multinucleate ; it is probable that the cells are multiplying by division ; the matrix presents a fibrous look, but whether it contains actual fibres, I am not sure ; between the decidual cells are a certain number of nuclei, some of which belong to leucocytes, others to blood capillaries, and still others, which I am uncertain about, which are few in number, and possibly belong to connective tissue corpuscles. The cavernous layer resembles now, in contrast to the first month, the upper layer of the decidua in histological constitution, but the decidual cells are smaller and at little wider intervals from one another ; the cavernous layer is especially characterized by the slit-like spaces in it ; some of these spaces, as indicated by the drawing, Cut 33, are undoubtedly blood-vessels or sinuses, but still others contain no blood, or at most three or four isolated corpuscles, although close to them are capillaries gorged with blood ; once in a while a few epithelioid cells can be seen adhering to the walls of the spaces. These spaces can hardly be assigned to the vascular system ; they have been held by Kundrat and Engelmann, 180, and various subsequent writers, to be the gland cavities ; we have not sufficient observations to establish the actual metamorphosis of the areolae of the one month's uterus into the slits, ^/, of Cuts 33 and 35, D^ , but there is no ground to question the occurrence of the change, which appears to be a necessary consequence of the stretching of the decidua due to the expansion of the uterus during pregnancy.

Cut 34. — Uterus about seven months pregnant; upper portion of decidua vera, with the chorion laeve in situ. 7>ies, mesodermic layer of chorion; ep, epithelial layer of chorion; D', decidua. X 340 diams.

Cut 35.— Section through a normal placenta of seven mov^hs, in situ. Am, amnion; Cho, chorion; Vi, villus trunk; vi, sections of villi in the substance of the placenta; D, decidua; Mc, muscularis; D\ compact layer of decidua; Ve, uterme blood-vessel (or gland?) opening into the placenta. The fcetal blood-vessels are drawn black; the maternal blood spaces are left white; the chorionic tissue is stippled, except the canalized fibrine, which is shaded by lines; the remnants of the gland cavities in i?" are stippled dark. (Drawn from nature by J. H. Eraerton.)

A complete section through the placenta in situ and uterus is represented in Cut 35, which has already appeared in my article, " Placenta " (Buck's Handbook, V., 696), and been sufficiently described. The chorion is separated by a dense forest of villi from the decidua, D ; the ends of some of the villi touch and are imbedded in the decidual tissue ; these imbedded ends are without epithelium, but their connective tissue is immediately surrounded by hyaline substance. The decidua is plainly divided into two strata — cf. infra. The section passes through a wide tube, Ve, which opens directly into the interior of the placenta and contains blood; in my article. I.e., this opening is referred to as that of a vein, the identification being in accordance with my understanding of the descriptions of Waldeyer,i 149. Professor Langhans has since informed me, that according to his own observations the opening of the arteries are characterized by the absence of villi projecting into their openings.

  • I am under much obligation to Professor Waldeyer for an opportunity to examine some of the injected specimens upon which his very important researches were conducted. His pupil, Raissa Nitabuch, has since published a dissertation, 117, confirming this opinion, according to which the vessel shown in Cut 35, Ve, is not venous, but arterial. Another possibility has occurred to me, viz. : that it is a gland duct ; in fact, it resembles very closely the undoubted gland ducts of the one month's decidua: there is no reason apparent why the gland ducts, which pass nearly vertically through the compact layer, should be obliterated ; on the contrary, one might expect to find them widened by the stretching of the uterus ; as there is

Cut 36. — Uterus of seven months, vertical section of the decidua serotina from near the margin of the placenta, nic, muscularis; D', D", decidua serotina; D', cavernous or spongy layer; D", compact layer; Vi, scattered chorionic villi. The intervillous spaces were filled with blood, which is not represented in the figure, X 50 diams.

blood in the intervillous spaces, it could easily make its way into the distended glands, and its presence there would not prove the glands to be blood-vessels. While, therefore, I accept Waldeyer's researches, 149, as well as those of Langhans and Nitabuch, 117, as verifying Farre's neglected account, 172, 722, of the placental circulation, I venture to express a note of caution as to the danger of mistaking glandular for vascular openings.

