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   Manual of Human Embryology I 1910: The Germ Cells | Fertilization | Segmentation | First Primitive Segment | Gastrulation | External Form | Placenta | Human Embryo and Fetus Age | Ovum Pathology | Integument | Skeleton and Connective Tissues | Muscular System | Coelom and Diaphragm | Figures | Manual of Human Embryology 1 | Manual of Human Embryology 2 | Franz Keibel | Franklin Mall | Embryology History

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The difficulties in the way of a comprehensive description of human placentation have been mentioned so often that a detailed re-enumeration of them is unnecessary here. The first stages, so necessary for the understanding of all the later ones, are lacking, just as they are in the case of the formation of the germinal layers, and, as in this case, must be conjectured by deduction and analogy. In the following description an endeavor will be made to state what has been determined with certainty, and, in connection with this, to call attention to disputed questions and to the probable significances of the phenomena described. A statement of our knowledge in the field of comparative placentation may also be dispensed with, since it has repeatedly been given in detail within recent years.^ The position which man oCoupies among the Mammalia on the basis of the structure of the placenta may, however, be indicated; and, in connection with this, the nomenclature employed in placental classification and the general morphological and histological processes involved in the formation of the placenta may be described. Placentation is (in mammals) the intimate union (apposition or fusion) of the mucous membrane of the uterus with the outer layer of the ovum, the chorion, which becomes vascularized from the allantois ^ for the purpose of providing for the respiration and nutrition of the embryo and for carrying away its waste products.

  • 0. Schultze: Grundriss der Entwicklungsjjeschichte, 1897; Strahl: Embryonalhlillen der Sau^r iind Placenta, in Hertwi^s Handbuch, 1902; Bonnet: Lehrbuch der Entwickliingfs^schicbte, 1907, and, most recently, 0. Grosser: Vergleichende Anatomie und Entwicklungsgreschicbte der Eihaute und der Placenta, Lehrbucb fiir Studierende und Aerzte, Wien, 1909. From this last work the majority of the illustrations of this chapter have been taken. A very complete li^t of the literature on human placentation is to be found in the work of F. Keibel and C. Elze: Normentafel des Menschen, Jena, 1908.

'In some mammals also from the yolk sack. ACoording to the view of Hubrecht (see especially Resink, Tijdschrif t Ned. Dierk. Vereen, 1903, 1905 : Hubrecht, Quart. Joum. Micr. Sc, 1909), however, the chorion possesses from the beginning a vasifactive mesoderm. Compare Grosser's Lehrbuch. 91

Since the union of the chorion and the uterine mucous membrane is either an apposition or a fusion, the expulsion of the chorion sack after birth either may take place without injury to the uterine mucous membrane or a portion of the latter, the decidual membrane (membrana decidual may be expelled with it; hence the old division of the Mammalia into the lower Adeciduata and the higher Deciduata. But tissue destruction frequently takes place during pregnancy in the former, and, on the other hand, in many highly organized forms the placenta contains no considerable quantity of maternal tissue, if the maternal blood be disregarded, so that in these it is hardly proper to speak of decidua (Strahl). ACoordingly, Strahl ^ has employed the relations of the maternal blood as a basis of classification and has designated these placentae *4n which post partum the spaces of the placenta which carry maternal blood are separated and expelled" complete placentce or placentce verce; while those simpler placentae, in which during and after birth the maternal blood-spaces remain intact, are termed half placentce or semiplacentce. The classification proposed by Robinson (1904) is practically the same, since his ^* apposed placentae" include those in which there is merely an apposition of the chorion to the uterine mucous membrane, while those in which there is fusion of the two he terms ** conjoined placentae." In the same way the two groups proposed by Assheton (1906), that of the placentce plicatce with simple, non-proliferating chorionic epithelium, and that of the placentce cumulated with a greatly proliferated and thickened chorionic epithelium, traversed by lacunae for the maternal blood, agree essentially with the two divisions of Strahl.

The idea of placental types which the author ^ has conceived takes its origin from another standpoint. The nutritive material which passes from the maternal blood into that of the fetus in the lowest types of placentae passes in suCoession through maternal endothelium, connective tissue, uterine epithelium, portions of the uterine cavity, the chorionic epithelium, chorionic connective tissue, and the endothelium of the chorionic vessels. At the commencement of development all the maternal walls are present in the highest types of placentae also. But while the fetal layers are always retained, or, in the highest types, are gradually formed, during the early stages of development, the maternal partitions disappear one after the other in the course of the phylogenesis or ontogenesis, the chorionic epithelium penetrating farther and farther toward the source of its nutrition, the maternal blood. But the blood spaces themselves, those of the mother on the one hand and those of the fetus on the other, remain sharply separated under all circumstances. The penetration of the fetal tissue may halt at any stage and so determine the structure of the mature placenta and also the name which may be applied to it, this indicating the maternal tissue which is in immediate contact with the chorionic epithelium. At the beginning of the series stand placentae such as those of the pig, in which all the maternal partitions are retained; the uterine and chorionic epithelia are in contact; and the placenta is a placenta epitheliochorialis. If the maternal epithelium disappears, at least to a considerable extent, as in the ruminants, the chorionic epithelium comes into contact with the connective tissue and a placemta syndesmochorialis is formed. If the connective tissue also disappears, so that the chorionic epithelium is in contact with the endothelium of the maternal blood-vessels, as is the case in the Camivora, aCoording to Schoenfeld, then the placenta is to be termed a placenta endotheliochorialis. And, finally, if the endothelium disappears, so that all the maternal partitions have vanished and the maternal blood directly bathes the chorionic epithelium, then the highest possible stage of placentation has been reached and the placenta is a placenta hcernochorialis. Thus the most important morphological character of a placenta is directly indicated by its name.

  • As a rule, however, the term decidua is not only applied to 'the superficial layer which is expelled, but also includes the entire thickness of the mucous membrane. This is the case with its application to the human placenta.
  • In Hertwig's Handbuch, 1902, and recently in a fuller somewhat modified statement in Der Uterus puerpuralis von Erinaceus europcetts, Verhandl. K. Ak. Wetensch., Amst., 1907.

'Verb, morph. Gesellsch., Wien, 1908; Zentralblatt fiir Physiologie, 1908; and Lefirbuch.

The further subdivisions may, following Strahl, be based on the form of the placenta. Thus there may be recognized a placenta diffusa^ in which the chorionic proliferations or villi are uniformly distributed ; a placenta multiplex, with the villi arranged in groups; a placenta zonaria, in which they have a girdle-like arrangement ; and a placenta discoidaUs, in which they are aggregated to form a disk-like structure. The last group, which includes the highest types of placentae (the haemochorial of the classification given above), may be divided, again following Strahl (1905), into labyrinth placentce, with narrow capillary-like channels for the maternal blood, and bowl placentce (placentce olliformes)^ in which the maternal blood has the form of a large sinus, the floor of the space (bowl) being formed by decidua and the roof by the chorion, from which the villi project, into the space. The human placenta is a placenta vera (conjugata, cumulata) discoidalis ollif ormis, or, aCoording to my nomenclature, a placenta hcemochorialis discoidalis olliformis. It represents the highest development of its type, a development which even the placentae of the anthropoid apes have not quite reached.

