Book - A Laboratory Text-Book of Embryology 7 (1903)

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Minot CS. A Laboratory Text-Book Of Embryology. (1903) Philadelphia:P. Blakiston's Son & Co.

A Laboratory Text-Book of Embryology: 1. General Conceptions | 2. Early Development of Mammals | 3. Human Embryo | 4. Pig Embryos | 5. Chick Embryos | 6. Blastodermic Vesicle and Ovum Segmentation | 7. Uterus and the Foetal Appendages in Man | 8. Methods | Figures | Second edition | Category:Charles Minot
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This historic 1903 embryology textbook by Minot describes human development.

This textbook was republished in a second edition 1917: Minot CS. A Laboratory Text-Book Of Embryology. (1917) Philadelphia:P. Blakiston's Son & Co.

See also his earlier 1897 textbook; Minot CS. Human Embryology. (1897) London: The Macmillan Company.

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

See also: Historic Embryology Textbooks

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Chapter VII. Study. Of The Uterus And The Foetal Appendages Of Man

Histology of the Uterus

In most mammals the uterus is double. This is the ease in the pig, the rabbit, and the mouse, the three species which furnish material for the practical study as planned in this book. In these animals each uterus is a long, more or less cylindrical tube. In primates the double uterus exists only during very early embryonic stages, after which the two are found united into a single median uterus. The mammalian uterus is always lined by a mucous membrane, consisting of a superficial epithelium, which forms glands, and of a deeper layer of reticulate connective tissue, in which there are lymph spaces, nerves, and a fairly abundant blood-supply. The mucous membrane is subject to very marked periodic changes in structure. It is enclosed by the muscular layers of the organ, the muscle-fibers being of the smooth type. In animals with double uteri the muscle-fibers form two distinct layers, an inner circular and an outer longitudinal layer. In the primate uterus the disposition of the fibers is far more complicated, and the two distinct layers cannot be identified. The surface.of the uterus, wherever it is free, is covered by a layer of peritoneum which consists of a layer of flattened epithelial cells and a thin underlying layer of fibrillar connective tissue.

The human uterus at birth has a mucosa which is about 0.2 mm. thick. The mucosa is soft, pale gray or reddish-gray in color; it consists of a covering of ciliated epithelium and a connective-tissue layer. It is without glands, the glands not appearing usually until the third or fourth year, and developing very slowly up to the age of puberty.


The function of menstruation involves great changes in the mucosa of the body of the uterus. We distinguish three periods : (1) tumefaction of the mucosa, with accompanying structural changes, taking five days, or, according to I Jensen, ten days; (2) menstruation proper, about four days; (3) restoration of the resting mucosa, about seven days. The times given are approximative only. The whole cycle of changes covers about sixteen days. Since the monthly period is about four weeks, the period of rest, as thus calculated, is only about twelve days.

1. Tumefaction

A few days before the menstrual flow the mucosa gradually thickens; the surface becomes irregular; the openings of the glands lie in depressions. The connective-tissue cells are increased in number, and it is said by some authors in size, but the increase in size is doubtful ; the number of round cells increases; the glands expand and become more irregular in their course; a short time before hemorrhage begins, the blood-vessels, especially the capillaries and veins, become greatly distended. We must assume that the connectivetissue cells proliferate, but we have no satisfactory observations upon their division. It was formerly asserted that the menstrual decidua contains decidual cells, but in all the specimens the author has studied there were none present.

2. Menstruation

When the changes just described are completed, the decidua menstrualis is fully formed, and its partial disintegration begins. The process commences with an infiltration of blood into the subepithelial tissues; this infiltration has hitherto been commonly explained as due to the rupture of the capillaries; but as no ruptures at this period have been observed, we may justly regard this explanation as inadmissible, and account for the infiltration per diapedesin. It lasts for a day or two, and is apparently the immediate cause of a very rapid molecular disintegration of the superficial layers of the mucosa, which in consequence are lost; the superficial blood-vessels are now exposed, and, by rupturing, cause the well-known hemorrhagia of menstruation. By the disappearance of its upper portion the mucosa is left without any lining epithelium and is very much (and abruptly) reduced in thickness. Its surface is formed by connective tissue and exposed blood-vessels.

3. Restoration of the Mucosa

At the close of menstruation the mucosa is 2 or 3 mm. thick ; the regeneration of the lost layers begins promptly and is completed in a variable time, probably from five to ten days. The hyperemia rapidly disappears; the extravasated blood-corpuscles are partly resorbed, partly cast off; the spindle-cell network grows upward, while from the cylinder epithelium of the glands young cells grow and spread up and out so as to produce a new epithelial covering; new subepithelial capillaries appear. The details of these changes are imperfectly known; they effect the return of the mucosa to its resting-stage.