The following additional points deserve notice : The serotina is about 1.5 mm. thick, and contains an enormous number of decidual cells, Cut 36. The cavernous, D\ and compact layers, n\ are very clearly separated ; the mucosa is sharply marked off from the muscularis, mc, although scattered decidual cells have penetrated between the muscular fibres. The muscularis is about 10 mm. thick, and is characterized by the presence of quite large and numerous venous thrombi, especially in the part towards the decidua. The decidua contains few bloodvessels. Upon the surface of the decidua can be distinguished a special layer of mingled hyaline and decidual tissue, which in many places is interrupted by the ends of the chorionic villi, as is well shown in Cut 36. The supposed gland cavities of the spongy layer, D\ are long and slit-like ; they are filled for the most part with fine granular matter, which colors light blue with hsematoxyline ; they also contain a little blood, sometimes a few decidual cells. I have seen in them also a few oval bodies several times larger than any of the decidual cells, and presenting a vacuolated appearance ; what these bodies are, I have not ascertained. In places the glandular epithelium is distinct ; its cells vary greatly in appearance, neighboring cells being often quite dissimilar ; nearly all are cuboidal, but some are flattened out ; of the former there are some with darkly stained nuclei, but the majority of the cells are enlarged, with greatly enlarged hyaline, very refringent nuclei.

The decidual cells are smaller and more crowded in the cavernous layer, and mostly larger in the compact layer — compare

Cut 37. — Decidual cells from the section represented in part in Cut 36. a, b, d,f, various forms of cells from serotina; c, giant cell from the margin of the placenta; e, clear cells from chorion; at a, seven blood globules have been drawn in to scale. X 545 diams.

Cut 36. The largest cells are scattered through the compact layer, but are most numerous towards the surface. The decidual cells exhibit great variety in their features, Cut 37 ; they are nearly all oval disks, so that their outlines vary according as they happen to lie in the tissue ; they vary greatly in size ; the larger they are, the more nuclei they contain ; but I observe no cells with more than ten nuclei. The nuclei are usually more or less elongated ; the contents of the cells granular. Some of the cells present another type ; these are more nearly round, clear, and transparent, e\ the nucleus is round, stained lightly, and contains relatively few and small granules ; such cells are most numerous about the placental margin.^

Uterus twelve hours after abortion at six months

For this specimen, also, I am indebted to Dr. W. W. Gannett. The woman was brought into the Boston City Hospital in a comatose condition ; the foetus, estimated to be about six months, was removed by the forceps ; the mother died twelve hours later ; the autopsy by Dr. Gannett showed death to have been caused by tubercular meningitis. The uterus is apparently normal ; I received it in a fresh state, and hardened it in Miiller's fluid. It was already very much contracted ; the mucosa measured about 2 mm. in thickness ; its surface was ragged and more or less covered with clotted blood, presenting very much the appearance so superbly figured by Coste {Developpement des Corps organises, PL X., Espece humaine).

Vertical sections. Cut 38, show that the surfaces of the mucosa are very uneven ; on the free surface there is a thin layer of clotted blood, coagl\ the upper or compact layer of the decidua has entirely disappeared, leaving only the deep portion, D, permeated by numerous large empty spaces, which I take to be in part gland cavities, in part blood sinuses, both changed from their slit-like form by the contraction of the uterus during and since the delivery of the child. Between the spaces are the brownish and hyaline cells, and a great many blood-corpuscles, which lie throughout the tissue itself as well as in the blood-vessels. In short, the conditions found agree with those described by Leopold as present in the uterus a short time after normal delivery at full term, 36, and accordingly, further details concerning my specimen may be omitted.