The nutrition of the embryo takes place, in general, in two ways : on the one hand, by the transference of nutritive material from the blood of the mother to that of the child ; and, on the other, by the direct absorption by the chorionic epithelium of products of the maternal mucous membrane, these products frequently being subjected to a kind of digestive process before they pass into the embryonic circulation. These maternal substances are partly products of secretion, partly waste products, together with extravasated maternal blood, and have been included by Bonnet under the term embryotrophe and by English authors have been designated pabulum. In the lower types of placentae the embryotrophe plays an important role throughout the entire duration of pregnancy; in haemochorial placentae, and therefore in man, we find (as has been noted, for instance, by Pfannenstiel and Jung), at the beginning of development, up to the establishment of a definite circulation in both the maternal and fetal blood spaces, a very distinct absorption of embryotrophe consisting of degenerated maternal tissues, while later, embryotrophe is entirely wanting, at least in the region of the placenta.^ In haemochorial placentae, therefore, two phases or stages may be distinguished : an embryotrophic phase, at the commencement of development; and a later hcemotrophic phase, not sharply distinguished from the former in time, but during which the nutritive material is received from the maternal blood exclusively.

This absorption of material cannot, however, be regarded as a simple process of diflfusion. This could be the case only with crystalloid substances at the most; colloids, on the other hand (such as the albumins, for instance), are taken from the maternal blood by a process of resorption, associated with a partly constructive and partly destructive activity on the part of the epithelium of the villi; and certain highly complex substances, such as many immunity substances, cannot pass the placenta at all. The chorionic villi of the placenta have a certain similarity to the intestinal villi (Hofbauer), the maternal blood corresponding to the digested food material. Up to the present the wandering of fat, glycogen, and iron, the last as haemoglobin or its derivatives, has been followed histologically from the maternal blood through the chorionic epithelium into the fetal vascular system. The fat, which penetrates into the chorionic epithelium in a state of solution (saponification), is reconverted into fat globules within the epithelium at the bases of its cells.^ The haemoglobin comes from the maternal blood-corpuscles which degenerate in the placenta itself, perhaps in contact with the chorionic epithelium. Oxygen is set free from the oxyhsemoglobin of the mother, probably by ferment action (for further consideration consult Hofbauer and Kehrer). The maternal blood, therefore, often assumes in later stages, even in placentae of the highest type, the role of the embryotrophe, although not in a manner easily recognizable histologically ; and also for this reason the term haemotrophic phase is justifiable.

  • A modification of this statement is necessary in connection with the maternal blood. See below.

' Holsti (1908) lays special weight upon fatty de^neration of the decidna, upon fat formation in the glands, and npon the transportation of fat from other orerans by leucocytes; this fat is directly absorbed by the chorionic epithelium up to the close of pregnancy.

In placentation many cytological phenomena oCour that are not observable elsewhere. The most striking are those that lead to the formation of multinucleated masses of protoplasm. Bonnet (1903) has brought order into the exceedingly confused nomenclature of these structures; he designates (Lehrbuch, 1907) as syncytia, ** deeply staining nucleated masses of protoplasm formed by the fusion of originally separate cells ; plasmodia, on the other hand, arise by repeated nuclear division unaCoompanied by corresponding cell division. . . . Syncytia and plasmodia are always living and active formations, endowed with especially energetic metabolism, together with histolytic or phagocytic properties, and also with the power of amoeboid movement. . . . They may subsequently split again into separate cell territories. . . . Quite different are the deeply staining nucleated masses produced by the confusion of originally distinct cell boundaries and by aggregation, but which show unmistakable signs of commencing degeneration,'^ Such masses are termed symplasmata.

Syncytia and plasmodia are chiefly formed by fetal tissues, namely, by the chorionic ectoderm ; symplasmata, on the contrary, arise from the maternal tissues. Yet, for a precise definition of the structure, mention should be made of its origin ; so we speak of a syncytium fetale epitheliale, of a symplasma maternimi conjunctivum, etc. If the masses in question remain relatively small they are known as multinuclear giant cells or simply as giant cells; the above classification is applicable to these also. Mononuclear giant cells, which, however, never reach a special development in the human placenta, are merely greatly enlarged cells, usually derived from the fetal epithelium.

While extensive histological modifications may affect almost all the constituents of the maternal mucous membrane and find expression there in the formation of the decidua mentioned above, these modifications affect only the chorionic epithelium among the fetal tissues, the chorionic connective tissue and vessels showing great uniformity of condition. The chorionic epithelium has been termed the trophoblast by Hubrecht; this distinguishes the ectoderm of the chorion from that of the embryo and that of the amnion. In the region of all placentae belonging to the higher types it shows, at least in parts, active proliferation phenomena. Where it comes into relation with the maternal tissues it usuallv becomes transformed at its surface into a syncytium (aCoording to Hubrecht's terminology, a plasmodium), and this portion has been termed the plasmoditrophoblast (Vernhout, the plasmodihlast of Van Beneden), now more properly the syyicytiotrophohlast ; while those portions in which the cell boundaries are still retained form the cytotrophohlast (the cytohlast of Van Beneden).

The trophoblast in Hubrecht's sense is a morphological concept, based upon the views of that author as to the phylogenesis of the Mammalia; it oCours in all of this group and covers the entire ovum: it may also enter into entirely passive relations with the maternal mucous membrane. The term is not used in Hubrecht's sense when it is applied to the proliferating trophoblast, as frequently happens in the literature.** This proliferating portion of the trophoblast, which is provided with histolytic properties and especially makes possible the formation of placentae of the higher types, is quite different from the portion known as the inactive trophoblast, and has been termed by Minot the trophoderm. Only in man (and the anthropoid apes) do the two ideas coincide, since in these cases the entire trophoblast undergoes lively proliferation and is, therefore, converted into trophoderm.

Finally, as regards the position of the ovum in the uterus, different types are recognizable (Bonnet, 1903). The union of the ovum with the mucous membrane is known as the implantation or nidation. If tlie ovum remains in the main cavity of the uterus, the implantation is termed a central one. This is the most frequent type {Adeciduata, Carnivores, the rabbit, the lower apes, etc.). If, however, the ovum becomes implanted in a furrow or diverticulum of the uterus and subsequently is shut off from the uterine lumen by a fusion of the lips of the furrow or diverticulum, then the implantation is of the excentric type (hedgehog, mouse). Finally, if the ovum penetrates into the mucous membrane by producing a destruction of the uterine epithelium and develops in the mucous membrane after the closure of the point of entrance, that is to say, outside the cavity of the uterus, as in the guineapig and in the rodent Geomys, then the implantation is of the interstitial type. In man the oCourrence of this last type has nowbeen almost certainly proved.

