Paper - Implantation of the ovum in mammals in the light of recent research (1937)
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Bryce TH. Implantation of the ovum in mammals in the light of recent research. (1937) Edinb Med J. 44(5): 317–332. PMCID 29647599
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Implantation of the Ovum in Mammals in the Light of Recent Research
By Thomas H. Bryce, M.A., M.D., LL.D., F.R.S.
- The Sir John Struthers Lecture delivered before the Royal College Surgeons of Edinburgh, 10 th December 1936.
Internal gestation was not a new discovery by the mammaalia. Cases of viviparity occur in lower vertebrates. It was the total loss of the yolk by the egg that produced the changes which opened the way to higher Eutherian development. The egg divides completely in higher mammals and the result is the formation of a sphere of cells of nearly equal size, which by the collection of fluid among the elements becomes converted into a vesicle, the d/astocyst. This is an embryological stage seen only among mammals. Its wall is destined to give rise to structures concerned with the nourishment of the embryo and fcetus during its sojourn in the uterus. We name it the trophoblast. The embryo with its yolk-sac and amnion arise from a knot of cells depending from the wall into the interior of the vesicle. To this the name embryoblast is given. The blastocyst varies considerably in size in the different orders of mammals, and the trophoblast shows differences in the mode and degree of its development; but generally speaking its physiological properties are two-fold, Tesorptive and histiolytic. It was the acquisition of an embryonic layer with such activities, and the specialisation of the lining membrane of the uterus into a food supplying tissue, in compensation for the loss of the yolk, that made internal gestation possible. But further new endocrine mechanisms must have been developed to regulate the activities of the reproductive tract under the new conditions. It is in connection with mechanisms of this order that so much advance has recently been made towards a deeper understanding of the physiology of reproduction.
The resorptive power of the trophoblast, and probably the Original one phylogenetically, is universally exercised at all Stages. The histiolytic activity reserved in deciduate mammals for the earlier phases of placentation varies to a remarkable degree. The extent to which the trophoblast invades the Maternal tissues is the basis for the classification of placentze Suggested by Professor Grosser? and now generally adopted.
Both maternal and embryonic tissues play a part in the process of implantation, and there are therefore two sets of phenomena to be analysed. The general functions of the trophoblast, and the mode in which it carries them out, are fairly well understood ; but we are entirely at a loss to explain the innumerable, seemingly accidental variations in the reciprocal relations of foetal and maternal parts in the development of the placente of the several orders of Eutherian mammals. A phyletic arrangement of placentze has not yet been achieved except on very broad lines. The question whether the invasive power of the trophoblast was an original property, or a character acquired within the mammalian class, is still debated. That it was a new feature seems the more reasonable conclusion.
The part played by the endometrium in implantation and placental formation is comparatively a passive one as compared with that of the trophoblast. Its primary purpose was to provide a food supply for the embryo, and it is of interest to trace how this has been modified or departed from in the mammalian series. In the present lecture I propose to consider implantation of the ovum from this point of view.
In the first place the endometrium is not permanently in a suitable morphological or physiological state to play its part in implantation, or in the nourishment of the embryo. Accordingly, at shorter or longer intervals, in different forms, and in somewhat varying ways, according to their anatomical and physiological characters, changes occur in the endometrium directed to these ends. With the uterine changes are associated processes in the vaginal canal especially connected with cestrus, and the whole cycle of events is known as the cestrous cycle. The phenomena are now known to be initiated and controlled by endocrine action. A hormone (or hormones) generated in the anterior lobe of the pituitary gland, acting in concert with hormones produced in the ovary, presides over the chain of processes in the genital tract. Co-ordinated with the cycle in the tract there is an ovarian cycle. It has two phases: one associated with the growth of the Graafian follicles during which a hormone named estrin is generated; and another corresponding with the development and regression of the corpus luteum during which a hormone named frogesterone is released. The endocrine activities of the ovary are correlated with that of the pituitary gland, and results vary apparently according to quantitative differences in the relation of the hormones. All are required for the full realisation of the normal activities of the genital tract, but they must be coordinated and present in due relative proportions.
