Talk:Book - Contributions to the Study of the Early Development and Imbedding of the Human Ovum

--Mark Hill (talk) 21:39, 31 October 2013 (EST): Early Draft of this historic 1908 text.

CONTRIBUTIONS TO THE STUDY

OF THE

EARLY DEVELOPMENT AND IMBEDDING OF THE HUMAN OVUM

AN EARLY OVUM IMBEDDED IN THE DECIDUA By THOMAS H. BKYCE, M.A., M.D.

LECTURER IN ANATOMY, UNIVERSITY OF GLASGOW

AND JOHN H. TEACHER, M.A, M.D.

LECTURER ON PATHOLOGICAL HISTOLOGY. UNIVERSITY OF GLASGOW

AN EARLY OVARIAN PREGNANCY By THOMAS H. BKYCE; JOHN H. TEACHER AND JOHN M. MUNKO KEKK, M.B.

OBSTETRIC PHYSICIAN TO THE MATERNITY HOSPITAL AND GYNAECOLOGIST TO THE WESTERN INFIRMARY, GLASGOW

WITH TEN PLATES AND TWELVE FIGURES IN THE TEXT

GLASGOW JAMES MACLEHOSE AND SONS PUBLISHERS TO THK UNIVERSITY 1908

PREFACE The following memoir combines in one publication two separate papers which deal with the processes involved in the imbedding of the human ovum. They have been associated because of the complementary nature of the evidence they afford regarding the histological characters and the activities of the trophoblast. Each breaks new ground, in respect that while the first paper embodies a description of the earliest phase of the human ovum yet recorded, the second deals with the earliest case of ovarian pregnancy hitherto reported.

The fact that the extremely early ovum described in the first paper is a unique specimen presenting features which have not up to the present been observed, necessitated profuse illustration both by coloured plates and photographic figures. This has rendered the production a costly one, and we desire to express our obligations to the Carnegie Trust of the Scottish Universities for giving us a grant towards the expenses of publication.

University of Glasgow, July 15th 1908.

CONTENTS

Introduction Paper No I. An Early Ovum imbedded in the Decidua History of the Specimen

Fixation of the Specimen

Dimensions of the Ovum

General Descriptions of the Sections

The Decidua

The Trophoblast

The Plasmodi-trophoblast The Cyto-trophoblast, - - - - - - - 19 Layer of Large Free Cells, - - - - - - - - 21 The Contents of the Blastocyst, ..---.- 22 The Mesoblast,- --------- 22 The Embryonic Rudiment, ------- 23 Discussion of Data, - - - - - - - - - - 27 Summary of Characters of the Ovum, ------ 27 Comparison with the Ova of Leopold and Peters, - - - 28 The Embryonic Rudiment, --------30 The Process of Imbedding, -------- 35 Comparative Data regarding Inbedding

Description of Imbedding of Human Ovum, - - - - 39 Analogy with Imbedding of an Embolus of Chorion-epithelioma, 42 Function of the Plasmodium, ------- 43 Fate of Early Plasmodium and Attachment of Ovum, - - 44

The Age of the Ovum and Relation of Imbedding to Menstruation, 47 Estimation of Age, - - - 48 Comparative Data, 50 Comparison with other Cases, - - - - - - - 51 Summary of Dat«, regarding Selected Ova, - - - - 52 Table of Selected Ova, -------- 59 Table showing Periods of Fertilization and Imbedding, - - 60 Relation of the Menstrual Cycle to Oestrus Cycle, - - - 61 Meaning of Menstruation, --._... g2 Table of Menstrual (^ycle, ------- 63 Ovulation and Menstruation, ------- 64 Nature of the Decidua, -------- 65

Paper No. II. An Early Ovarian Pregnancy

Introduction

History of the Case, ----.--._ 70 Naked-eye Description of the Specimen, ------ 72 Description of the Sections, ------..74 Discussion of Data, ----81 Summary of some other Cases of Ovarian Pregnancy, - - 81 Relations of Ovum to Lutein Tissue, - - - - 85 Origin of Lutein C'ells in Human Subject, . . . - 85 Process of Imbedding in Ovarian Pregnancy, - - - 87 Origin of Plasmodium

List of Works Cited

Introduction

Very early stages of the human ovum are necessarily extremely rare. It is only by fortunate and fortuitous circumstances that an occasional specimen comes into the hands of the investigator. Within recent years a number of young ova have been described, which have considerably extended our knowledge, and have served to show that in certain respects the early stages of development in man differ materially from those in lower mammals. The ovum of Hubert Peters, of which an account was published in 1899, still represents the youngest phase known. A specimen described by Leopold in 1906 is certainly earlier than that of Peters, but no embryonic rudiment was present, and in several other respects it must be considered abnormal. On the other hand, the ova described by Graf v. Spee (1905), Beneke, and Jung amply confirm, though they do not extend, the data provided by Peters' specimen.

Considerable light has been thrown on the problems involved in early human development by recent comparative work, more especially that of Selenka and Keibel on monkeys and apes, and of Hubrecht on Tarsius spectrum. It is now known that the Primates, including Tarsius in that category, form embryologically a group by themselves. All have certain common and peculiar features. There is always present a mesodermic connecting-stalk (Ha/tstiel), through which the vessels of the embryo and chorion are connected without the medium of an allantois ; the yolk sac is very minute and is not coextensive with the blastocyst ; there is a precocious extra-embryonic coelom lined by middle-layer cells, which are present at a very early period before the appearance of the primitive streak or embryonic axis, and therefore before the formation of the dorsal mesoderm of the embryonic body.

There are other features, however, in which the several orders of the Primates differ inter se. In Tarsius the amnion is formed as in the rabbit, dog, etc., by secondary folds, while in monkeys, apes, and man it is already closed in the earliest stages known. The placentation again, in the monkeys (Old and New World), differs from that in the anthropoid apes and man. While the early phases in apes and monkeys, described by Selenka, confirm and explain the corresponding phases in the human subject, none of the stages known reach to the initial stages of the blastocyst, and therefore much is still left for conjecture. The extremely young ovum, which is the subject of the first of the papers in this memoir, represents the earliest stage of any primate form except Tarsius yet recorded, and merits careful and det<iiled description in respect that it pushes back the limits of the unknown in a sensible degree.

The age of young human ova is, of course, from the nature of the case, quite uncertain. It is usually calculated in terms of the conventional rule formulated by Professor His, but the results of the rule as applied to the youngest know^n specimens are unsatisfactory and contradictory. In the present case we are fortunate in possessing very accurate data, and an effort will be made by correlating the facts with those known for other specimens to revise the basis on which the age of early ova is calculated.