In the last two types of implantation the ovum is separated from the uterine cavity by a layer of maternal tissue, the decidua capsularis. It arises in the first case by a fusion of the margins of the walls of the furrow or of the lips of the diverticulum; in the second case, by the fusion of the lips of the implantation cavity ; and, in later stages, when the ovum bulges out toward the lumen of the uterus, it covers like a shell the part of the ovum turned away from the placenta.

•Hubrecht, however, in his earlier works (Plaeentation of the Hedgehog, 1890) employed the expression to denote only the proliferating chorionic ectoderm. Compare Hubrecht, Science, 1904, and foot-note 2.


Menstruation, which oCours at regular periodic intervals in man and the apes, is the expression of changes in the uterine mucous membrane which are associated with preparations for the reception of a fertilized ovum. A consideration of it is therefore necessary as an introduction to an aCoount of placentation. The mucous membrane of the corpus uteri has, in general, a very simple structure. A single-layered, cylindrical or cubical surface-epithelium, with varying amounts of ciliation (Mandl, 1908), simple or sparingly branched tubular glands varying considerably in number in different individuals (Hitschmann and Adler), a stroma with fine connectivetissue fibrils which are diffi^ cult to demonstrate by ordinary histological methods (Bjorkenheim, Hitschmann, and Adler), and, finally, the entire absence of a submucosa — these are its most important characteristics. The ends of the glands frequently penetrate to between the irregularly defined innermost layers of the muscularis and so obtain for the mucous membrane a firm adhesion to the muscularis and a wellprotected position for their basal portions, a circumstance of considerable importance both intra and post partum. Hitschmann and Adter • have shown by their comprehensive, recently published observations, upon which are based the statements that follow, that this mucous membrane is never in a completely resting condition in a fertile female. Growth and degeneration alternate regularly and form together a menstrual cycle of normally twenty-eight days. The cycle may be divided into certain more or less clearly marked periods or phases. The longest of these is the interval (between two menstruations) or the intermenstrual period, which lasts for about fourteen days, during which the mucous membrane is almost at rest and only undergoes a very gradual increase in thickness. Upon this follows, without a sharp limitation, the premenstrual period, which lasts about six or seven days and is characterized by intensive proliferation and swelling of the mucous membrane, finally leading to hemorrhages. These last for about three to five days, the period of menstruation, during which the mucous membrane again decreases in thickness and undergoes extensive degeneration. In the remaining period of the cycle, the postmenstrual period, of about four or six days duration, the mucous membrane is regenerated.

•These authors cite the literature of the question. Vol. I.— 7

The mucons membrane during the interval is in the condition usually described as normal (Fig. 80). The mucosa is, on the average, about 2 mm. tiiick ; in the fresh condition it is grayish red and rather smooth. The glands have a slightly spiral course and, for the most part, are directed obliquely to the surface, their lower ends being, as a rule, bent upon themselves. Their lumina are oirfiular in transverse section and at first empty. The gland cells are at first small with elosely-set nuclei ; but in the second half of the period they become enlarged, their plasma becoming homogeneous aod acidophilous. The stroma cells (Fig. 89) are fusiform or stellate, with large nuclei, richly provided with chromatin, and possess but little plasma, so that the tissue resembles adenoid or embryonic connective tissue. Lymphocytes and small lymph-nodes oCour in it.

++++++++++++++++++++++++++ Fig. 81. Fro.S2. FiOB. 79-82.— Fiein*s "f the uterine mueoun membrftne in the various pluue*. Fig. 79. PoetmeaBtruiil mucous membrBne. one dsy after menstruation. Fig. 80. Tlie condition during the interval. Fig. SI. Prementtnial condition. Fis. 82. Condition on the third day of meastruation, shoving sepaiatioD of the superlicial layer. (After HitschDiann and Adier.)



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Already toward the close of the interval the gland cells begin to produce secretion granules (Fig. 84), which are also expelled into the lumina of the glands; and the stroma begins to show a diminution in compactness and some oedematous infiltration. In the premenstrual stage (Fig. 81) the mucous membrane rapidly thickens to two or three times its previous thickness. This depends partly upon an increase of the oedema, and partly upon an enlargement of the individual elements. This shows itself in the gland cells (Fig. 85) by a swelling of the nuclei and of the plasma and by abundant secretion, which produces a frayed appearance on the inner surfaces of the cells; the secretion, which is also to be found in the lumen of the uterus, is now clearly recognizable histologically as a mucous secretion and contains flakes of the older, still acidopbilous secretion. The enlargement of the cells produces, on the one hand, a formation of folds and ontpouchings of the walls of the glands, the stroma projecting like papillae into the folds of the mucous membrane; on the other hand, in conjunction with the secretion, a great enlargement of the liunina of the glands results. In consequence, tbe walls of neighboring glands are brought nearer together, the stroma being compressed between them; and an appearance as if there was an increase in the number of the glands is produced. The glandular changes are most striking in the deepest layer of the mucous membrane; in the superficial layer the stroma cells enlarge (Fig, 90) and become roundish or polygonal with a clear, feebly staining plasma and large, also feebly staining nuclei; they represent a preliminary stage of decidual cells (Fig, 91). By the localization of tbe glandular changes, on the one hand, and those of the stroma cells, on the other, the mucosa is differentiated into two layers, which, as in the case of the mucous membrane of the gravid uterus, are designated, the deep one as the spongy and the superficial one as the compact layer (compare also Fig. 92). The former acquires a spongy consistency as the result of the great enlargement of the glands, and the latter is characterized by the closely packed, decidua-Iilce stroma ceils and the straighter course of the glands. Toward the end of the premenstrual phase there oCours an engorgement and dilatation of the blood-vessels; and the mucous membrane, which at first was pale, becomes bright red in color.


Fig. R8-ei.— The cyclic chansei of ths Btroma ceLli of the utBiine muooH. Fig. SB. PoAmenBtnul coni^tion. Fig. SO. Tbe CODdition oCoumng in the interval. Fig. 90. Premengtnjiil auiditioa <BvB t« nx daya before the menBtruatioii). Fig. 91, Condition immediklely before meantnistioa. (Afler HiliKhmanii ud Adier.) ++++++++++++++++++++++++++

Small hemorrhages oCour at the same time ; these become confluent and destroy the continuity of the tissue, subepithelial haematomata are formed, portions of the epithelium are torn away, blood makes its way to the surface of the mucous membrane, and the actual menstruation begins. With the onset of this there is an effusion of blood and of the oedema fluid, on the one hand, and an expulsion of the glandular secretion, on the other, whereby a rapid shrinkage of the mucous membrane oCours. Frequently, but not always, there is also a desquamation of the glandular epithelium. The outpouchings of the glands rapidly disappear and these assume an almost straight form with narrow or collapsed lumina (Figs. 82, 86, and 92) ; the emptied cells become low and small. Of the stroma cells those that have been most profoundly altered break down and are expelled or carried away by leucocytes; the rest again diminish in size. The surface epithelium may be for the most part retained, or, even in normal menstruation, may be expelled together with the greater part of the compact layer (Fig. 82), painful contractions of the musculature aiding in the separation of this rigid swollen layer; yet the epithelium is in all cases regenerated before the close of the menstruation.