The correlation between the ovarian and uterine cycles has been worked out in a number of forms, and it may now be accepted as a general proposition that the cestrous cycle in all mammals has two phases—the follicular phase before ovulation, when cestrin is the presiding influence, and the luteal phase after that event, in which the corpus luteum is active and progesterone assumes the dominant part. In animals in which mating occurs only at definite times the phenomena of cestrus fall into the follicular phase (Fig. 1, cycle of Cavia).
Fig. 1. — Scheme of the cestrous cycle in Cavia founded on Professor Nicol’s observations and experiments. The continuous line represents the follicular phase, the interrupted line the luteal phase.
The pabulum supplied to the developing embryo may be, from the point of view of its origin, placental, z.e. derived direct from the maternal blood circulating in the placenta, or paraplacental, t.e. derived from the endometrium. The Paraplacental pabulum may be of two kinds: (1) foodstuffs elaborated or stored in the endometrium by the activities of Its cells, and (2) detritus, the product of the breaking down of degenerated maternal tissue. The term embryotrophe Invented by Bonnet included, according to his original definition, all food material which could be identified by morphological as distinguished from physiological methods. For the purposes of this lecture I propose to distinguish pabulum supplied by living cells as embryotrophe, and pabulum in the shape of detritus as histiotrophe, to borrow Professor Grosser’s word.” His term hemotrophe will be applied to food material supplied direct from the blood.
There is great variation in the degree to which the several modes of supply are utilised in different orders of mammals ; and in all mammals with a true placenta there is a period when embryotrophe and histiotrophe are the only sources of food supply pending the establishment of the vascular nexus between the maternal and fcetal blood in the placenta.
The simplest mode of nourishing the embryo in utero, and in all probability the most primitive, is met with among the Eutheria in the Ungulata, and in the lemurs among the Primates. The blastocyst remains a relatively long time free in the uterine cavity, indeed until the chorion is formed and vascularised. It must therefore, in the first instance, receive nourishment by imbibition from the secretions in the cavity. When the vascular chorion has become apposed to the uterine epithelium, maternal and foetal vessels are brought into sufficiently close relation to one another to permit of gaseous exchanges through their walls. This is of course a primary purpose of placentation, but here I shall consider only the other end in view, the supply of pabulum to the embryo.
The arrangements for this purpose occur in a very uncomplicated form in the sow. The outer ends of the low columnar cells of the uterine epithelium are received into small recesses between trophoblastic processes which grow into it. In the follicular phase of the cestrous cycle the epithelium, by a series of changes, is converted from a neutral or resting condition into an actively secreting phase characteristic of the luteal phase. There is an active invasion of leucocytes into the epithelium; some degeneration of individual basal cells in the epithelium takes place but there is no desquamation and no hemorrhage. The glands show no marked changes. If pregnancy supervenes, the condition of the epithelium persists, otherwise there is a return to the neutral state.
Some additional features are to be observed in the ruminants. In the follicular phase just before cestrus the endometrium becomes thickened, the vessels are engorged, and small hemorrhages occur under the epithelium. Invasion of leucocytes takes place, but in this case, reaching the zone of hemorrhages, they take up hemoglobin products from the broken down extravasated red blood corpuscles. The embryotrophe which collects between the trophoblast and the uterine epithelium consists of the secretion of the glands and lymph. It contains much fat—hence the name “uterine milk” — glycogen and other chemical substances, besides red and white blood corpuscles, and some epithelial debris.
This last comes from the gland ducts, and is produced apparently by a mechanical process. The epithelium proliferates actively and forms masses of cells which are shed into the lumina of the ducts, and there degenerate. The detritus and the blood corpuscles are taken up into the trophoblast by phagocytosis, and the erythrocytes and leucocytes with hemoglobin pigment furnish iron to the metabolic process in the embryonic tissues.
The Carnivora furnish examples in which embryotrophe, histiotrophe and hemotrophe are all available for nourishment. In the follicular phase of the cycle the endometrium becomes thickened and very vascular. Hzemorrhages occur under the epithelium ; there is desquamation and some destruction of the stroma, quickly repaired. Blood gets into the uterine cavity and there is some external bleeding. When pregnancy occurs embryotrophe is secreted and the blastocyst, when it has reached the uterus, gets a thick covering (prochorion) of this material. Meantime the epithelium proliferates and becomes converted into symplasma. The ducts of the glands, blocked with shed epithelium and secretion, also degenerate and give rise to symplasmic masses. The trophoblast having consumed the prochorion, and caused the disappearance of the epithelial symplasma, attacks in turn the glandular masses and absorbs them. The stroma between the invading trophoblastic villi is left intact, with the vessels and their endothelium. As a result, when mesodermic villi invade the trophoblastic villi, foetal vessels and maternal vessels come to lie side by side. The deeper portions of the muscosa, with the bases of the glands, are not involved in the destructive process.