  • This and the preceding paragraph are taken with sundry alterations from my article on the placenta, l.s.c.

Origin of Decidual cells

Besides the erroneous hypothesis of Ercolani, there are three views as to the origin of the decidual cells known to me, to wit : 1°, they are modified leucocytes (Hennig, Langhans, et al) ; 2°, they arise from the connective tissue cells of the mucosa (Leopold, 36) ; 3°, they are produced by the epithelium (Overlach, 39). The first view is not supported by observation, even by its advocates, and may be dismissed. Overlach' s observations certainly favor the third view, but inasmuch as he has studied only one uterus with pseudo-menstruation from acute phosphorus poisoning, his theory cannot be accepted definitely until verified by further observations on normal uteri. Overlach found in the cervix of the uterus in question, the lining epithelial cells to contain an endogenous brood of small cells, one to fifteen in each parent-cell ; the daughter-cells begin as nuclei, around which there gathers a protoplasmatic body for each. The cells are like the young decidual cells just below, so that the latter may be assumed to have wandered forth from the epithelium. I may recall that in the normal menstruating uterus I find no true decidual cells, and consequently I must regard Overlach' s find as pathological.

The observations of Creighton, of Masquelin and Swaen, and of myself may be fairly considered to establish the fact that in rodents, at least, the decidual cells arise from the connective tissue cells of the mucosa. That they arise from the same cells in man is rendered extremely probable by the investigations of Leopold, which have been confirmed and extended by the observations' recorded in §§ i6, 17, and 18, of the present article. Accordingly I assent to the second of the views above enumerated.

Ercolani erroneously regarded the decidual tissue as a new formation, arising after the total destruction of the mucosa. He observed the degenerative processes of the uterine epithelium, and the arrangement of the decidual cells around the vessels of the placenta in rodents and other mammals ; he inferred that the whole mucosa was degenerated and lost, but he never established the inference by observation ; he also inferred that the perivascular cells, being different from the surrounding tissues, were a new formation, but he never traced the actual genesis of the cells. In spite, however, of the absence of the observations necessary to establish his double thesis of the total destruction of the mucosa and the new formation of the decidua, he advocated his doctrine with the greatest earnestness, even to the last — see 91, 92. The failure of his hypothesis to find acceptance has been due not to any unreadiness to bestow merited acknowledgment upon his researches, but to the incompatibility of the hypothesis itself with the ascertained facts of the structure and development of the placenta. While, therefore, we utilize Ercolani's numerous and valuable observations, it will be a distinct gain for science to set aside his theory of the new formation of the decidua.

General considerations

We are now in a position to compare the changes in the uterus during menstruation and gestation. In both cases the processes begin with tumefaction and hyperaemia of the mucosa ; they continue with hyperplasia of the connective tissue (the decidual cells being regarded as modified connective tissue corpuscles) and with hypertrophy, accompanied by distention and contortion of the glands ; they both close with casting off the superficial layers of the mucosa, after which follows the regeneration of the membrane. The essential steps, then, are the same in both cases. The difference is, that during the long life of the decidiia graviditatis, changes supervene in the tissues which do not take place during the rapid menstrual cycle ; the mucosa of gestation is distinguished by the loss of both its surface and glandular epithelium, and by the enlargement of its connective tissue cells into so-called decidual cells. We must accordingly view the changes in the uterus during gestation as a prolonged and modifi-ed menstrual cycle. The relation in time between menstruation and the commencement of pregnancy is attributable to the menstrual process rendering the uterus receptive ; that is to say, capable of receiving and retaining the ovum. We must conceive that the ovum has no power of initiating the development of a decidiia, but only of modifying the menstrual process ; hence pregnancy can begin only at a menstrual period. The ovum, too, exercises this influence at a distance, for in all mammals, the earliest development of which is known, the ovum passes through its segmentation in the oviduct (Fallopian tube), and takes from three to eight days to reach the uterus ; but during this period the change in the womb is going on. The most plausible explanation of this action of the ovum at a distance is a reflex stimulus passing from the oviduct to the central nervous system of the mother, and thence back to the uterus ; the validity of this hypothesis must be tested by physiological experiment. That the nerves are able to effect morphological changes is already abundantly proven, not only by the influence of the secretory nerves upon gland cells, by the degeneration of muscular and other tissues, when their nerves are severed, but also by certain embryological observations tending to show that histological differentiation does not progress very far until the tissues are joined by the outgrowing nerves.