In the postmenstrual stage the mucous membrane is thin, with almost straight glands (Fig. 83) and long, narrow, fusiform, closely packed stroma cells (Fig. 88) ; little remains of the hemorrhages, and even these remnants quickly disappear. After a few days the glands again become larger and begin to assume a wavy outline, the stroma cells become more suCoulent, and the mucous membrane returns to the relatively quiescent stage of the interval ; nevertheless, in the first half of the interval numerous mitoses are to be found in the glandular epithelium. The similarity of the premenstrual mucous membrane to that of the decidua indicates that the premenstrual changes (the loosening up of the tissue, enlargement, increased glandular activity, swelling of the stroma cells, formation of two layers in the mucosa) are a ripening process, a preparation for the reception of a fertilized ovum, and that they are physiolo^cally the most important part of the entire cycle, while menstruation itself is only a secondary process, a degeneration of the mucous membrane, which from a failure of pregnancy has not been able to fulfil its purpose.

As to the relation of menstruation to " heat " of animals, as well as concerning the question of the oCourrence of re^lar cyclical changres in the genital mucous membranes, see, for example, W. Heape : " The Sexual Season of Mammals and the Relation of the ' Prooestrum ' to Menstruation," Quart. Joum. Micr. Science, vol. xliv, 1906 ; M. Van Herwerden : " Bydra^ tot de Kennis van den menstrueelen Cyclus, Tijdschr. Nederlandsche Dierkundi^ Yereeniging, Deel, x, 1906: and the auto-abstract of the author: "Beitrasr zur Kenntnis des menstruellen Zyklus," Monatsschrift fUr Geburtshilfe und G>Tiakologie, vol. xxiv, 1906.

Bryce and Teacher (1908) lay special weight upon the possibility of an implantation in any portion of the intermenstrual cycle (compare Sect. Ill), and so reach the conclusion that the " menstrual decidua is not a preparation for the reception of an ovum and that menstruation cannot be regarded as " the abortion of an unfertilized ovum." The object of menstruation is merely to maintain the endometrium at all times ready for the formation of a decidua; the premenstrual tumidity and decidua are by chance similar, but actually are merely degenerative phenomena of an over-ripe mucous membrane. Both periods, the menstrual and the premenstrual taken together, are compared by these authors, in agreement with Heape, to the phase of animal " heat " that the latter author has termed the " prooestrum." During this the vulva is swollen and red, blood and mucus exude from the vagina, but the animal is not capable of conception. The time for conception, the " oestrus," corresponds to the postmenstrual period, with its somewhat increased libido, observable also in the human species; while the interval is equivalent to the resting stage in animals, the " metoestrum." This view of the matter is difficult to reconcile with the histological phenomena of menstruation; see also later. Section III. III. OBSERVATIONS ON YOUNG OVA. {Implantation^ the Embryotrophic Phase of Placentation^ and Transition Stages.) Ova which, indeed, do not directly reveal the processes of implantation, but are young enough to permit definite cod elusions concerning it, are that of Bryce and Teacher (1908) and then that of Peters (1899) and that of Leopold (1906). These preparations show, on the one hand, the so-called implantation opening,^^ and, on the other, the extensive proliferation of the chorionic ectoderm or trophoblast which precedes the development of true chorionic villi containing mesoderm. They do not suflSce, however, for a certain solution of all the questions which suggest themselves.^ ^ Thus confirmation, based upon the study of older ova, is much needed of the views regarding the mode of development of the extensive intervillous space and of the formation of a doublelayered epithelium on the villi. Of modern, well-described preparations the thoroughly studied and very beautiful ovum of Jung

The implantation opening is also evident in some older ova (Graf Spee, Beneke) ; in others, some of which are very young (such as that of Jung), it is no longer so. " The ovum of Leopold is undoubtedly extensively altered, so that while it is of value for the confirmation of ideas derived from other ova, it is in itself of little significance; the Peters ovum, whose discovery has effected a revolution in our ideas of placentation, and the more recent ovum of Bryce and Teacher are the most important sources of our infonnation concerning the beginning of human development. That the ova described by older writers (Breuss, Allen Thomson, and especially the celebrated ovum of Reichert) were younger, as Stratz, for example, supposes, is very improbable, since the measurements of their egg capsules were much greater. The methods by which these ova were studied were too imperfect to allow wide-reaching conclusions; and the ova themselves need not be further considered here. Furthermore, as regards the Reichert ovum, Kolliker and later Hofmeier (1896) have, on sufficient grounds, reached the conclusion that it was not normal. (See also note, p. 104.)

(1908) is the most important; then the equally well-preserved ovum of Siegenbeek van Heukelom, the first that was described under the influence of the newer ideas concerning implantation and placentation ; and the preparation of Frassi (1907 and 1908), the study of which has led to conclusive information concerning many of the processes suCoeeding implantation. Other important objects are the young ova which Graf Spee (1905) and Beneke (1902) exhibited at Congresses, but concerning which only quite brief notices exist ; and, further, the thoroughly described preparations of Friolet (1904), Rossi Doria (1905), and Cova (1907), as well as those of Pfannenstiel (1903) and Marchand (1903). A very young, but unfortunately poorly preserved, ovum is that of Stolper (1906). For a number of questions the older preparations of Merttens (1894), Graf Spee (1896), and Leopold (1897) are of interest.^ ^ The preparations considered here are arranged aCoording to their size in the appended table. Certain diflSculties, however, become apparent in the arrangement, since the measurements employed by the authors are not identical. ^^

AiifK/^I Dimensions ^^^^<'^' iu mm.


Bryce-Teacher 0.63 ( XO.77) Petera 1.6X0.9X0.8

Leopold Stolper. Graf Spee

1.4X0.9X0.8 2.5X2.2X1.0 2.5X1.5

Exclusive of epithelium (trophoblast). Internal space of the ejrg capsule (exclusive of trophoblast) . Diameter of cavitj'. Diameter of egg capsule.

Jung I 2.5 X 2.2 ( X 1.1) I Exclusive of epithelium (trophoblast) . Beneke ' 4.2 X 2.2 X 1.2 Measurement of cavity of ovum. Siegenbeek I 5.5X4.5 Including epithelium between the bases of the I villi, but excluding the villi themselves. Rossi Doria 6 X5 Frassi ' 9.4 X 3.2 Diameter of the cavit v. Friolet 11-12x9 Cova I Em>)ryo with open auditory vesicles, anlagen of ! liver, hypophysis, etc.

" The number of young ova described in the literature is much greater. Of well-preserved ova there may be mentioned especially those of Etemod and Hitschmaim and Lindenthal (figures of the latter in Schauta: Lehrbuch der gesamten Gynakologie; and in the Lehrbuch by the author, already mentioned); but of these thorough descriptions have not yet been published. The numerous ova described before the publication of the works of Siegenbeek and Peters, few of which were observed in situ, the majority being aborted or separated from the egg capsule, are for the most part of little interest in connection with the questions under discussion here, since up to that time the problems were imperfectly understood. A mention of these ova would oCoupy too much space; compare the comprehensive reviews of Peters, Pfannenstiel, and Frassi.