Not only do the structural features of the early primate blastocyst remain unknown, but the process of imbedding and the initial phases of placentation are also merely matters of surmise. All the ova described before the appearance of Hubert Peters' monograph were found completely imbedded in decidua, and the hypothesis that the ovum becomes surrounded by a process of circumvallation was generally accepted, though in more recent times the results yielded by comparative embryology had caused some doubt on the matter in the minds of a few observers.^ Several of

• In William Hunter's Anatomf/ of the Oracid Utervs^ 1774, Plate 35, there is figured a complete decidual cast, in which an ovum about the size of a pea lies imbedded. In his diagrams William Hunter clearly indicated that the ovum is at this stage completely surrounded by the decidua, but he expresHed no opinion as to how it Injcomes implanted therein. The theory that th(^ decidua covei-s the orifices of the Fallo])ian tubes, and is pushed out by the ovum as it entei-8 the uU.'rus, lias been erroneously attributed to him. (See Historical Introduction to the Catalogue of Um Ati^ttomical ami Pat/uklotjical Preparations of Lh\ William Hunter. John H.

'*ie ova, such for instance as that described by Reichert, showed a small Ti^rea of the decidua capsularis (formerly called reflexa) over the blasto teyst, which was of a different nature from the rest of the capsule, and ^jipparently composed of cicatricial tissue. They were, notwithstanding, r^ completely enclosed by organised tissue. In Peters' ovum, however, and Ij^so in one described by Graf v. Spec (1905) there is a relatively large ^. area from which decidua is absent, and its place is occupied by a mass of fibrin and blood-clot (the *' Gewebspilz "). The aperture in the wall of the implantation cavity occupied by this mass was considered by Peters, and also by Graf v. Spec, as the point of entrance of the ovum into the substance of the mucosa, but their preparations do not by themselves conclusively demonstrate the actual process by which the ovum is implanted. To prove how this is effected still earlier stages are required. Our young ovum is a further step in the direction of assured knowledge, and as w^ill be seen later necessitates some modification in the interpretation of the "Gewebspilz" completing the capsule in Peters' specimen, while our ovarian ovum, which is the youngest hitherto described implanted in the ovary, throws considerable light on the nature of the imbe^lding process.

In the absence of the early stages in the human subject it is necessary to make use, for the purposes of interpretation, of the data provided by Comparative Embryology, but the remarkable variability in the methods of implantation and in the details of placentation in the diflFerent mammalian orders, speaks for a certain specific character of the embryological processes involved. Caution, therefore, is required in grafting any data derived from the investigation of the conditions in lower mammals on to the facts known for the human ovum, and the more so as the young ovum we have to describe accentuates the very special features of the human blastocyst in its early phases.

The only competent analogy with the higher primate ovum among the lower mammals is to be found among the forms in which there is likewise a decidua capsularis, for instance the hedgehog among the insectivora, and the mice, rats, and guinea-pig among the rodents. It is to be noted that in these forms, as in the Primates, the amnion is closed from the first, and that the blastoderm shows the phenomenon, to a greater or less degree, of "inversion of the germinal layers." Two methods of imbedding, which will be dealt with in greater detail later, have been described in animals with a decidua capsularis. In the hedgehog, mice, and rats, the ovum is said to be received into a recess or fissure of the mucous lining of the uterus ; the epithelium disappears round the blastocyst ; the mucous membrane becomes greatly thickened to form the decidua capsularis ; and the fissure is cut off from the general cavity of the uterus by the fusion of the lips of the decidual swellings from which the epithelium has likewise vanished. In the guinea-pig the observations of Graf V. Spce seem to prove that the ovum, while still in the early blastocyst stage, destroys the epithelium of the surface at the spot where it becomes implanted, by the activity of its ectodermic cells, and then, by a continuance of the process of destruction and solution, imbeds itself in the connective tissue of the mucosa. One or other of these alternatives must apply to the human ovum, and we submit our two communications as a contribution towards the solution of the problem.

In regard to the initial stages of placentation a very large body of data has been accumulated by comparative embryology, and our views as to the deciduate placenta have undergone considerable modification. Apart from the general character of the placentation in the different orders of mammals, debate has centred on the nature of certain layers of cells which separate the foetal from the maternal blood in the placenta. It is unnecessary here to enter on any detailed account of the various and contradictory opinions which have been held on this histological detail, or of the several theories which have been put forward on the subject of the origin of these layers.^ It is now very generally admitted that the evidence afforded by both human and lower mammalian material is in favour of the foetal, i.e. chorionic origin of both layers covering the villi in the human placenta. No doubt appears to exist in the mind of anyone as to the cellular layer, generally known as Langhans* layer, but there is still a lack of decisive proof regarding the plasmodial investment of the villi, or syncytium. Certain authors have maintained that it owes its origin to the maternal tissues — some deriving it from the epithelium either of the surface or of the glands of the decidua, others holding that it represents maternal endothelium spread over the villi in the interlocking of foetal and maternal tissues, which has long- been considered to take place in the development of the placenta.

• The different hypotheses are fully set forth by Hubert Peters, by Webster {Human Placentation)^ and by Strahl {Hertwt'ffs Uandhuch der EntwiclcduiKjitleJin') ; they have also been dealt with by Teacher in his papers on "Chorion-epithelioma," and are briefly summarised by Bryce in Quain^s Anatomy^ vol. i. 11th ed. 1908.

The idea that uterine epithelium is necessary for the production of the Plasmodium was finally excluded by Catherine Van Tussenbra^k when she demonstrated the existence of a syncytial layer on the villi of a chorionic vesicle imbedded in the ovary, and her observation has been confirmed by several investigators. It is just conceivable, however, that if a fertilized ovum developed in the interior of the GratHan follicle, the follicular epithelium might be responsible for the production of the Plasmodium ; but this cannot be the case if it be proved that the blastocyst may be imbedded in the ovarian stroma outside the corpus luteum. In the second communication embodied in this publication further proof will be provided that such a case may occur.

The theory that the plasmodial layer on the villi is derived from maternal endothelium has become practically untenable in view of the characters of the early ova described in recent years, but it is not quite so certain that it may not owe its origin to the maternal connective tissue in which the ovum is imbedded, altered by the biochemical activities of the ectodermic cells of the blastocyst In this connection earlier stages in the development of the plasmodium than have been hitherto available in the human subject, are required for the complete demonstration of its foetal origin in the human placenta. Our young uterine ovum, in virtue of its stage of development, and our case of ovarian pregnancy, being a crucial experiment both on the nature of the imbedding process and on the origin of the plasmodium, bring critical evidence to bear on this question.