When the ovum reaches the uterus, it appears to exert a more direct influence, for one set of changes occurs in the placental area, where there is concrescence of foetal and maternal parts ; another in the region around the placenta (peri-placenta, decidua reflexa), and still another in the rest of the uterus (decidua vera, ob-placenta). Whether the three zones enumerated can be distinguished in the pregnant uteri of all placental mammals, and whether they have more features in common than appear from a direct comparison between man and the rabbit, are questions to be decided by increased knowledge. However, it already seems very probable that the decidua reflexa and peri-placenta are homologous at least in rodents.

Concerning the evolution of the amnion nothing definite is known, nor do the speculations of Balfour {Cotnparative Embryology, II., 256) nor of van Beneden and Julin, 44, 425, seem satisfactory, although the view of the latter is suggestive. They say : —

"Dans notre opinion, la cause determinante de la formation de renvellope amniotique reside dans la descente de I'embryon, d^terminde elle meme par le pois du corps. Cast par une acceleration du developpement que la cavity amniotique en est venu a se former quand I'embryon ne possede encore qu'un pois insignifiant." The chief objection to this theory is that it really gives no cause for the expansion of the somatopleure and chorion ; there is no proof that a mere strain of weight can cause the cells of a membrane to proliferate, and since such proliferation is the immediate cause of the growth of the amnion, van Beneden and Julin must assume for their theory that the strain of weight does cause proliferation ; but this assumption lacks support. Moreover, they give no evidence to show that the embryo in utero is situated in the primitive amniota upon the upper side of the ovum, although it is probable such is the case."[2]

Ryder's theory,[3] of the origin of the amnion, like that of van Beneden and Julin, to which he does not refer, is purely mechanical ; but Ryder seeks the cause in a rigid zona radiata, forcing the embryo down into the yolk. See his summary. I.e., p. 184. So far as we know, however, the embryo of the Sauropsida cannot be said to sink into the yolk, and so lead to the development of an amnion ; but, on the contrary, the amniotic folds rise up clear above the yolk. Moreover, the formation of the amnion is really a very complex process, part arising from the pro-amnion, part by a dilation of the pericardial cavity {ParietalJwhle), and part as the extra-embryonic tail folds. These facts speak, in my judgment, unequivocally against the amnion having arisen by the sinking of the embryo into the yolk sack. Nor is there any justification, I think, for seeking these simple mechanical explanations, which are worthy of Herbert Spencer, since the formation of the amnion depends upon inequalities in the growth power of the germ layers, and only such explanation can be valid as explains that inequality — which Ryder's hypothesis fails to do, so far as I can see.