The breaking down of the maternal tissues and its conversion into histiotrophe is generally attributed to enzyme action, the enzyme being produced by the trophoblast, or in the endometrium in consequence of its presence. Phagocytic action of the trophoblast seems to be preceded by histiolysis. In this connection Bonnet, in his classical paper on the Processes in the dog, pointed out that the circulation in the endometrium is in a condition of stasis, and he suggested, as a possible explanation of the degeneration, that the hyperemia would produce growth of the tissue, while the stasis and thrombosis of the vessels would be followed by breaking down of the proliferated tissue and its conversion into symplasmic masses,
In the carnivores the blood supplies a far greater amount of pabulum than in ungulates. In the dog and ferret, for instance, large extravasations of blood take place between the chorion and the endometrium along the borders of the zonular placenta. Where they occur the chorion becomes much folded and the spaces between the folds are filled with blood corpuscles and other blood derivatives. The trophoblast is in direct contact with this material derived from the blood. These extravasations yield pabulum of the histiotrophic order, the corpuscles break down and pigments are produced which give rise to the “ green border ”’ of the placenta.
Fig. 2. — Diagram of a late blastocyst stage in a mammal in which implantation is central; the outer circle represents the trophoblast thickened at the future placental site ; the inner circle represents the outline of the large yolk-sac. For comparison with Fig. 3 showing the condition in rat and guinea-pig.
It will be observed that both in carnivores and ungulates there is hemorrhage which precedes ovulation.
In the mammals in which the placenta is of the hamochorial variety, paraplacental pabulum becomes largely replaced by direct supply from the blood (Aemotrophe). In the lower orders, however, embryotrophe plays a very considerable part. In the rabbit, for instance (Fig. 2), in which implantation is central, z.e. within the uterine cavity, the yolk-sac is large, and has a well-developed vascular area reminiscent of the avian arrangement, but it has no vitellus. In early stages it must, however, resorb the embryotrophe in the uterine cavity which consists of glandular secretion and shed blood. By a series of changes the vascular area is brought into apposition with the ventral part of the vascular chorion, and a yolk-sac placenta is formed which serves for absorption until the allantoic circulation is established in the placenta. — ‘The decidual cells are active in the metabolism of nutritive substances, fat, glycogen and no doubt others, while iron is again derived from the hemoglobin of the red blood cells. A considerable thickness of decidua is retained throughout gestation, especially under the placenta where the endometrial cells continue to supply pabulum. The bleeding which occurs into the cavity of the uterus about the time of attachment of the blastocyst is specially noteworthy, as will appear later.
In the rodents in which the condition known as “ inversion of the layers” occurs, things go differently, for the ventral trophoblast layer sooner or later disappears and the resorption of embryotrophe is now done by the inner layer of the doubled up yolk-sac.
The case of the guinea-pig was worked out very fully in my own laboratory, }*1* and is instructive in many ways. The intimate detail of the process of implantation has an important bearing on the question of what may occur during this episode in the human subject, and the work done has provided objective evidence of the action of the ovarian hormones on the endometrium.
The cestrous cycle has an average duration of 16 days. Towards the end of the second week of the cycle changes set in which affect both vagina and uterus. The histological changes in the vagina are very important, but I shall refer here only to the endometrial phenomena. The cells become loaded with mucus, and there isan active migration of leucocytes toward the epithelium ; the vessels become congested, the leucocytes invade the epithelium, and its cells become vacuolated and disorganised ; desquamation takes place and hemorrhages occur under the epithelium. Repair quickly takes place, the whole process occupying only some 24 hours. Mating occurs towards its close and ovulation has taken place by the second day from the onset of the symptoms. In the case of multiparous females mating occurs immediately after Parturition, and the stages noted above as they were described by Stockard and Papanicolaou 38 are replaced by those of post-partum repair reported upon by Dr W. J. Hamilton.8 These resemble closely the changes which occur at oestrus; but while the epithelium is restored in a few hours during that episode, it is not completely replaced post-partum for several days.