" The determination of any measurements from the figures is hardly ever possible; the authors almost never state the magnification. A statement of the objective and ocular, which is generally preferred, is worthless, since for determination of the enlargement the tube length, the height of the stage, and the kind and dimensions of the drawing apparatus are also necessary.

Finally, some preparations of the attachment of the ovum in atypical situations, such as tubal and ovarian pregnancies, are of importance, since they offer opportunities for observing, as in an experiment, tlie development under modified external conditions and for separating fetal and maternal derivatives. To this group belong, for example, the tubal ova of Fiith (1898), Pfannenstiel (1903), etc., and the ovarian ova of Freund and Thome (1906), Busalla (1907), and Bryce, Kerr, and Teacher (1908; tliis also contains literature references),


Tlie youngest known human oviim, that of Bryce and Teacher, was already embedded in the mucous membrane. It consisted of a loose, almost spherioal mass of mesodenn, avera^iiiK 1-63 mm. in diameter, with wide intercellular spaces, but no rColoui; in this mass were two small epithelial cavities (probably the medullo


Fig. 93.— Transvi

rw Motion of the Bryce-Teacher nv»m (Verb. Aunt. Ges., 1908)

»how« "the point otealninoe, with a eonital man of fibrinous di

it, pmnting U> blutocya

the implajilation csvily it bounded by a necrolic dCoiduA laye

with UUUmtl blood. «b

amniotic and yolk-sack cavities) and it was enclosed by a tliick investment of tissue, wliicii is probably to be regarded as chorionic ectoderm only, the trophoblast shell (Fiff. 93). This shell, the blastocyst wall, consists (see Verk. Anat. Geseltsck., 1908) (1) of an inner lamella in which the cell outlines are not sharply detiued, the nuclei are *ery irregular in size, and many cells show double, treble or even multiple nuclei; (2) of an extremely inegnlar formation which has definitely plasmodtal characters. These two layers differ very markedly in the characters of the nuclei and in tlie staining' reactions of the protoplasm, but they clearly form parts of one formation. The cellular layer we name, after Hubrecht. Ilie cytotn)phoblast, and the plasmodial layer, the pi a smodi trophoblast. The cytotrophoblast is confined to the immediate wall of the blastocj'st, and tiiere is no sign of protrusions of tlie cellular layer into the strands of the plasmodiuni, although at one or two points a minute bud of eytotrophoblast is seen estending outwards.

" The Plasmodium " foniis an extremely irregular network, the spaces of which are Dlled with maternal blood (Fig. 94). Isolated masses of the formation show a]l stagpes of vaeuolation, from niulliple small vacuoles to a spun-out reticular condition. This vacuolation of the plasm odium is probably produced by the secretion of a fluid containing digestive ferments, which cause coagulation necrosis followed by solution of the decidua, thus leading to enlargement of the implantation cavity. As the vacuoles enlarj-e tlie plaj^modium is reduced to fine strands, and when these break throtigh, the maternal blood lakes the place of the secretion in the spaces of the mesh-work."

The oval cavity of the decidua, in which the o\'um lay, had diameters of 1.9 X 0-95 X 1-1 mm, A small opening, closed by fibrin, and about 0.1 mm. in diameter, placed the ca.vity in communication with the lumen of the uterus; the opening was not covered by a blood-clot. The wall of the implantation carity, except at points where maternal vessels opened into the cavity, was formed of necrotic decidua and fibrin deposits. Only at individual points did the Plasmodium quite reach the wall. The glands were enlarged and filled with blood, their epithelium haWng separated; and the greatly dilated vessels form, especially beneath the ovum, a regular cushion. The decidua is traversed by numerous leucocytes, and all tlie portions of the uterine mucous membrane that were esamiaed showed decidual changes. In the necrotic decidua zone around the ovum and also lying free in the blood -containing implantation cavity was an almost continuous peripheral layer of large, mostly mono-nucleated cells (Fig. 95), which are perhaps to be regarded as degenerating decidua cells set free by the breaking up of the necrotic zone." "The comparison of these cells with a layer of fetal cells which oCours upon the surface of the placental anlage in the guinea-pig, a comparison drawn by the authors on the bafli.s <)f a demonstration by Graf Spec, does not seem to be justified, since the cell layer in question (Duval's ectoplacental entodenu; see also the author's Lehrbuch) owes its existence to tlie greatly modified inversion of the germinal layers in the guinea-pig.

++++++++++++++++++++++++++ Fig. 04. — BIsatoFyst wnll Kith cytotiophoblaiit and eyncytiiim, decidun, sod opening o( m dilated nnu»-1ike rapilLary in the implantation CHVjty. cyl., eylatrophoblsxt: dtc., decidua; end., endoIhe)>iim of s iDaternal capillBry; n. I., necrolic tone of the decidual p'-. [da>niadlum (siiicytium). X 2S0. (From Bryce-ToMher, Plat* V.)


" Corresponds to the syncytium of Bonnet (p. 95), since it contains no nuclear

DlaoUtiooWiiy filled with blood. ,■; 3M. (F^[Il Bryoe-Teacher, FIbib VI.)

The preparation was obtained from an abortion which oCourred sixteen and a half days after the only eohabitation that needs consideration and ten days after the failure of the expected menstruation. Tlie microscopic picture is very Strang* and striking and cannot be compared with any stages of placenta formation known in animals;" it ii liowever, as the niithors state, qnite reconcilable with the newer theoretical deduclions concerning implantation and the commencement

++++++++++++++++++++++++++ Fig. 86.— A wclion thnnuch Ihe Peters ovum and ihe ramninding ponionsof the uleriue inuwiu membrsnc. Bl.. blood iBeunir: Ca„ capMuJariK; m.CAi., mesoderDiBlBxia of ihe first chorionic vifli; Co.. dfadua comparia; Dr., tilande; £., «mbryo; G.. malernal veiuels; Sc„ clof-rng onisuJuni IPeten<'s fungoid capoulamexWDdafromalob. -50. lAfier Pelen^ ISOU. CompKrE' slw Fig. »7.> of the placenta formation. Among the most sinking peculiarities in cumparisoa with what is found in older preparations are ; ( 1 ) the structure of tlie trophoblast shell; (2) the sniallness nf the implantation oj>eninp; (3) the necrotic character of the wall of the egf; eliaraber and the absence of a mutual penetration of the fetal and maternal elements. Whether the preparation can be regarded as absolutely Donnnl must provigionally be left undecided; it comes, on tbe one hand, from an abortion, and, on tbe other, it was preserved only after having remained for twenty hours in a mixture of urine and blood serum. Nevertheless, mitoses are still distinguishable la the cytotrop ho blast cells, and the geaersl impression furnished by tbe preparation, which was demonstrated at the Congress of Anatomists at Berlin, 1908, is distinctly favorable. In the absence of other equally young ova our views concerning placentation must, for the time being, be brought into harmony with this preparation. The conditions in the Peters ovum are quite different. The cavity of the ovum contains the magma reticulare with the antage of the embryo and the body cavities" (Figs. 96 and 07), and has diameters of 1.6X0-9X0.8 mm.; external to the chorionic mesoderm is a layer of closely packed cells, which is traversed by wide blood spaces and surrounds the entire ovum like a shell or mantle having a thickness of 0.5 mm. or more. Into this cell mantle, which is thicker toward the muscularia than toward the surface of the mucous membrane, there project everywhere short, stout processes of the mesoderm, the anlagen of the mesodermal axes of the villi. The cell mantle, on aCoount of its relation to the mesoderm of the ovum, can hardly be interpreted otherwise than as the chorionic ectoderm, trophoblast, and trophoderm." Peripheral to this trophoblast shell lies a layer of tissue which Peters terms the transition zone and which contains, imbedded in an (edematous stroma, a confused mass of maternal, and apparently also of fetal, cells, together with a large number of free blood-corpuscles. The entire ovum, without projecting beyond the level of the mucous membrane, lies beside a fold of the membrane.