Decidua Menstrualis

Specimens from the first day of menstruation are the most instructive. They should be preserved in Zenker's fluid; sections may be made perpendicular to the decidual surface from blocks io to is mm. cube, and stained with alum hematoxylin and eosin. The use of Mallory's triple connective-tissue stain will demonstrate the fibrillar tissue in the decidua and the very large amount of the same in the muscularis.

The accompanying figure is from a uterus in active menstruation. The decidual membrane is from i.i to 1.3 mm. thick; its surface is irregularly tumefied; the gland openings lie for the most part in the depressions. In the cavity of the uterus there was a small blood-clot. The demarcation between the decidua and the muscularis is sharp. The upper fourth, d, of the decidua is broken down and very much disintegrated ; its cells stain less readily than those of the deep portion of the membrane ; the tissue is divided into numerous more or less separate small masses. Some of the blood-vessels are ruptured. The superficial epithelium, ep, is loosened everywhere ; in places fragments of it have fallen off, and in some parts it is gone altogether; it stains readily with alum hematoxylin, differing in this respect from the underlying connective tissue. The deeper layer of the decidua is dense with crowded well-stained cells, which lie in groups and are probably proliferated connective-tissue cells. They have small oval or elongated darkly stained nuclei, with very small granular protoplasmatic bodies. There is no indication of any enlargement of the cells, such as occurs in the production of true "decidual" cells. There are very few leucocytes. The glands are enlarged somewhat, and are lined by a normal cylinder epithelium, which offers no obvious change as compared with that of the glands of the resting uterus.

The Pregnant Uterus: the Two Stages

When the ovum implants itself in the uterine wall, it becomes covered by a growth of the mucous membrane or decidua which we know as the decidua reflexa. For an account of this process see pages 118 to 120, where also proper definitions of the terms decidua reflexa, serotina, and vera are given. As the ovum increases in size the decidua reflexa also increases, and gradually becomes thinner and thinner, until it ultimately disappears. The exact date of its disappearance is not known; it falls somewhere within the fifth month. Accordingly, we may divide the period of pregnancy into two phases or stages, each comprising about half of the whole period. During the first stage the decidua reflexa is present, during the second stage it is absent, so that the chorion laeve comes into direct contact with the decidua vera. In the following sections a typical uterus of the first and second stages each is described.

Human Uterus Three Months Pregnant

The uterus measures about 34 inches in transverse diameter, and shows well-marked venous sinuses on its external surface. It should be opened by a crucial incision on the anterior side ; its walls will be found about an inch or more in thickness; it contains a grayish-red bag (decidua reflexa), which nearly fills the cavity of the uterus and encloses the embryo, so that upon opening the womb we do not encounter the foetus directly. The inner bag has a smooth surface, but shows a few small pores; it is without blood-vessels and is attached to the dorsal wall of the uterus. The inner surface of the uterus shows a rich network of blood-vessels, many of which are large, irregular sinuses. The uterine walls consist of an outer muscular layer, and an inner decidual layer, which takes up nearly half the thickness of the wall, and is known as the decidua vera. Comparison with the seventh month uterus shows that the proportion of the layers changes, because during gestation the muscular layer increases and the decidual layer diminishes in thickness. The inner bag, when opened, shows the large cavity in which the embryo lies, floating in amniotic fluid. The bag is formed by three very distinct membranes, of which the outermost, the decidua reflexa, is opaque and the thickest ; the two inner ones are thin and transparent ; the innermost is the delicate amnion; the middle membrane is the chorion, and is quitedistinct from both the amnion and reflexa ; with the latter it is connected by a number of small branching villi scattered at some distance from one another over the surface; the villi adhere firmly to the reflexa by their tips. The embryo (Fig. 94) resembles a child in its general appearance; the length of the head and rump together is about 8 cm., and the head is approximately of equal bulk to the rump. The umbilical cord is from 5 to 7 mm. in diameter and usually about 1 2 cm. long. From its distal end the blood-vessels spread out over the placental area, and around the edge of the area rises the decidua reflexa, which does not extend on to the placenta. Floating in the amniotic fluid is a pear-shaped vesicle, the yolk-sac, which is about 7 mm. in diameter; it has a fine network of blood-vessels upon its surface, and is connected at its pointed end with a long slender pedicle, the yolk-stalk, which runs to the placental end of the umbilical cord, there enters the cord itself, and runs through its entire length to its attachment to one of the coils of the intestine of the embryo.* Over the whole of the placental area the chorion gives off large villous trunks, each of which has numerous branches, with ramifications of the foetal vessels; the villi fill a space about one centimeter wide between the membrane of the chorion frondosum and the surface of the uterine decidua serotina, to which the tips of some of the villi are attached. With care the villi may be separated from the decidua, which is seen, when it is thus uncovered, to be cavernous ; the caverns are rounded in form and part of them may be followed, on the one hand, until they connect with the blood sinuses of the uterus, and, on the other, until they open into the intervillous spaces, which therefore receive a direct supply of blood from the mother.