Dr Thomas Nicol,4® now Professor at King’s College, London, when working in my laboratory on the phenomena of intra-vitam staining with trypan-blue, discovered that large numbers of dye-laden cells were collected in the endometrium. They were massed mainly in the antimesometrial section of the uterine wall in the region where implantation of the blastocyst occurs, and their incidence had a cyclic character. They were most abundant in the 12th day of the cycle, and persisted in fair amount through the cestrous phase until ovulation occurred, after which event their number fell away almost entirely. This incidence seemed to correspond with the follicular phase of the cestrous cycle, and that this was so was proved by Dr Nicol in a striking manner by a series of experiments on ovariectomised animals injected intravitally. Always absent in the controls, the dye-bearing cells appeared after the administration of cestrin. He also found that fat has a cyclic distribution corresponding to the period of the activity of the corpus luteum. That the luteal hormone was actually at work was demonstrated by another series of experiments, in which progesterone was administered to ovariectomised animals injected intravitally with trypanblue. The hormone had no effect unless the uterus had previously been sensitised by the administration of small doses of cestrin for several days, when a rich deposit of fat, or material staining like fat, was found in the mucosa and especially in the epithelium. The large cells containing dye also showed droplets of fat in the cytoplasm.
As the dye cells were never wholly absent at any time, and as the fatty material also appeared all through the cycle, although at times only in minimum amounts, it seemed that the determining factors were in action in all the phases, but more powerfully at particular times stimulated by cestrin in the follicular and progesterone in the luteal phase. These experiments proved conclusively that the endometrium is actively concerned in the metabolism of food materials and that the process is under the control of the ovarian hormones. _Incidentally they also afford a confirmation of the fact that the luteal hormone requires, in order to be effective, the previous action of the follicular hormone.
Implantation occurs early in the seventh day after parturition. By this time the endometrium is more cellular than before and becomes thickened. Embedding takes place in a crypt placed at a variable distance from the antimesometrial pocket of the slit-like uterine lumen. The epithelium disappears, and there is also histiolysis of the subepithelial connective tissue, so that the embryo comes to lie in a small subepithelial space. After implantation the ventral trophoblast wall disappears or is dispersed into its constituent cells. The result is that in guinea-pigs the ‘ventral trophoblast and the Outer wall of the doubled-up yolk-sac are wholly absent, Whereas in mice and rats they persist in attenuated form for a time (Fig. 3). In Arvicola a condition exists which is a half-way house between that in rat and that in guinea-pig. The space in which the embryo lies occupies the heart of a pearshaped endometrial mass, in which the cells are enlarged and closely packed together (Fig. 4). At 9} days the endometrial cells in trypan-blue injected animals were found loaded with blue granules. Dr Nicol also detected fat, but could not identify glycogen. Here we have a large store of embryotrophe and it is made available to the embryo by its being broken down and gradually absorbed. In the absence of the ventral trophoblast the high columnar and obviously resorbing endoderm cells of the inner wall of the doubled-up yolk-sac take up the material stained blue, and convey it in turn to the vitelline vessels which develop in the mesodermic lining of the wall of the sac.
Fig. 3. — Diagrams of early stages, A in mouse or rat, and B in guinea-pig. They show the condition known as “ reversal of the layers.” In A the interrupted outer line represents the ventral trophoblast, the dotted line the outer layer of the doubled-up yolk-sac; both these disappear later. The inner layer of the sac is represented by a continuous line which is thickened in the region in which the endoderm is resorptive. In B the ventral trophoblast and outer wall of yolk-sac is absent; the first disappears very early and the second never forms at all; the inner wall of the yolk-sac is represented by a thick line where the endoderm is resorptive. In A the inverted ectoderm is represented with the inverted yolk-sac; the cellular part is embryonic ectoderm. In B the ectoderm layer connecting ectoplacenta with embryonic layer is absent.
Fig. 4. — Drawing of the embryo of Cavia immediately after implantation (T. H. Bryce). With permission of the Council from the Zransactions of the Royal Society of Edinburgh, vol. viii., pt. II. (No. 19).