"Whether the extensive syncytial formation described by Strahl (1906) in young stages of Miirmecaphaiia, Dasypus, Dendmhiirax. and Aluala, and by Duckworth (1!)07) in Maeacus, is comparable with that in the hnman ovum cannot be determined, since Strahl gives no figures and (hose of Duckworth concern a somewhat later Nlnge.


Fig. 07. — Cytotropboblut and lyncytiuDi of the Peon ovum. The embryonie >truiituT«9 an shown dii«nunm«ticallv. Ah,, uvmaiie avity: Dt., yolk mck; Ic.-H.. inUrcellulmr c&vitiaa ol the meiodemi; U.. body cavitiei (cf. Groaser: Lehrbuch); M. r„ tn««au reticulare; Sy.. lyueytium: Tr., (cytoltrophobilM. (From Peten. Plate I, copied under control of the preparatioD ilMlf.)

" For details concerning the formation of tbe body cavities of the Peters embryo consult Grosser's Lehrbuch. "Compare, however, the ovum of Beneke described below, and Disso's interpretation of it.

imbediJed in the decidua compacta, which over tbe dorsal surface, the summit of the ovum, is defective over an area of about 1 mm,; throughout this region the uterine epithelium, elsewhere well preserved, is wanting. The egg does not project through this defective area freely into the uterine lumen, but is separated from it by a fibrin clot that closes the opening in the compacta and spreads out lateralis like a fungus growth (Figs. 96 and 98). This clot b termed by Peters the fungoid tissue or blood-fungus, and later by Bonnet tbe dosing coagulum. The decidua over tbe entire surface of the uterus is high and swollen, and is divided by furrows into distinct areas; its separation into compact and spongy layers is distinct only in the neighborhood of tbe ovum, for, although enlarged glands with epithelial papillffi oCour elsewhere, yet these oCoupy almost the entire thickness of the mucous membrane, so that a superficial compact layer is not distinct. Typical decidua cells cannot be found, although some large cells of irregular shape and with large, deeply staining nuclei oCour in the vicinity of the ovum; the significance of these is, however, obscure. The entire mucous membrane, in which very greatly enlarged blood-vessels oCour, especially in the neighborhood of the ovum, shows signs of edematous infiltration, which increases in distinctness nearer the ovum; in this region extravasated red and white bloodcorpuscles also oCour. A new formation of blood-vessels oCours especially in the zone of tissue which intervenes between the ovum and the uterine lumen. The glands in the neighborhood of the ovum cun'e around this and open near it upon the surface of the utena; beneath the ovum are closed glandular spaces filled with blood, which show no connection with the e^ capsule.

++++++++++++++++++++++++++ Fig. 08.— Summit o( lli« PoWn ovum. BL, blood Ucun«; Co., capnuluie: &., oli _ _ _ . St., iUiUlk; Sv., syncytium; rr., trophoblut; (7*.. uterine epithelium: l/c.R., the crumpled bolder of this; a., cropboblKt aucleue in the syncytium; b. sod c, preparatory stagea of the syncytium (wreath-like depont in ■ blood taauna-J (From Peters, ISBB.J ++++++++++++++++++++++++++

A number of important points are slill to be noticed concerning the trophoblast layer. The principal part of tbe layer consists of completely separated cells with pale protoplasm and large, deeply staining, round or oval nuclei. On aCoount of the size And staining properties of the nuclei the entire trophoblast shell appears dark even under weak magnification. Throiighout its entire extent it is traversed by blood lacunffl, some lai^ and some small, which are continuous one with the other; these lacune, some of which approach so closely to the chorionic mesoderm as to be separated from it only by one or two layers of cells, are everywhere completely filled with well-presen-ed matertiat blood. At various places they are in cotmection with venous vessels, which possess an endotlielial wall only in the transition zone; the opening of arteries or capillaries into the lacunee cannot be made out. " The most peripheral lacun» are, for the most part, separated from the decidual tissue by a thin covering of ectoblast arranged in concentric layers; but in places diverging tracts of trophoblast stream out into the compacta, and the blood spaces lying between these lack the ectodermal covering on their peripheral surfaces," In the most central portions of the trophoblast cells are to be found wilh feebly staining and distended nuclei, with vacuoles, nuclear fragments.

++++++++++++++++++++++++++ Fig. «9.— A portion of the peripherjof the trophublMt sh»ll of the Peten ovum. Degenerating patsheB of syncytium with greatly enlnrged nuclei; tbe blood-corpusclea, for the mont part, only mtiag npon tbe lyooytium. Tr., cytotropboblaat; Ui., encloaiug lone. x 360. ++++++++++++++++++++++++++