As regards the evolution of the placenta, we are in the dark. Contrary to prevalent opinion, it is not an organ of the allantois, nor is it an organ of the yolk sack. On the contrary, it is always, so far as we know, an organ of the chorion, and begins its development by a differentiation of that membrane. The allantois is a secondary and later structure. Its primitive role is apparently only that of a stalk of connection between the chorion and embryo. There is no evidence to show that the tissue of the allantois spreads out over the chorion to form the mesodermic layer thereof, but the mesoderm of the chorion is proper to it as much as to any part of the somatopleure the mesoderm thereof. When the allantois becomes a large sack, we have a subsidiary change, so that we are brought squarely to the conclusion that the foetal placenta is chorionic. From this premise phylogenetic speculation must start. Further, we know through the discovery of fundamental importance by His that the allantois cavity is at first a small entodermal tube lying in a posterior prolongation of the body {Bauchstiel), and that at this time the so-called allantoic vessels run to and branch out upon the chorion ; the placental differentiation of the chorion has already begun, without participation of the allantois, the enlargement of which, when it occurs at all, occurs at a later stage. To speak, therefore, of an allantoic chorion as do Balfour and Selenka {Studieii iiber EntwicJiclungsgcs., p. 135) is unjustifiable. Nor can we trace the origin of the placenta to the yolk sack, since in most mammals the mesoderm does not spread over the yolk until quite late, so that the yolk sack consists, as in the rabbit and opossum, in large part of ectoderm and entoderm only, and is without vessels, and therefore unable to form a placenta, which, however, is developing meanwhile from the chorion.

We seek nowadays, following the lead of Professor Cope, to deduce mammalia from the reptilia. Since the reptilia have a free allantois, it is a temptation for embryologists to seek to trace the placenta to a modification of the allantois ; but the placenta of mammals appears in the embryo before the allantois becomes free, and the great size of the allantoic vessels is connected primitively not with the allantois, but with the already important chorionic circulation. The placenta is interpolated in the ontogeny of mammals before the specialization of the allantois, which functions as the vascular pathway between the embryo and the chorion, both primitively and permanently. The enlargement of the allantois, which takes place in certain mammals, is a supervening change, probably a survival of reptilian ontogeny. The question is, not how is the connection of the allantois with the placenta (chorion) established in mammals, for it exists from the start,i but what becomes of it in reptiles and birds.

Ryder's theory, of the origin of the discoidal placenta[4] by constriction of the villous area of the zonary placenta, is difficult to accept. The placenta, being chorionic, cannot of course develop, except so far as the chorion is differentiated ; that is to say, so far as the ectoderm (exochorion) is underlaid by mesoderm. Now, in mammals, the chorion, as mentioned above, does not go at first but part way over the yolk sack, even at the period when the development of the placenta has begun. Accordingly, so far as our present knowledge enables us to judge, the discoidal is probably the primitive placental type. If the chorion is completed by the further extension of the mesoderm around the yolk sack, then the placental formation also may spread, and a diffuse type arise. At present, the whole subject is very obscure, but there is certainly no sufficient evidence to prove that the diffuse placenta is the primitive type.

In conclusion, let me point out that we have no satisfactory knowledge of the nutrition of the embryo. We know positively scarcely more than that the maternal and foetal circulations are brought very close together in the placenta. We infer that there must be a transfer of nutritive material from one blood to the other. As to what material is transferred and how, we have only theories, but of them an abundance. Under these circumstances, the best beginning is undoubtedly a frank acknowledgment of our ignorance.


The following paragraphs attempt to give the more important of the conclusions reached in the second part of this paper.