The early embryo occupies a space separated from the uterine cavity; on its right is the epithelium of a part of the lumen distal to the site of implantation; on the left the remains of a gland duct; below and on each side the endometrium consisting of closely packed cells with large vesicular nuclei. These cells, at a slightly later stage, are packed with trypan-blue coloured granules in animals injected intravitam with the dye; below unaltered endometrium with capillaries and gland ducts.
As the endometrial tissue breaks down the periembryonic space enlarges, and is found to contain masses of debris resulting from the degeneration of the epithelium lining the section of the uterine lumen distal to the site of implantation and that of the gland ducts which opened into it. This mass of histiotrophe is greatly added to when the whole of the epithelium proximal to the point of implantation falls into degeneration. The maternal capillaries are not opened during the breaking down of the embryotrophic mass, for no blood can be detected in the periembryonic space. This is in contrast to what is seen in the mouse, in which there is blood round the embryo. Kolster and Sobotta detected by special staining Pigment granules in the cells representing haemoglobin derivatives from broken down erythrocytes.
Ultimately the whole of the embryotrophe is removed, and all that remains of the ventral decidual tissue is a thin layer—constituting the so-called decidua capsularis—which Separates the periembryonic space from the uterine lumen now restored round the antimesometrial part of the uterus.
The pabulum here provided for the embryo is clearly Produced by the continuing influence of the ovarian hormones, and the behaviour of the endometrial cells entitles them to be included in the reticulo-endothelial category.
What is the actual agent in the breaking down of the maternal tissue is a question to which it is not easy to give a categorical answer. In the mouse and rat large cells are found on the decidual wall of the periembryonic cavity. Some authorities trace these to the trophoblast, others refer them to the endometrium. The fact, determined in my laboratory, that these cells, in rats stained by trypan-blue, take up the dye in the form of small granules—is some support for their maternal origin. In guinea-pig, in the early stages, no embryonic cells, save the endoderm cells of the yolk-sac, ever become stained. In the absence of any such cells in guineapig or of such giant cells as occur in Arvicola (Sansom 1"), we reached the conclusion that in this case the histolysis was brought about by enzymes of maternal origin.
Coming now to the Primates, in Tarsius spectrum™ at the base of the order, there is evidence that although there is a small hemochorial placenta, there is a considerable supply of embryotrophe of both categories. The yolk-sac as described by Professor Hill 14 has a much folded wall with an outer covering of mesenchyme cells which are packed with granules. He concluded that it was endowed with considerable resorptive activity, and noted that the exoccelom was full of coagulated material. The gland ducts in the placental area are resolved into symplasma by hypertrophic degeneration and this is taken up by the trophoblast. Again round the placenta the endometrium, thin elsewhere, is thickened and occupied by enlarged glands, while the surface is covered by a layer of modified epithelium which is in contact with the decidua.
In the New World monkeys the endometrium is greatly thickened.*!. There are two placente, a primary dorsally, and a secondary ventrally placed. The uterine cavity is thus open and into it, in the early stages, the glands secrete. In Ateles they are specially well developed in a periplacental ring. Here the placenta has grown over the endometrium, leaving a space, triangular in section, between it and the decidua. Into this recess the glands secrete, and continue to do so for a considerable part of the period of gestation. In early stages embryotrophe consisting of disintegrating mucosa, mucus and extravasated erythrocytes, and secretion from the glands, occupies the meshes of the invading strands of trophoblast. When the expanding chorion comes into contact with the endometrium, and thus obliterates the uterine cavity, the epithelium disappears, the endometrium degenerates, and the whole is converted into histiotrophe. In the Catarrhines 7! the mucosa is also thickened, but the chorion comes earlier into contact with the uterine wall and seals the gland openings. Hill44_ remarks on the _precocity of the placental development in Catarrhine as compared with the Platyrrhine monkeys, and there is, presumably on that account, less of embryotrophic and histiotrophic pabulum.