  • At this Stage a large part of tile yolU-stalk within the umbilical cord has degenerated and usually disappeared by resorption.

The principal difference to be noted between the uterus before and that after the fifth month in the relations of parts is the presence or absence of the decidua reflexa as a distinct membrane. During the fourth month the reflexa stretches as the membranes expand, and becomes thinner and thinner until by the end of the fourth month it is as delicate and transparent as the chorion and lies close against the decidua vera.

Human Uterus Seven Months Pregnant

If we examine a pregnant uterus at any time during the sixth to ninth month of gestation, we find essentially the same relations of the parts — the most marked difference being in the size of the uterus, which increases with the duration of gestation, to correspond to the growth of the foetus. A description of a uterus seven months after conception will suffice, therefore, for our present purpose.

Such a uterus is a large, rounded bag, with muscular walls, and measures seven or eight inches in diameter. Examined externally, it is remarkable especially for the numerous large sinus-like blood-vessels ; its surface is smooth ; the texture of the walls is firm to the touch, but the walls yield to pressure, so that the position of the child can be felt. As the placenta is generally upon the dorsal side, it is usual to open the uterus by a crucial incision upon the ventral side. The walls are about one-half of an inch thick, sometimes more, sometimes less, and as soon as they are cut open we enter at once into the cavity of the uterus containing the foetus and nearly a pint of serous liquid— the amniotic fluid. The foetus normally lies on one side, has the head bent forward, the arms crossed over the chest, the thighs drawn against the abdomen, and the legs crossed; it resembles closely the child at birth, but is smaller; its head is, relatively to the size of the body, larger; the abdomen is more protuberant, and the limbs proportionately smaller. The inner surface of the uterus is smooth and glistening; if it is touched with the finger, it is found to be covered by a thin but rather tough membrane, called the amnion, which is only loosely attached. Examination of the uterine wall, where it has been cut through, shows that its thickness is formed principally by the muscular layer, which is made up by numerous laminae of fibers, between which are the large and crowded blood sinuses, similar to those distinguishable on the external surface of the uterus. About one-fifth or less of the wall inside the muscularis has a different texture and can be partly peeled off as two distinct membranes, the innermost of which is the amnion already mentioned, and the outer is the chorion united with the deeidua. The amnion and chorion are appendages of the embryo; the decidua is the modified mucous membrane of the uterus. Let us return to the embryo. From its abdomen there springs a long, whitish cord, known as the umbilical cord; it is usually from about one-third to one-half an inch in diameter and 40 cm. long, but its dimensions are extremely variable; it always shows a spiral twist, and contains three large blood-vessels, two arteries and one vein, all of which can be distinguished through the translucent tissue. The distal end of the cord is attached to the wall of the uterus, usually near the middle of the dorsal side of the organ. It is easily seen that the blood-vessels of the umbilical cord radiate out from its end over the surface of the uterus underneath the amnion, branching as they go; they spread, however, only over a circumscribed area, the placental, where the wall of the uterus is very much thickened. A vertical section through the placental area shows that the amnion and chorion are widely separated from the decidua and muscularis by a spongy mass soaked with maternal blood. This mass consists of numerous trees of tissue, which spring with comparatively thick stems from the chorion and branch again and again. In these stems and branches are to be found the final ramifications of the vessels of the umbilical cord; the trees are known as chorionic or placental villi. Some of their endtwigs are very closely attached to the surface of the decidua. In the center of the placental area the villi form a mass about three-fourths of an inch thick, but toward the edge of the area the mass gradually thins out until at the very edge the chorion and decidua come into immediate contact. The mass of villi, together with the overlying portions of the chorionic and amniotic membranes and the underlying portion of the decidua, constitutes what is known as the placenta. The decidua of the placental area is called the decidua serotina; the chorion of the placenta is known as the chorion frondosum. When birth takes place, the whole placenta is expelled after the delivery of the child; the placenta of the obstetrician is therefore partly of foetal, partly of maternal, origin.

Decidua Vera of the First Stage in Section

Specimens may be preserved in Zenker's or Tellyesnicky's fluid, or they may be preserved with less good results in Midler's or Parker's fluid or in picrosulphuric acid. Sections may be made of the entire wall in celloidin, or, if it is desired to get thinner sections, in paraffin, in which case it is advantageous to remove as much as possible of the muscular coat so as to cut only the decidual membrane.