I AN EARLY HUMAN OVUM IMBEDDED TN THE DECIDUA

By THOMAS H. BRYCE and JOHN H. TEACHER

AN EARLY HUMAN OVUM IMBEDDED IN THE DECIDUA

History Of The Specimen

The ovum was found by Dr. Teacher in a portion of membrane sent to him for examination by Dr. T. Doughis Brown, in a mixture of urine and l)lood-clot. The membrane had been expelled by a young woman who had been married for about two years, but who had not before been pregnant. Dr. Brown recognised a portion of fawn-coloured membrane among the blood-clots, which he considered to be probably a portion of the decidua of pregnancy, as the patient had passed the date on which menstruation was expected by about ten days. By the time the specimen reached Dr. Teacher the haemoglobin had diffused out of the clots, and the membrane was no longer distinguishable from them. In order to differentiate the tissue from the clot, about one-third of its volume of 90% alcohol was added to the fluid, and an hour later the mixture was decanted and fresh 30% alcohol substituted. Two hours later, on examining the material in a white porcelain dish, the membrane was readily identified by its somewhat lighter colour. It was roughly quadrilateral in shape, and somewhat broader at one end, measuring 3*8 cm. in length and 2 cm. in breadth. It represented the greater part, if not the whole of the mucous membrane of one wall, prol)ably the posterior wall, of the uterus. It presented the characters of decidua and was mapped out into areas by shallow furrows. Its outer surface had the characteristic appearances of the detached surface of shed decidua.

The margin was in parts rather thick and rounded, in other parts it faded off into very thin shreds. One of the areas near its centre stood up somewhat prominently from the rest of the membrane. The papilla BO produced (Figure i) at its right m.irgiu slightly overhung a deep furrow, but at the ends and on the left side it sloped gradually down to the general level of the surrounding membrane. This elevated area mcEtsured roughly 6 ram. by 3 mm. ; it was nearly 3 mm. in thickness, while the rent of the membrane did not exceed 2 mm., and was for the moat part thinner. About the centre of the papilla there was a circular mark like a very shallow dimple, and of lighter colour than the rest of the surface. The adjacent tissue was of deep red colour as if congested, and a still darker patch above the circular mark was regarded as probably due to haemorrliiige into the memlmme. The area thus described is very similar in appesiraiK-e to the lol)ule of deridua in which the ovum investigated by Leopold in 190G was enclosed.

FiGURE I. Portion of Decidual MEMKn*KK I'he prominent lolinle ia tim actuni nitc uf impliinLition. The iudiattncb (limplcliko mark slightly belcm- the centre of the laliule oorreB|MiniU to the position of the lilustocyst. The darker shading represents congestion, and the dark apot above the dimple is a haemorrhage.

Although it seemed improbal)lo that after twenty hours' immersion in a mixture of blood aud urine the fixation of the preparation could be satisfactory, the membrane was placed in abaolntu alcoliol, this medium having before been found to be the most satisfactory fixative in similar circumstances. After thirty hours the lobule was excised, imbedded in paraffin, and cut by William Price, the laboratory attendant, into a perfect series of sections 7 microns thick, vertically to the surface of the decidua and in the supposed long axis of the uterus.

The probability of finding a valuable ovum being somewhat remote, thirteen sections, at intervals, were cut out of the ribbon, mounted and stained. In three of these Dr. Teacher recognised what seemed to be an ovum of about 1 mm. in diameter, and the remainder of the ribbon containing about 400 sections was then mounted and stained with haemalum and eosin. At a later stage the first series was restained by Weigert's fibrin stain in order to obtain a more accurate difierentiation of the Plasmodium from the decidua.

The following details relative to the history of the case have been supplied by the husband of the lady, himself a man of science and therefore alive to the importance of exact data. They can be implicitly relied upon.

The patient and her husband are both healthy. There has never been any uterine disorder. Menstruation has always been regular, the intervening period averaging twenty-six days with an occasional variation up to two days in either direction, the irregularity being however usually compensated ; if one interval were short the succeeding interval was usually longer by a corresponding amount, and vice versa. The data may be summarised as folio w^s : September 2nd, 1907. Menstruation began.

September 27th, 1907. Menstruation began.

October 2nd to 3rd or 3rd to 4th. Coitus.

October 19th to 20th. Coitus.

October 25th. Menstruation was expected but did not appear. In the succeeding days the patient felt particularly well ; she had no symptoms suggestive of pregnancy except the absence of menstruation.

November 3rd to 4th. Coitus.

On the morning of the 4th of November a discharge appeared like the commencement of menstruation.

12 AN EARLY HUMAN OVUM On November 5th the bleeding became more profuse than at an ordinary menstrual period, and the clots and membrane were passed towards evening with considerable pain.

The patient made an uneventful recovery, and menstruation reappeared twenty-six days after the fourth of November.

Fixation of the Specimen

Notwithstanding the circumstances under which the ovum was obtained the state of preservation of the tissues is wonderfully good. The mixture of blood serum and urine apparently behaved as a neutral fluid, which destroyed the tissues far less than they would have been destroyed by water or even by ordinary normal saline solution. Though we cannot claim that the fixation is perfect, we are satisfied that it is sufliciently reliable for all practical purposes. The nuclei are on the whole well fixed, but the protoplasm is less well preserved and has in parts a slightly macerated appearance. The colouring matter of the blood has been almost entirely dissolved ; the red corpuscles are, therefore, shadowy, but they have for the most part retained their normal shape. There is further a certain amount of granular debris in the blood spaces. The effects of precipitation and maceration are not, however, sufficient to invalidate conclusions regarding the essential facts of the histology of the ovum and of the decidua. The embryonic rudiment is somewhat torn, but it has been possible by reconstruction to make good the defect in this respect. It has generally been assumed that the only hope of getting a young normal ovum, is the chance of an operation or of a suicide. The present case shows that even in the case of an early abortion, an ovum may be cast off* intact in the mucous membrane, and recovered in quite a satisfactory state of preservation. There is no reason for supposing that the mucous membrane was diseased in this case, and it is probable that the abortion was due to mechanical causes, seeing that it ensued immediately after coitus.

DESCRIPTION OF SECTIONS 13

DIMENSIONS OF THE OVUM.