  1. The umbilical cord is not covered by the amnion, but by an extension of the foetal epidermis. Its coelomatic cavity is completely obliterated during the third month, and a little later the stalk of the yolk sack is resorbed. The allantoic epithelium persists as a tube or cord of cells for a long period. The blood-vessels have specialized walls derived from the surrounding mesoderm, but have no true adventitia. Connective tissue fibres begin to develop during the third month.
  2. The amnion is covered by a single layer of ectodermal cells, which are connected by conspicuous intercellular bridges. It has no true stomata. Its mesoderm consists of anastomosing: cells, with a dense matrix ; it is imperfectly divided into three strata, of which that next the ectoderm is without cells, that furthest from the ectoderm is often of a loose texture.
  3. The chorion consists of two layers, mesoderm and ectoderm, both of which are present over all parts of the chorion throughout the entire period of pregnancy. The mesoderm has at first a dense colorable matrix, with cells, which color very sHghtly. During the second month the matrix loses its coloring property, and subsequently the cells acquire a greater affinity for coloring-matters ; the matrix assumes a fibrous appearance, and ultimately in the region of the chorion frondosum connective tissue fibrils appear in it, most numerously next the ectoderm, so that the mesoderm is differentiated there into an outer fibrillar layer and an inner and thicker stroma layer. The ectoderm during the first month divides into two strata, an outer dense protoplasmic layer and an inner less dense cellular layer. In the latter part of pregnancy the whole ectoderm of the chorion laeve has acquired the character of the cellular layer, except close to the margin of the placenta ; at the same period the cellular layer forms a number of irregular patches over the chorion laeve, while the protoplasmic layer remains over the entire surface, both where the cellular is present and where it is absent ; the protoplasmic layer may undergo complete or partial degeneration into canalized fibrine, which is developed in irregular patches. The cellular layer remains on the villi only in a few patches iZellknoteii) and over the tips of certain villi ; the protoplasmic layer of the villi remains everywhere and develops numerous nodular thickenings ; it changes partially into canalized fibrine. It is probable that the fibrine covering the surface of the decidua serotina is derived from the ectoderm of the ends of the villi imbedded in the decidua. The villi are at first of awkward and irregular forms, but their branching gradually becomes more regular, and the twigs acquire a slender and more uniform shape.
  4. The menstruating uterus is characterized by hyperaemia, by hyperplasia of the connective tissue of the mucosa, and by hypertrophy of the uterine glands ; the upper fourth of the mucosa is loosened and breaks off : there are no decidual cells.
  5. The uterus one month pregnant has lost its epithelium from its surface, and from the ducts of its glands ; owing to the dilatation and contortion of the deep parts of the glands, it is divided into a lower cavernous or spongy layer and an upper compact layer ; the connective tissue of the upper layer is transformed into decidual cells ; in sections the glands of the lower layer appear as crowded areolae, which are lined by a cylinder epithelium more or less disintegrated, or else filled with isolated enlarged epithelial cells.
  6. The uterus seven months pregnant is without epithelium either on its surface or in the glands, except a few isolated patches in the deep parts of the latter ; there is no trace of the decidua reflexa ; the decidua vera is covered by the epithelium of the adherent chorion laeve ; the decidual serotina is covered for the most part by a layer of fibrine, which is probably derived from the degeneration of the chorionic ectoderm covering the imbedded ends of the villi ; the decidua is divisible into an upper compact and a lower cavernous layer, in which latter the gland cavities are reduced to slits ; the decidual cells are very numerous and crowded ; the larger ones lie near the chorion ; the multinucleate decidual cells are found chiefly in the serotina ; at the edge of the placenta decidual cells are found in the chorion.
  7. The decidual cells arise by direct enlargement of the connective tissue cells of the mucosa. All parts of the decidua and placenta arise in place by metamorphosis of the tissue ; the mucosa is preserved, and there is no production of placental tissues by new formation.
  8. The changes of the uterus during menstruation and gestation are homologous, the menstrual cycle being prolonged and modified by pregnancy ; hence it is that conception takes place only at the menstrual period, for the ovum can only modify the menstrual change, not initiate the formation of a decidua. No satisfactory explanation of the origin of the amnion has yet been offered. The placenta is an organ of the chorion ; its evolution cannot be traced to modifications of either the allantois or the yolk sack ; the allantois is originally the intestinal canal of the Baiichstiel, which serves as the means of vascular communication between the chorion and embryo ; the enlargement of the allantois is secondary. We possess no positive information as to how the placenta performs its nutritive functions.

Boston, Aug. 3, 1888.

  1. Kolliker's Handbuch der Geiuebelehre, 5te Autl., p. 563
  2. Quoted from Buck's Reference Handbook, I., 140.
  3. This is beautifully shown by Selenka's investigations on the opossum, cited in the text.
  4. The human placenta is not discoidal, but metadiscoidal.

Minot, C.S. Uterus And Embryo (1889) - I. Rabbit II. Man - Plates

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