and flakes. More peripherally the distention and degeneration of the nuclei increases, the cell boundaries vanish, and there are formed very irregular, large, vacuolated masses of protoplasm, with numerous, irregularly contoured, and exceedingly large nuclei (Figs. 99 and 102). These masses constitute the syncytium of the Peters ovum, which is, aCoordingly, united by all possible transitions with the cellular trophoblast; it is never separated from this by a limiting membrane. Prickle processes cannot be seen; at most there is " a delicate and thin, strongly refractive deposit, slightly frayed at the edges, on the surface of thfe syncytium." The syncytium completely clothes, except at a few ]>laces, the blood lacunie with a thin layer; indeed, aCoording to Peters, the formation of the syncytium seems to be produced by the contact of the trophoblast with the maternal blood. Furthermore, it would seeni, aCoording to his ideas, that degenerating red and white bloodcorpuscles may be " transformed " into a syncytium, which applies itself to that formed by the trophoblast, so that the blood with its own stnictural elements may be concerned in the formation of the syncytium (compare Fig. 98, " wrealh-like deposits in the lacune representing preliminary stages of the syncytium ")." Tlw syncytium oCours, as a rule, only at regions where there is contact with the maternal blood. Trophoblast and syncytium are frequently mingled with elements of the maternal tissues in the transition zone, and, like these, undei^ degeneration in that region, since free maternal and fetal nuclei can be found in it; the trophoblast and syncytium also frequently replace the wall of a gland and project into its lumen, and they may form the walls of the maternal blood-'Vessels in the peiipheral portions of the trophoblast shell — sometimes by forming one wall of the vessel while Ihe opposite one remains formed fay normal epithelium, sometimes in that over the entire wall only the epithelium, either intact or in fragments, separates the syncytium from the cavity of the vessel (Fig. 100). A transition between the endothelium and the syncytium is never recognizable. Peters's preparation was obtained from the uterus of a suicide, poisoned by caustic potasli on the third day after the omission of a menstrual pmod. The through At two places (a And endothelium; En., eudotheli ium," (After Peters, 1890.) mode of death may not have been without influence on the blood engorgement of the uterine mucous membrane. Peters estimates the duration of the pr^nanc; at from three to four days; details concerning this are given elsewhere. The preservation of the ovum (the autopsy was performed a few hours after death) with the exception of the caudal end of the embryonic aniage, was very good, even although mitoses are not recognizable in the trophoblast cells. The Leopold ovum, which was obtained from a case of phosphorus poisouing, concerning which further data are not available, differs in several points from that of Peters; Thus, the trophoblast growth was less extensive ; and the blood lacunte, which were in open conneetioii with the neighboring capillaries, were unusually wide. The trophoblast cords were, for the most part, reduced to one or two layers of cells, and were to a lai^ extent covered by syncytium. In addition to the ovum, separated portions of the syncytium were also to be found in the decidua,

" Peters, in a private communication, now regards these and similar structuree as rather the expression of embiyotrophic processes.

having evidently wandered into it; but, on the whole, there was less syncytium than in the Peters ovum, and its individual parts were smaller. It was everywhere clearly distinguishable from the vascular endothelium. The contents of the trophoblast shell, the mesoderm of the ovum, were irregularly shrunken and contained maternal blood-corpuscles; an embryonic anlage could not be distinguished (it had probably been already destroyed). The internal diameters of the cavity of the ovum were 1.4 X ^-^ X 0.8 mm. This ovum was also imbedded, somewhat superficially, in the mucous membrane in the neighborhood of a furrow, but a differentiation of the mucous membrane into compacta and spongiosa was not distinct. The regions surrounding the ovum were very rich in glands, which curved around the ovum. Dorsal to the ovum the decidua, in contrast to that of the Peters ovum, is closed except for a small opening, so that a decidua capsularis is present. Extending outward from the opening upon the surface of the mucous membrane is a clot, consisting of blood and fibrin, which corresponds to the fungoid tissue or closing coagulum and is termed by Leopold the fibrin cover. On aCoount of the absence of the embryonic anlage the stage of development cannot be aCourately determined; the measurements are not sufficient for this purpose, since, as has been pointed out, the contents of the ovum were shrunken and the blood lacunae enormously distended. The relatively scanty growth of the trophoblast may indicate, as Bryce and Teacher remark, that the ovum was younger than that of Peters; the small size of the opening in the capsularis may also have the same significance (see below, p. 117). However, definite conclusions cannot be drawn from the preparation. The ovum described by Stolper (1906) seems to represent a very young stage, but it had evidently died some time before its abortion. The diameters of the egg capsule (2.5 X 2.2 X 1-0 mm.) are, at all events, smaller than in the two ova to be described next, but for which the corresponding measurements are not given. The embryo was macerated. The ovum is characterized by a very extensive development of syncytium. Wide blood spaces, probably intended for a diminution of the blood pressure in the communicating vessels, are regularly arranged around the inten'illous space. The results obtained by Graf Spee (1905) from a young ovum studied by him have been stated only briefly, and until a thorough study of the ovum and figures are available, a comparison with the ova already mentioned cannot be made. The ovum was obtained from a case of oxalic acid poisoning; the mucous membrane of the uterus " showed the areas, di\'ided by furrows, that are characteristic of pregnancy," and in one of these at a point marked by a slight depression of the surface was the ovum. " Beneath about two-thirds of the free surface of the prominent area of mucous membrane, imbedded in a cavity in the interglandular connective tissue of the uterine mucosa, was an ovum measuring 1.5 X 2.5 mm. in its greater diameters, poorly provided with villi, and with very small embryonic structures in the anterior. Between the surface of the chorion and the uterine tissue were here and there small quantities of blood from open bloodvessels. The walls of the egg chamber consist throughout of elements of the interglandular connective tissue of the uterus. The lumina of all the glands open into the uterine lumen; none into the egg chamber. The portion of the mucous membrane (serotina) intervening between the ovum and the muscularis holds a large mass of blood (just as in the ovum of Peters) contained in enormously enlarged endothelial canals and apparently stagnant even in life ; it may very well have furnished nutrition to the ovum and at the same time have served as a rampart protecting the parts of the mucous membrane near the muscularis from the destructive contact action of the ovum. The walls of the egg chamber, separating the ovum from the lumen of the uterus, consisted of a thicker or thinner layer of the interglandular connective tissue next the ovum "and a single-layered epithelial covering next the uterine ca\nty. Only in the region of the surface Vol. I.— 8

114 HUMAN EMBRYOLOGY. depression is the uterine tissue interrupted by an opening; which may be regarded as the point of entrance of the ovum iato the uterine mucous membrane, the implantation opening; it is closed only by a flat expanded blood-clot (fibrin with enclosed leucocytes and red blood-corpuscles). The conditions are, aCoordingly, very similar to those oCourring in the human ovum described by Peters. '^ The implantation opening, at this stage 0.8 mm. in diameter at the most, has probably increased somewhat in size from what it was when first produced by the ovum by stretching and growth, and perhaps also by histolysis of the chamber wall, for I imagine that the ovum during the seven days which probably intervene between fertilization and implantation cannot have increased much in diameter and therefore cannot have measured much over 0.2 mm." Among these data the most striking are the small amount of blood in the immediate neighborhood of the ovum and the small number of villi. Nothing is stated concerning the character of the chorionic epithelium, the syncytium, and the intervillous space. The ovmn, nevertheless, does possess villi and in this respect is further developed than Peters's preparation. It is questionable, however, if it is to be regarded as quite normal. The ovum described by Jung agrees excellently in its general character with that of Peters. It was obtained from a curetting, and, completely surrounded with mucous membrane, was preserved, while still fresh, in 80 per cent, alcohol. Its age was not determined. The egg capsule was completely separated from the lumen of the uterus, but was situated somewhat superficially. The somewhat compressed but uninjured cap of the ovum was composed of coagulated blood and tufts of fibrin, together with numerous leucocytes and degenerating decidua cells, and passed gradually over into the transition zone. The diameter of this necrotic cap was 1.7 mm.; it corresponds, aCoording to the opinion of the author, to the closing coagulum of other ova and its extent indicates that of the distended implantation opening.** The diameter of the ovum exclusive of the chorionic epithelium was 2.5 X 2.2 (X1.1) mm. The mesoderm of the chorion had already sent processes, the mesodermal axes of the villi, into the extensively developed trophoblast (Jung avoids the use of the term trophoblast and speaks only of ectoblast). The ovum was completely surrounded by rudiments of vilH, all of about the same length; and on the chorion membrane and at the roots of the villi was an epithelium, consisting of two layers, a basal and a covering layer. The basal layer, composed of distinctly defined cells, passed over into stout columns of cells, which frequently united and so formed the shell around the ovum. Only oCoasionally did free cell-columns oCour, the representatives of free villi. In the peripheral portions the individual cells were somewhat larger and clearer, but everywhere abundant mitoses could be observed and there were no signs of degeneration in the peripheral elements. Individual mitoses were so placed that one of the daughter cells passed into the covering layer, this, the syncytium, showing no mitoses although the nuclei were, for the most part, well presented. The protoplasm of the covering layer presented, in general, a foamy structure; prickle processes projected toward the intervillous space, at least in certain places. Only at certain regions of the periphery was the covering layer, which frequently streamed into the maternal tissues, in degeneration and forming a symplasma syncytiale in Bonnet's sense, perhaps as the result of the action of maternal leucocytes. The vacuoles of the covering layer at places contained what seemed to be altered maternal blood, but everjrwhere a continuity of the syncytium with the maternal tissues, that is to say, with endothelium, was lacking. The boundary between the maternal and fetal tissues was almost everywhere easily recognizable, with some difficulty only in the regions where symplasma structures oCourred.