The ovum is completely surrounded by decidua and lies in a blood filled space excavated in it. The dimensions of the cavity at their maximum are 1*95 mm. in its longest diameter (parallel or approximately parallel to the vertical axis of the fragment of decidua figured on page 10), '95 mm. in depth, and Vl mm. in its third dimension. The last measurement is arrived at by summing the number of sections 7/^ thick in which the implantation cavity appears. The space enclosing the ovum is thus oval in shape, with its long axis parallel to the surface and about twice the length of its vertical axis, or depth from the surface. The dimensions of the blastocyst itself are rather difficult to determine with exactness owing to its wall being a little folded at several points. In Plate I, Fig. 1 and Plate in, Fig. 3, which represent a section well to one side of the equator of the vesicle, the cavity measures about '36 mm., and it is approximately circuLir. In Plate ii, Fig. 2 and Plate iv, Fig. 4, which is a section cutting the equator of the blastocyst, the internal measurements are '77 by *63 mm. in its two dimensions. The shorter measurement may be taken to represent the maximum size of the cavity, seeing that the blastocyst is obviously compressed and folded at one side ; it is probable that the vesicle is spherical at this stage, because the contour of the unfolded wall is approximately the arc of a sphere.

GENERAL DESCRIPTION OF THE SECTIONS.

Plate I, Fig. 1 shows the general relations of parts as revealed by a low power of the microscope. The section cuts the whole length of the decidual lobule described above. It is marlced off above by a fissure which indicates the upper limit of the tubercle seen in Figure 1 ; below it ends in a rounded and slightly overlapping projection which corresponds to the lower edge of the papilla. The free surface of the lobule shows above the mouth of a gland, and over the ovum an irregular depression which is the orifice of the space excavated in the decidua. This does not correspond to the dimple seen on the surface of the lobule ; that is represented by a shallow depression which appears on the surface immediately over the centre of the blastocyst, and is seen in Plate ii, Fig. 2. The blood-vessels of the decidua are greatly dilated, more especially on the deep aspect of the blastocyst, where they form the same "blood cushion" seen in the ova of Peters, Ercole Cova, and Graf v. Spec (1905). The glands are enlarged and irregular. Beneath the blastocyst there is great extravasation of blood and deposit of fibrin, while the upper part of the tubercle is seen to be occupied by a haemorrhage.

The implantation cavity is clearly marked off all round, but more especially at its outer part, by a darker staining band which, as will be seen presently, represents a layer of necrotic tissue. The cavity is occupied by very irregular strands of tissue, the spaces of which are densely packed with blood corpuscles. Above, the space is encroached on and the zone of necrotic tissue broken up by the haemorrhage already alluded to.

Under a higher power (100 d) the blastocyst wall (Plate iii, Fig. 3) is seen to be composed of a lightly staining lamella of protoplasm, in which the cell outlines are indistinctly defined, while the very irregular meshwork occupying the space in the decidua is observed to have plasmodial characters.

In describing the histological details of the several parts, we shall first take the decidua, then the trophoblast, and lastly the embryonic rudiment.

THE DECIDUA.

The surface epithelium has completely disappeared. Whether this be the result of maceration, or is a normal feature, it is not possible to ilotermino. The fact that it is also absent from the outlying parts of the decidua, while it is present in the cases of Peters and Leopold, suggests that it is due to superficial maceration.

There is no coagulum or organised thrombus adhering to the surface over the ovum. The mucous membrnno shows all the well known characters of the menstrual decidua. The cells are clear and swollen, the tissue spongy : the vessels aiv givatly dilated; the glands arc enlarged, while their epitbeliuQi is desquamating and their lumen is occupied by extravasated blood. The whole membrane is thickly studded with leucocytes, mainly of the polymorphonuclear variety. All round the ovum the marginal lamella of the decidua is in a state of advanced coagulation necrosis, appearing as a hyaline, darkly-staining, and nearly nuclear-free zone dotted with polymorphonuclear leucocytes. Stained by Gram's method this zone stands out as a deeply staining purple band, forming the wall of the implantation cavity. The plasmodiuni is not everywhere in contact with this layer. Here and there masses of it lie directly against the necrotic tissue, but round the rest of the circumference the two are separated by a space occupied by large mononuclear cells, the nature of which will be discussed below. The necrotic zone is broken up by the haemorrhage in the upper part of the section, and here and there, but in relatively few places, by masses of plasmodium, which are seen spreading outwards along the walls or into the lumina of blood-vessels. The mass of cells seen in Plate ii. Fig. 2 and Plate iii. Fig. 3, occupying the centre of the space between the cyto-trophoblast and the wall of the implantation cavity, corresponds to the layer of cells seen lying elsewhere immediately within the necrotic zone ; the cells have here been displaced inwards by the haemorrhage which has broken into the implantation cavity.

At either extremity of the implantation cavity an interesting condition of the glands is to be observed, from which it would appear that the gland w^alls resist the disintegration longer than the genera] tissue of the mucous membrane. The sections across each pole of the chamber show a gland in section with necrotic tissue and plasmodium on either side of it, as if the process of destruction were extending round the gland wall and isolating it. The implantation cavity is, as already stated, completely closed. There is no large aperture filled with blood clot such as is seen in Peters' ovum, or Graf v. Specs youngest specimen. Over the blastocyst, but distinctly nearer one end of the implantation cavity, is a well marked pocket, '1 mm. in diameter, partially filled with what appears to be thrombus (Plate iii. Fig. 3). Directly continuous with this, and reaching into the implantation cavity, is a spur-like projection of hyaline material only distinguishable histologically from the necrotic, zone of the decidua by being devoid of nuclei.

This depression is (dearly the mouth of the space in the decidua, but there is no direct evidence to show whether it represents the point of entrance of the ovum, i.e. the point where it first began its destructive action on the decidua, or the closed mouth of a fissure into which the ovum had been received. This problem will be discussed in a later section.

The Trophoblast

The term trophoblast, i.e. trophic epiblast, will be used here in the original sense in which it was employed by Hubrecht, to designate that part of the ectoderm which does not share in the upbuilding of the embryo, or of the amnion in the human subject, but only in the attachment and nourishing of the ovum. The trophoblast includes the whole thickness of the wall of the blastocyst, and is differentiated into two parts — the Cyto-trophohlast or cell layer, in which the cell outlines are more or less preserved, and the Plasmodi'troj^hohlast or plasmodium, in which they are wholly lost.^ THE PLASMODT-TROPHOBLAST.

The plasmodi-trophoblast forms an extraordinarily extensive spun-out investment for the ovum. It occurs in masses, bands, or threads. It is difficult to differentiate, in places, the fine threads from fibrin filaments. The plasmodial masses are distinguished by the dark, slightly rusty-red tint with which they st^iin, forming a sharp contrast with the blue-pink of the immediate wall of the blastocyst, and the red-pink of the necrotic layer of the decidua (Plate in. Fig. 3). The nuclei differ from those of the cyto-trophoblast in respect that they are invariably small and stain darkly. This latter character is due to the finely granular nature of the chromatin network, which in the nuclei of the cyto-trophoblast is more open, loose, and reticular.