In man, and presumably also in the anthropoids, placentation is still more precocious. The blastocyst becomes entirely surrounded by the endometrium, and in the early phases lies in an enclosed space cut off from the general cavity of the uterus. The same is true for the hedgehog among the insectivores, and for the forms with so-called inverted layers among the rodents, but the space is produced in a different manner. We do not know in what form the ovum reaches the uterus, nor how long it remains free therein. Teacher ! thought that a considerable time might elapse before embedding occurred, and that during this period the ovum might be nourished by imbibing the secretions in the uterine cavity. That it is so fed for a certain time is, of course, certain, but it is probable that implantation takes place quite quickly. Once embedded the blastocyst must be cut off from the supply of embryotrophe in the general cavity, and the amount of histiotrophe, as compared for instance with that provided in guinea-pig at a comparable state of development, is very small. On the other hand, the periembryonic space is flushed with blood. It is this early relationship of the embryo to the living blood of the mother that seems to be the distinguishing feature of the process of implantation in the human subject.
In view of the part the guinea-pig has played in theories of human implantation, I must bring out here the sharp contrast between the two forms in respect of the condition of the endometrium. In the guinea-pig the early pregnant endometrium consists of a great mass of decidual cells closely packed together. The glands are not dilated and the capillaries are inconspicuous ; there are no small hemorrhages. In the human subject the endometrium at the time of implantation is oedematous, there is great hyperplasia of the glands and injection of the vessels. The arterioles as shown by Bartelmez ? have a curious coiled course through the mucosa towards the surface. They end in the subepithelial layer in a leash of branches which no doubt form an anastomosing capillary network in this situation. It is in this layer that embedding takes place, and as the threshold for extravasation in the endometrium is very low (Hartman ®) the process is almost certainly accompanied by some hemorrhage. This would not, however, correspond with the endometrial hemorrhage noted above in the cases of the lower mammals. This occurs at cestrus, not at the time of implantation. We now know from the work of Corner ° and Hartman ? on Macacus rheseus that ovulation occurs in monkeys about the middle of the cycle, and the modern view seems to be that the same is the case in the human being, even if a somewhat wider range is allowed than in the lower primates. In normal circumstances the ovum finds the endometrium prepared for its reception. In lower forms the preparation involves the storage or manufacture of pabulum for its early preplacental development. In man the preparation is such that the embryo receives, at an extremely early phase, a direct supply of living blood as the universal purveyor of foodstuffs of-every kind.
The actual mode of implantation of the human ovum is unknown, but the process as exemplified in the guinea-pig has been long used as an illustration of how it may occur.
The comparison is cogent to a very limited degree only. As I have already shown, the condition of the endometrium is utterly different in the two cases. The blastocyst becomes implanted in Cavia with the ab-placental pole leading, whereas in the human subject the placental pole is the one which penetrates first. The ultimate position of the embryo in guineapig is exactly the same as in other rodents of the same type, the final separation of the periembryonic space from the uterine lumen being effected in the same way, viz., by fusion of the epithelial walls of the uterus mesometrial to the site of implantation. In respect of the blastocyst itself, it is necessary, having regard to later stages, to conclude that the trophoblast in the human subject very quickly proliferates to form a thick layer all over the surface of the vesicle, whereas in Cavia the ventral layer disappears, and the active trophoblast is confined to the placental pole and shows no proliferation until a later phase.
Nevertheless the guinea-pig blastocyst does illustrate how the human blastocyst coming to rest on the surface of the endometrium will cause a disappearance of the epithelium, and also histiolysis of the subjacent stroma, and come to occupy a subepithelial space partly by its own activity, and partly perhaps by the turgor of the lips of the original pit of implantation. Once embedded, rapid proliferation of the trophoblast associated with necrotic changes in the endometrium will quickly produce an implantation cavity such as is seen in the earliest known embryos.
The least destruction in an endometrium in the premenstrual condition would almost certainly cause extravasation, whether the damage or stimulus proceeded directly from the trophoblast, or indirectly in consequence of the presence of the ovum.
A reconstruction of this sort is quite in keeping with the view propounded by Hartman® that menstrual bleeding corresponds to implantation bleeding, or the placental sign in lower forms, all the more were the first effusion of blood referred to a local rupture of the capillary bed, rather than to an opening of the vessels by the trophoblast.
The general result of a survey of the behaviour of the endometrium in the light of new information confirms the old view that the cyclic changes imply a preparation of the tissue for implantation, but not of a corollary that menstruation represents an abortion, as it were, of an unfertilised ovum. Such an idea is untenable in view of the facts now known regarding the occurrence of menstruation in the absence of ovulation.