"In this case the capsularis would not actually be closed. The gradual transition of the cap into the transition zone on the lateral portions of the ovum seems to be opposed to Jung's view. It is possible that the capsularis had at one time been complete, but was again undergoing degeneration.

Between the trophoblast columns, that is to say, the anlagen of the villi, there was a very irregular intervillous space ; this was abundantly tilled with blood, was in continuity with the maternal vascular system by means of gaps in the sieve-like trophoblast shell, and was lined by maternal tissue (endothelium) only in the neighborhood of these gaps. The communications with the maternal vessels were always narrow and the circulation must have been very slow. In the transition zone, situated outside the trophoblast shell, degenerating maternal tissue oCourred, partly associated with the formation of symplasmata, but always without signs of active proliferation; also no new formation of blood-vessels could be found. Around the ovum was a strip of fibrin of varying thickness, produced by degeneration of the maternal tissue (stroma, endothelium, gland epithelium). Leucocytes oCourred abundantly in the transition zone, but not in the fetal tissues. The glands were frequently destroyed by the trophoblast, but appeared to withstand its attacks for a longer time than the rest of the decidua; none of them opened into the intervillous space, but they took a curved course around the ovum. External to the transition zone a separation of the compacta and spongiosa had taken place. The former was oedematous, beset with numerous lymphocytes, and its gland ducts were contorted; typical decidua cells and hemorrhages were wanting, although the glands frequently contained clotted blood. The representatives of the later decidua cells showed numerous mitoses. No oedema oCourred in the spongiosa. The ovum of Beneke was obtained from a curetting twenty-five days after the omission of a menstrual period and was fixed in alcohol. It contained an embryo measuring 1.86 mm. in length, and had a cavity of 4.2 X 2.2 X 1*2 mm., surrounded by a trophoblast measuring 0.4-1.0 mm. in thickness. " As regards the structure of the trophoblast the author can only confirm in general the observations of Siegenbeek, Peters, Marchand, and others.** The syncytial giant cells are throughout of fetal origin and symplasma formation is not recognizable. The sjmcytia had encroached upon the endothelium of the decidual vessels and also upon the epithelium of the glands; by the development of extensive clefts in the interior of the giant cells the intervillous blood spaces are being formed. Scattered giant cells with prickle processes oCour in the chorionic connective tissues of the investment of the oviun; they wander to a certain depth into the decidual tissue, where they may be recognized by their characteristic nuclei and by containing glycogen. The decidual cells, extensively swollen, take part in the formation of the so-called transition zone to a greater extent than has been supposed by Peters, for example. The closing * tissue plug ' which fills the opening in the reflexa corresponds in general in its histological constituents, blood, fibrin, leucocytes, etc., with what Peters has described." The measurements of Siegenbeek's ovum (4.5 X ^-^ mm.) were not made directly, since the ovum had been opened by a tear at one spot and was collapsed^ but were estimated from the perimeter. It was obtained from a woman who had met an aCoidental death from burning; the entire uterus was preserved in formalin fourteen hours after death. The ovum was completely covered with villi, those on the basal surface being stronger than the peripheral ones, and those about the equator of the ovum the strongest of all. Free villi oCourred; the majority were continued into cell columns (ectoblastic trabeculae), which united "Disse, who has subsequently studied the specimen, regards the entire trophoblast of the ovum as maternal tissue and also transfers this same interpretation to the Peters ovum.

together peripherally and formed an ectoblast shell traversed by large and small spaces, and varying in thickness in different regions. In general it was thicker on its peripheral than on its basal side. The ectoblast cells situated near the maternal tissue were larger than those having a more central position and frequently showed degenerating nuclei, but mitoses oCourred in all portions of the ectoblast shell. The boundary between the fetal and maternal tissues was difficult to make out in certain regions. The intervillous space was formed by blood-filled lacunie which were lined only by cellular ectoblast or by syncytium; the endothelium was also frequently wanting in the blood-vessels at their communication with the intervillous space. In the space were very many leucocytes and perhaps also special nucleated elements of the maternal blood. The syncytium was only to be found in the region of the blood paths; it showed no prickle processes and no cuticula on the side next the ectoblast. The derivation of the syncytium from maternal tissues (endothelium, epithelium, or connective tissue) was excluded, but its continuity with the cellular ectoblast could not be made out, so that the origin of the tissue could not be determined.

The ovum lay in the compacta, whose basal portion had the same structure as the capsularis (reflexa), except as regards the oCourrence of glands. The capsularis, for the most part, lacked an epithelium and contained in its interior fibrin stria?. Basally there were greatly distended glands filled with blood; around the periphery of the ovum the glands were arranged concentrically, and the glandular epithelium did not fomi syncytia. Characteristic decidual cells were nowhere present. A sharp separation of the compacta and spongiosa had not yet oCourred in the decidua vera; the compacta was (Edematous."

The ovum of Rossi Doria was obtained from an abortion. It was injured by a tear and does not seem to have contained an ernbiyo. The egg membranes were rather well presened. The theoretical considerations of the author will be discussed in note 23. Frassi's ovum was obtained from an operation fourteen days after the omission of a menstrual period; the unopened uterus was preserved in formalin. The ovum of Friolet was also obtained from an operation and fixed in the unopened uterus. Both ova already showed, for the most part, a twolayered epithelium over the villi; a criticism of the observations made upon these ova will follow in the resume.

The ova of Peters, Jung, Beneke, and Siegenbeek, with their extensive development of the trophoblast, fonn a single harmonious group, which may be derived from conditions such as Bryce and Teacher have described. To the older stages, on the other hand, a natural and easy transition is formed by the Siegenbeek ovum.

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