• The terms oytobljust and plasnuKlihlast were suggested hy Van Beneden. The terms used in the tt»xt aiv tht^se now employtnl by Hubinjcht. In so far as there jK'i-sists a certain i^mnant of doubt ivgaitling the origin of the plasmodium in the human ovum, the use of these terms at this iK)int of our inipiiry involves in some sense a /n^tifio j)rincij)n'.

The central strands of plasmodium are arranged round the cytotrophoblast in many places as an apparently laminated formation, with numerous spaces or clefts, which give an appearance of sharp separation of the layers, but between the spaces the cellular layer passes directly into the plasmodial. In some of the isolated masses of plasmodium vacuoles occur, which are either empty or partially filled with granular material. The vacuoles occur in some instances as single spaces, but more frequently they are multiple, and all intermediate stages are seen between masses in which vacuolation is commencing, and the spun-out plasmodial reticulum, the meshes of which are filled with maternal blood corpuscles. Plate v. Fig. 5, shows the characters of the plasmodium as revealed by a higher power of the microscope. To the left and below, a mass of plasmodium is seen lying free in the blood space, and in the early stage of vacuolation, while further to the right, interposed between two portions of decidua, is a larger and more vacuolated portion which is in direct contact with the necrotic zone of the decidua. It lies in the lumen of a greatly dilated capillary which has become directly continuous with the blood space round the ovum by the destruction of its wall. The endothelium of the vessel still persists on the surface of the detached mass of decidua to the left. At first sight it might be inferred that this mass of plasmodium was a portion of the closely adjoining network of the same tissue, but this is not so. When traced through the series of sections it was found to occupy the lumen of the vessel for a considerable distance, and to spring ultimately from the general plasmodial mass much nearer the pole of the blastocyst, where it could be seen entering a gap in the vessel wall.

In Plate viii, Fig. 10, an irregular mass of vacuolated plasmodium is seen lying in a bay in the necrotic zone ; it is extending into the decidua in close relation to a vessel which appears in the adjoining sections. In other situations large masses of plasmodium are spread out against the inner face of the necrotic zone — as if anchoring the ovum in the cavity.

The great inequality between the plasmodial mass in the upper part and that in the lower part of the section figured in Plate iii, Fig. 3, is clearly due to the haemorrhage into the upper part of the implantation chamber. The effect of the blood extravasation comes out rather more clearly in Plate ir, Fig. 2, in which the plasmodial meshwork can be observed to be pressed down to some extent on the wall of the blastocyst, while the layer of large cells elsewhere lying immediately within the necrotic zone is displaced inwards. The larger masses and bands have, as might be expected, resisted the pressure, and one process remains applied to the decidua as what appears to be an anchoring strand (Plate iii, Fig. 3). The inner part of the plasraodi-trophoblast has the appearance of irregular branching lamellae laid down round the wall of the blastocyst. In no section, or part of a section, is any portion of the wall of the vesicle left uncovered by a plasmodial layer. Here and there in the formation there are masses of what appears to be coagulum, but it is not impossible that some of these, at any rate, represent portions of necrosed Plasmodium.

The distinctive dusky-red colour of the plasmodium is brought out in Plate V, Fig. 5. At this magnification a granular structure is revealed in the protoplasm which is absent from the cyto-trophoblast, and under a still higher power this appearance is discovered to be due to an alveolar structure in the protoplasm.

The whole appearances presented by the plasmodium lead one to infer that the extraordinary irregularity in the disposition of the layer is due to a process of vacuolation which has broken up the larger solid masses into a sponge-work, and that the trabeculae of this sponge-work have broken down so as to allow the blood shed into the implantation space by the opening of the vessels, to piiss into its meshes.

We thus reach a conception of the origin of the primitive blood lacunae in the trophoblast not unlike that of Peters, but it will be observed that the spaces are ])roduced, in the first instance, entirely in the plasmodi-trophoblast. In this respect our ovum reveals a condition of the human blastocyst hitherto unsuspected ; its walls are at this early stage almost wholly plasmodial, with the exception of a thin germinal layer or mother-zone of cyto-trophoblast forming the immediate wall of the vesicle.

While our specimen speaks for the production of the blood lacunae by the formation of spaces in the trophoblast into which the extravasated blood is shed, it cannot be concluded that they are formed in the first instance in a uniformly thick lamella of plasmodium constituting a tolerably regular wall to the blastocyst. The plasmodi- trophoblast from its very nature is probably highly irregular from a very early stage, and therefore the blood lacunae may in part represent spaces intervening between outgrowing phismodial masses — between what may, in fact, be termed primitive plasmodial villi.

The Cyto-Trophoblast

The cy to- trophoblast constitutes a relatively thin lamella which forms the immediate wall of the vesicle. The lamella is sharply diflFerentiated from the plasmodi- trophoblast by its staining reactions. It is tinted, in haemalum and eosin preparations, a delicate blue-pink colour. The cell outlines are nowhere sharply defined, but they can be readily made out in sections in which the blastocyst wall has been tangentially cut. Elsewhere the appearance is rather that of a zone of protoplasm with embedded nuclei. As has been already mentioned spaces occur between the cytotrophoblast and the inner laminated layer of the plasmodium, but between the spaces the two formations are directly continuous. While at these points the cell-layer and plasmodial layer are distinguished in histological characters, the transition from the one to the other is not sharp and defined; the cell-layer changes its tone to pink, and passes uninterruptedly into the dusky-red plasmodial layer. The nuclei are extraordinarily irregular in size, though all show the same loose character of the chromatin reticulum, with one or two chromatin nucleoli (Plate vii. Fig. 7). They differ markedly, as already stated, from the nuclei of the plasmodium, which stain deeply and have a granular appearance. Here and there the innermost nuclei tend to be arranged in a row for a short distance, as if the cells next the cavity were assuming the epithelial disposition which characterises this zone in Peters' ovum. This arrangement of the nuclei is also seen in Leopold's youngest ovum, but in our specimen it is much less definite even than in that extremely early stage.