In choosing the theme for this lecture I was influenced in part by a consideration of a practical kind. The practitioner interested in the preventive treatment of early abortion cannot exert any influence from the embryonic side of implantation, but something is possible from the maternal side. An understanding of the meaning of the processes in the endometrium from a comparative point of view may be of some value towards this end.
Only a few works and papers specially cited are enumerated.
Allen, G. E., 1932, Sex and Internal Secretion, Williams & Wilkins Company, Baltimore. (Literature up to 1932.)
Bartelmez, G. W., 1931, “ The Human Uterine Mucous Membrane during Menstruation,” Amer. Journ. Obstet. and Gyna@col., V., 21.
Bartelmez GW. Histological Studies on the Menstruating Mucous Membrane of the Human Uterus. (1933) Carn. Inst. Wash., Publ. No. 443; Contr. to Embryol., No. 142.
Bryce TH. and Teacher JH. Contributions To The Study Of The Early Development And Imbedding Of The Human Ovum 1. An Early Ovum Imbedded In The Decidua. (1908) James Maclehose and Sons. Glasgow.
Corner GW. Ovulation and menstruation in Macacus rhesus. (1923) Contributions to Embryology, vol. 15, Carnegie Inst. Washington Pub. no. 332, 75-101.
Grosser, O., 1927, Lruhentwicklung Ethautbildung und Placentation des Menschen und der Saugetiere, Bergmann, Munchen.
Hamilton, W. J., 1933, ‘‘ The Restoration and Regeneration of the Epithelium and Endometrium of the Uterus of Cavia Post-Partum in Non-Pregnant Animals,” Trans. Roy. Soc. Edin., vol. lvii., pt. 2.
Hartman, C., 1930, ‘‘ The Corpus Luteum and the Menstrual Cycle, together with the Correlation between Menstruation and Implantation,” Amer. Journ. Obst. and Gynecol., v., 19.
Hartman, C., 1932, Studies in the Reproduction of the Monkey Macacus (Pithecus) rhesus, with Special Reference to Menstruation and Pregnancy, Carn. Inst. Wash., Publ. No. 433; Contr. to Embryol., No. 134.
Long J. and Evans HM. The Oestrous Cycle in the Rat. (1920) Anat. Rec. 18. ; Proc. Amer. Assoc. Anat., p. 231.
Maclaren, N. H. W., 1926, " Development of Cavia: Implantation," Trans. Roy. Soc. Edin., vol. lv., pt. 1, No. 5.
Maclaren, N. H. W., and T. H. Bryce, 1933, "The Early Stages in the Development of Cavia,” Trans. Roy. Soc. Edin., vol. vii., pt. iii. Marshall, F. H. A., 1922, The Physiology of Reproduction, 2nd Edition, London, Longmans, Green & Co.
Nicol, T., 1935, “ The Female Reproductive System in the Guinea-Pig : Intravitam Staining; Fat Production; Influence of Hormones,” Trans. Roy. Soc. Edin. vol. Wiii., pt. ii. (No. 19).
Sansom, G. S., 1922, “‘ Early Development and Placentation in Avvicola (Microtus) amphibius, with Special Reference to the Origin of Placental Giant-cells,” Journ. Anat., vol. lvi.
Stockard, C. R., and Papanicolaou, G. N., 1917, ‘‘ The Existence of a Typical Oestrous Cycle in the Guinea-Pig, with a Study of its Histological and Physiological Changes,” Amer. Journ. Anat., vol. xxii.
Teacher JH. On the implantation of the human ovum and the early development of the trophoblast. (1924) J. Obstet. and Gynec., Brit. Emp. 31: 166.
Wislocki GB. Ox the Placentation of Primates, with a Consideration of the Phylogeny of the Placenta. (1929) Carn. Inst. Wash., Publ. No. 394; Contr. to Embryol., No. 111.
Wislocki GB. On a Series of Placental Stages of a Platyrrhine Monkey (Ateles Geoffroyi), with some Remarks upon Age, Sex and Breeding Period in Platyrrhines. (1930) Carn. Inst. Wash., Publ. No. 414; Contr. to Embryol., No. 133.
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