The uiiclei of the uyto-truphobhist are clearly in very active division. Mitotic* tigures are not numerous, nor are they very well preserved, but their presence in<licates that the ovum was in all probability in active i^rowth diortlv before beincj cast oft' The cell characters are best made out in the tangential sections which cut the poles of the blastocyst. Plate VIII, Fig. 11, is a photograph of one pole and Plate vii, Fig. 7, is a drawing of the other. A great many of the cells have either double, triple or even multiple nuclei. This may be observed Ijoth in the drawing and photograph. In the photograph (Plate viii. Fig. 11) the cell outlines are clearly distinguishable ; there is evidence of cell division ; and it may be pointed out that the large dividing cells belong to the innermost layer of the cyt«)-trophoblast. The absolute continuity of the cyto-truplioblast and plasmodi-trophoblast is brought out <-learly in Plate vii, Fig. 7. The a])pearances here, and all round the blastocyst wall, altogether preclude any other conclusion than that the cyto-trophoblast, by active and continuous proliferation of its cells, is gradually diflerentiated at its outer mar»n*n into plasmodi-trophoblast. The cyto-trophoblast is, in short, the terminal zone of the trophol)last.

At one or two places the cyto-trophoblast shows minute buds extentlin<^ from its outer aspect. In one situation the bud has taken the form of a narrow cell column, on each side of which, but separated from it bv a s])ace, is a strand of plasniodium. The space does not contain blood, 1 it corresi)onds to the clefts seen elsewhere between the cell-layer and il'ismodium. These buds, though very rare, we take to indicate a com

niff m-oliferation of the cellular layer, which will ultimately lead to m^ n»- ■^o 11 he formation of the cellular villi of Peters stage. The grounds on whieli . . •„ kosed will be discusse<l hitcr; meantime it may be stated •iji< opinion IS uiii^ >'* ] ^-^ buds arc clearly outgrowths of the cell-layer, and not plasmodial ^"^^.r^',,.|^i^.h have reverted to a (rellular condition.

. ^ ^^Q^ the .cyto-trophol)last show any such columnar . 11 ^j^ would indicate the i)icscnce of a layer of '-'■"'" - ,^^ :„ ..,iv part of the wall of the Mastocyst, nor any 11 ^.,,.rpcont the formative cell mass continuous with

It will be convenient at this point to consider the origin of the large cells which are seen lying free in the blood space within the necrotic layer of the decidua. These cells closely resemble bodies which are certainly cross sections of plasmodial strands, but when traced through the sections it becomes quite certain that they are not continuous with the Plasmodium, but are really isolated cells. Plate vi, Fig. 6, shows them at a point where they form almost a continuous layer for a considerable extent. The necrotic zone of the decidua is depicted. The inner edge of the dead or dying tissue is extremely irregular and is excavated into bays, many of which include one or more of the cells under consideration. Within the layer itself are seen spaces inclosing cells in different phases of degeneration. The nuclei are identical with those of the completel}^ free cells, and the protoplasm stains the same dusky-red in both cases. There are two ways of interpreting these appearances.

1. The cells within the spaces in the necrotic tissue may be foetal, i.e. trophoblastic, derivatives which, having wandered outwards, are here caught in the act of attacking the necrotic wall of the implantation space and so causing its gradual enlargement.
2. The cells may belong to the decidua itself, and are being set free from the necrotic zone as it is absorbed and the implantation cavity is enlarged.

The following considerations are in favour of the latter alternative :

1. The cells cannot be cross sections of plasmodial strands because they cannot be traced back into the general plasmodial meshwork.
2. The outer shell of trophoblast is entirely plasmodial. No cells which can be definitely identified as embryonic, occur anywhere except in that part of the wall of the blastocyst which we have called the cytotrophoblast.
3. The cells, both in the character of the nuclei and reactions of the protoplasm, agree with cells further out in the decidua, which are clearly degenerating maternal cells, and are distinguishable from the trophoblast in both these respects. They are quite different from the wandering trophoblast cells of later phases.
4. The Plasmodium and necrotic zone of the decidua are not everywhere directly in contact with one another; a union between the two occurs only here and there ; elsewhere a space is left between the two containing red blood corpuscles and leucocytes. There is no indication that the wall of necrotic tissue is being absorbed by phagocytosis in the strict sense of the term ; the appearances suggest rather solution by enzymes produced by the trophoblast.

On the whole we are inclined to conclude that these elements are derived from the necrotic zone of the decidua; that they are ifn short decidual cells set free in the process of absorption of the necrotic tissue. It must be admitted however that there is no histological criterion by which it can be absolutely determined whether they are maternal or foetal derivatives.^

The Contents of the Blastocyst

The Mesoblast

The cavity of the vesicle is occupied by a very delicate cellular reticulun), or loose syncytial tissue which has the characters of mesenchyme. It represents the earliest stage yet observed of the mesoblast. This mesenchymatous tissue shows no signs of cleavage into a parietal and a visceral layer ; it is not yet arranged in a definite and denser layer round the wall of the vesicle, nor are there any processes of it indenting the wall.

The constituent cells of the mesoblast are minute, rounded or stellate elements united together by very delicate protoplasmic threads. They are brought out in the drawing reproduced in Plate ill, Fig. 3, but the network which they form is somewhat obscured by the delineation, in the interests of exact reproduction of what the section shows, of an extremely delicate reticulum, which is clearly an artefact due to precipitation of the fluid basis of the tissue. The coagulation of the tissue has clearly caused some contraction of the mesoblast, which has resulted in its withdrawal from the centre of the vesicle so as to leave a clear space in which the embryonic rudiment is situated. This retraction of the mesoblast has caused some degree of tearing of the embryonic rudiment.

• Since the above wjis written we have had the opportunity of seeing a demonst ration by (iraf V. 8pee of the eaily ])haHes of ])laeentation in the j]ruinea-pig, at the meeting of the Anatoniische Ciesellseliaft in l^rlin (Apiii 23rd, IDOH). (iraf v. 8i)ee showed that in the guineapig there is a layer of cells of fotjtal origin oiitj^ide a ])l{isni(Klial formation, which, though lesM extensive, has the same character as the ])lasmodinm in our human ovum, — in short, that there are three layers of the trophoblast. Until his account of his preparations is published we can do no more than j)oint out that (Iraf v. Spee's researches may possibly necessitate some modification of our interpretation of these elements.

The Embryonic Rudiment

In the absence of any spot in the wall of the vesicle which could by any possibility be regarded as embryonic ectoderm, and of any thickening which could represent the inward projecting embryonic knob, we must recognise the embryonic rudiment in two closed vesicles which occupy the central retraction space in the mesoblast; There is a larger vesicle and a smaller. It is unfortunate that the larger is collapsed and considerably torn, while the smaller, though complete, is probably slightly displaced, as it is not directly in contact with the larger sac.

The larger vesicle extends through 24 sections (Figure ii, 1-24), and therefore measures in this axis '168 mm. The cells forming its walls are small compared with those of the trophoblast, and are cubical rather than columnar, but the protoplasm is frayed, and it is evident that the cell bodies have suffered considerably from defective fixation. The nuclei are rounded and fairly regular, though an occasional flattened nucleus occurs in what appears to represent the roof of the cavity. The vesicle hangs free in the central space, being definitely attached at one point only, where presumably the mesenchymatous tissue was more resistant.

A reconstruction in wax (Figure iii) makes it quite certain that we have to do with a closed but torn and collapsed vesicle with uniform walls; there is no distinction between the cells of the roof and those of the floor. There is no indication of a passage from the cavity towards the surface of the blastocyst.

The smaller sac extends through six sections (Figure ii, 26-31), and measures therefore '042 mm. The cells forming its walls are more flattened than those of the larger vesicle. It is quite certain that though it is of very miuute aize the formiition is n closed sac, and not an accidental groujiing of the mesoblaat celU. It is inipoitant to note that while the larger vesicle is attached definitely only at one point, the smaller is closely surrounded by nicsoblast strands. These are absent only in one section, and on the side looking towards the position in the central space which is occupied in other seetioua by the larger vesicle.

Figure II. OUTLINK DkaWINCS OF rOHTlONS OK THK EMBHYONIC RUDIMKNT AS IT AITKAIW IN THIRTY succKsmvK SECTIONS. X 320 I). The exact histological characters are shown in Plate VII, Figs. 9 and 10.

Flqdbk IIL Dbawini

tliR ayto-trophoblast : the inner lighter, the

the meBoblaBt. It uonlajna two venicles : the larger is the toi-n and coUnpeed amnid-embryoDio vesicle, the smaller ia the entodermio vtsiole.

The model was uunstructed by liiitlilitig up wax pUteN to re[ireaeilt what was really n cut of the cavity of the blutocyat, with a npnce in ita centre cautaining the veaiclea. The model wm then cat acroaB ao as to preaeot to view the portinu of the retraction cavity containinc both vesicloa. I'his portion of the model wag then uaBt in plaster of Paria, and an outer enBo conatruct«d whii:h represents in A diBf^rammatlo fashion the cellular layer ol the blastocyst wall.

The model {Figure iii) shows the relative sizes of the two sacs, and their position in regard to one another. The space between the two is obviously considerable, but aa it contained no mesohlast it is probable that there baa l)een a certain amount of accidental displacement. The appearances strongly suggest that the larger vesicle has suffered most in the shrinking of the tissue surrounding it. It seems to have collapsed and drawn away from the smaller vesicle. Two forces must have operated upon it — the centrifugal traction of the shrinking mesoblast, and a centripetal traction due to the precipitation of its fluid contents and contraction of its walls. The result of these opposing forces is the extensive tearing of the walls of the vesicle that has taken place. The other vesicle being very much smaller, and not so directly in the centre of the retractive force, luis suffered much less.

After careful consideration of the sections and model, the conclusion is inevitable that the larger vesicle represents the amnio- embryonic cavity, and the smaller the entodermic vesicle or future yolk sac. The data on which this interpretation is based will be discussed in the next chapter; meantime it may be said that it is in harmony with what we now know of the early primate blastocyst, and with the recent views as to the development of the amnion in several lower mammals. It may also be pointed out that this is the earliest phase of the human embryo yet observed. In Peters' ovum the rudiment consisted likewise of two closed vesicles, but the larger or amuio-embryonic vesicle showed a differentiation of the floor into embryonic ectoderm, and the roof into the amnion. Our ovum shows a still earlier condition of the amnio-embryonic cavity, in which there is as yet no distinction between embryonic and amniotic ectoderm.

Discussion Of Data

From the foregoing description it will be apparent that the present ovum differs in certain important respects from any human ovum hitherto described. It is also in some of its features unlike any ovum with which we are acquainted among the lower mammals.

The special characters of our specimen may be summarised as follows :

1. The blastocyst is completely enclosed in decidua except at one point, where there is a small gap closed by a mass of fibrin and leucocytes. The wide gap closed by the mushroom-like mass of fibrin and blood clot seen in Peters' ovum is entirely absent.
2. The ovum lies, bathed in blood, in a relatively large implantation chamber with the walls of which it is not united. There is no interlocking or mixing of maternal and foetal tissues. The innermost layer of the decidua lining the cavity is in a state of advanced coagulation necrosis, and this, together with a certain amount of fibrinous deposit, forms a layer of dead material which is practically complete except at one or two points where blood-vessels have been opened up, and at one end where a haemorrhage has broken into the implantation chamber.
3. The wall of the blastocyst consists of an inner lamella (cytotrophoblast or cell layer) composed of cells rather ill-defined from one another, and continuous externally with an extremely irregular formation which has definitely plasmodial characters (plasmodi-trophoblast). This forms a straggling reticulum, the meshes of which are filled with maternal blood (primitive blood lacunae). There is no protrusion of the cytotrophoblast into the plasmodi-trophoblast strands.
4. The cavity of the blastocyst is filled by a delicate tissue having the characters of mesenchyme. There is no cleavage of this early mesoblast into a parietal and a visceral layer ; it does not form a distinct lamella round the wall of the cavity ; and there are no protrusions from it representing future mesoblastic villi.
5. The embryonic rudiment is represented by two eccentrically placed vesicles slung in the mesenchyme by fine protoplasmic threads. They are quite separated from the wall of the blastocyst by mesenchyme, and the cells forming the two sacs have definite and difierent characters, but inter .se show no differentiation. The cells of the larger (amnioembryonic) vesicile are cubical ; those of the smaller (entodermic) vesicle are flattened.

Remarkable as some of the features of this new ovum are, there is no reason to suppose that it is in any way abnormal or pathological. Every one of its characters, as we shall now proceed to show, harmonises admirably with known later stages. It is in no sense contradictory or bizarre. It is not only consistent in itself, but is also consistent both with admitted facts and with inferences founded on these facts.

In the following discussion we shall first establish the position of our ovum relative to the earliest ova hitherto recorded, more especially those of Iieo])old (1906) and of Peters, and consolidate the basis of our interpretation both of the trophoblast and of the embryonic rudiment.

Comparison of the Present Ovum with those of Leopold and of Peters

In Leopold’s ovum nothing like an embryonic rudiment Wtis found. Towards the centre of the implantation cavity is a sac which Leopold regarded as the blast o(*yst. It is a thin walled and irregular vesicle containing tissue similar to the mesenchyme in the present ovum and in that of Peters, but infiltrated with red-blood corpuscles, obviously maternal, and separated more or less from the wall of the vesicle by a s})ace containing l)lood corpuscles (cf Leopold. I DOG, Plate x, Fig. 18). The wall of the blastocyst is described as a thin mantle of ectoderm showing two layers of cells, the inner consisting of rounded or oval elements closely appjied t<3 one another and almost filled by large darkly staining nuclei, the outer of larger granular cell-masses with one or several nuclei, having thus the charactei-s of plasniodium. The cells of the inner layer he distinguishes as Langhans* cells. Here and there the layer thickens into buds of Langhans' cells covered by plasmodium. These buds stretch out into long processes containing cells of both types, but there is never any outgrowth of mesoblast into them. Both cell layers are certainly of ectodermic origin, indeed Leopold regards any separation of them as inconceivable in light of the appearances in his specimen. The processes are attached here and there to the decidua, the terminal cells insinuating themselves among the tissues of the latter. The implantation cavity is a globular space filled with maternal blood in whi(*h the ovum floats freely, being anchored only by the tips of the trophoblastic processes.

There are distinct points of resemblance between our ovum and that of Leopold, but the differences are also strongly marked. The first striking point of contrast, apart from the absence of an embryonic rudiment, is that in our case the trophoblastic processes are entirely plasmodial ; any cells which could bear comparison to Langhans cells are confined to the thick wall of the vesicle, and the anchoring strands of Plasmodium are fewer in number. The undifferentiated condition of the blastocyst wall in our ovum, and the absence of a cellular layer in the trophoblastic processes, lead one to infer that it is probably younger than Leopold's.

While the characters of the blastocyst, the haemorrhage into its interior, and the absence of an embryonic rudiment are abnormal features in Leopold's ovum, on the other hand, in respect of the trophoblast it very possibly represents a normal intermediate stage between the present specimen and Peters' ovum. It is known that in later stages the villi develop practically normally in the absence of an embryo, and doubtless the trophoblast may also do so at this early stage.

The blastocyst wall in Leopold's- ovum, while it has a general likeness to that in the present ovum, is a less definite and a thinner structure, and has a greater resemblance to the layer of cells, with commencing cubical arrangement, covered by endothelium-like syncytium, which constitutes the greater part of the blastocyst wall in Peters' specimen (cf. Leopold, Figure 18, with Peters' Figures 1 and 22). Leopold's ovum further resembles Peters more closely, (1) in having two kinds of cells in the trophoblastic processes, and (2) in the characters of the mesoblast. The principal difierence between the two is that the trophoblastic processes are more numerous and much thicker in Peters' ovum, so that, as compared with Leopold's specimen, the blood lacunae are much reduced in size, and the trophoblast takes the form of a thick layer containing blood spaces. The attachment of the primitive villi to the decidua appears to be very similar in both cases, but the degree of intermingling is much greater in Peters' ovum, and the condition of the decidua is also diflfierent Whereas in Leo})old's ovum (see his Figures 23 and 24) the zone of decidua next the trophobhist is largely in the state of coagulation necrosis, characteristic of the present specimen, in Peters' case the decidua has a much more living appearauite ; there is more mingling of living cellular elements, and the jiinount of necrotic material visible is relatively small. There is evidence of active reaction on the part of the decidua in Peters' case, as shown by the })rescnce of large numbers of polymorphonuclear leucocytes and formation of new vessels; this is less marked in Leopold's case, but the invasion of leucocytes is very striking in the present specimen. All three ova however show a very distinct reaction as far as dilatation of bloodvessels is concerned, and this is greatly exaggerated in Leopold's case by congi'stion whiith is probably due to the mode of death.

In respect therefore of the trophoblast the ovum of Leopold may with some confidence be considered as a stage intermediate between the present ovum and that of Peters' ; but as there was no embryonic rudiment the comparison cannot be carried further.

When the embryonic rudiment in the ])resent case is compared with that in Peters' blastocyst, it is at once apparent, that if our interpretations be correct, this ovum is at a considerably earlier stage of development.

The Embryonic Rudiment

It will be necessary to explain at this point the data on which our interpretiition of the embryonic rudiment is based. The mammalian blastocyst is a hollow vesicle with a knob of cells projecting into its cavity from one point on the wall (Figure rv). The ectoderm of the wall or trophohlast is concerned in the processes of imbedding and placeutation, the inward projection constitutes the embryonic blastema. Whereas in the rabbit the cells forming the embryonic knob become spread out flat at the upper pole of the blastocyst (Figure v, a), and are soon exposed on the surface, by the disappearance of the thinned out trophoblastic covering (Rauber's layer), in another series of mammals the knob remains iuturned and a cavity appears among the cells of « >, >^^-,^/-~A— '-•■■■ ■■■■•'1® "%«•'

the knob. In some cases the roof of this cavity early breaks away {Figure v, h), and the embryonic ectoderm, becoming flattened out, is exposed on the surface just as in the rabbit. In other instances the roof of the cavity persists and forms the definitive amnion (Figure v, e, f). There is very good reason for believing that this is the case in the liuman subject. Owing in all probability to the nature of the processes of imbedding, the embryonic knob or formative-cell mass remains in its original position, and the trophoblast l)ecomes uniformly thickened all over the sphere. The embryonic rudiment lags behind in development, and there is a relatively enormous expansion of the trophoblast shell, which is concerned at first in the excavation of the implantation cavity and then in the nourishment of the embryo.

The entoderm is probably split off, to judge from the stages in

AN EARLY HUMAN OVUM

Tareius, before the embryonic ectoderm is differentiated from the amniotic. Owing probably to the relatively great expansion of the trophoblaat

Akk* it. H. Bhh'r,

lii> troplinblnit in repreBented by coDtinuoua black lines II liy intorrupteil lines, the eniliryonic ecUxiGrm, aad i Intio ecUxlorm by epithelial cells. Kauli ligure represents the blMtoHennic vesicle, n, of the

rtain figures the sbbit; 6, of the inole 1 r, nf the bat ; r/, of the mouse or rat ; r, of the guinea-pig ; /, of the kalong (I'loropu* oiluliB).

Ill the rnbbit (a) the cells of the embryonic knob early become arranged as an oiiithulial plate at the upjier pole of the blastocyst : the covering layer of trophoblaat (Riiiiuer's layer) disappears and it is exposed on the surface— there is do umiiiO'i'iiibryonic cavity.

in Ihe mole (6) the embryonic plate is for a short time ioturned. The hallow is HUihI with trnpholilast cells which disappear, and the plate straightening out is uxiKmud uD the surface as in the rabbit.

In the bat (ir) a more distinct cavity appears ithis