The following description is based upon a uterus one month pregnant. Figure 184 was obtained from a vertical section of the decidua, by drawing the outlines of the glands or gland spaces, gl' and gl" , and by dotting the areas occupied by the connective tissue. The blood-vessels are indicated by double outlines. The artery shown in the figure, owing to its spiral course, is cut repeatedly. The figure demonstrates very clearly that the gland cavities are so arranged that the decidua is divided into an upper compact layer and a lower cavernous layer, the difference being due to the size and number of the gland cavities. The amount of epithelium to be observed at this stage varies greatly. It is some times wholly absent from the surface, in other cases absent or present in patches. In the glands the epithelium has undergone many modifications. In some parts the original cylinder epithelium of the glands is well preserved in patches, and such patches of epithelium are found at every stage until after delivery. It has been observed that these patches serve to regenerate the epithelium of the glands, and by spreading from the glands on to the surface, to regenerate also the epithelial covering of the uterine mucosa But for the most part the glandular epithelium is considerably altered. We find places in which the cells, though attached to the surrounding connective tissue, are separated from one another by small fissures. In other places the cells are a little larger (Fig. 185), each for the most part cleft from its fellow, and some of them loosened from the wall and lying free in the cavity. Apparently the cells which are thus freed become swollen, probably by imbibition, both the protoplasm and the nuclei becoming enlarged (Fig. 186). The cells lie separately and almost completely fill the gland cavity. They are no longer cylindrical in shape, but irregular. Their protoplasm is finely granular and stains rather lightly. The nuclei are rounded, granular, and with sharp outlines. In somewhat older stages one finds the cells replaced by a granular material. The obvious interpretation of the appearances described is that the glandular epithelium is breaking down and disintegrating, or, in other words, passing through a special form of degeneration which is highly characteristic. In later stages some of the broken-down material forms hyaloid rounded concretions, which, owing to their deep staining, are somewhat conspicuous.

File:Minot1897 fig184.jpg

Fig. 184. Vertical Section of a Human Uterus (Decidua vera), One Month Pregnant.

Comp, Compact layer. Cav, Cavernous layer. D, Gland-duct. Art, Spiral artery. Gl, Spaces occupied by epithelial glands. Muse, Muscularis. (For clearness all the glandular epithelium has been omitted from the drawing.)

File:Minot1897 fig185.jpg

Fig. 185. Human Uterus, One Month Pregnant. Section of Gland from the Cavernous Layer, with the Epithelium Partly Adherent to the Walls. X 445 diams.

File:Minot1897 fig186.jpg

Fig. 186. Human Uterus, One Month Pregnant. Section of a Gland from the Cavernous Layer with the Epithelium Loosened from the Walls. The Epithelial Cells are Swollen.

The formation of decidual cells has already begun in the upper or compact layer (Fig. 187). They are modified connective-tissue cells, which have grown in size and altered their structure. Their bodies stain deeply with eosin; the nuclei are round, oval, slightly irregular in shape, coarsely granular, and sharp in outline. The cells themselves, though irregular and variable in shape, are all more or less provided with processes running off in various directions. Scattered between the cells are many sections of the processes. Occasionally it may be seen that two cells are connected. Later on the decidual cells acquire smoother and more rounded outlines, and appear to lose altogether their connections with one another. In the cavernous layer there are no decidual cells.

File:Minot1897 fig18.j7pg

Fig. 187. Uterus One Month Pregnant ; Portion of the Compact Layer of the Decidua Seen in Vertical Section. Coagl, Coagulum upon the surface. d,d', Decidual cells. x 445 diams.

Decidua Reflexa of the First Stage

The decidua reflexa may be preserved in Zenker's fluid, Parker's fluid, or picro-sulphuric acid. It should be hardened with the portions of the chorion and chorionic villi adherent to it. It may be imbedded in celloidin and the sections stained with alum hematoxylin and eosin, with Beale's carmine, or with a so-called fibrin stain.

As stated above (page 319), the presence of the decidua reflexa distinguishes the first stage of pregnancy from the second, in which the reflexa is absent, having disappeared by degeneration and absorption. To observe this process of the disappearance of the reflexa, membranes from the second and third months should be examined.

Section of Decidua Reflexa of Two Months

At this time the reflexa starts from the edge of the placental area as a membrane of considerable thickness, but it rapidly thins out, the very thinnest point being opposite the placenta. Examination of sections shows that the entire reflexa is undergoing degeneration which is found to be more advanced the more remote the part examined is from the placenta. The chorion lseve lies very near the reflexa, being separated only by the villi, which are already very much altered by degeneration. In the region half-way between the base and the apex of the reflexa the tissue (Fig. 1 88) shows only vague traces of its original structure. Only here and there can a distinct cell with its nucleus be made out. Most of the cells have broken down and fused into irregular hyaline masses without organization. Ramifying through the fused detritus appear strands and lines, which are more darkly stained by both carmine and hematoxylin. On account of their fibrous appearance, these strands are often spoken of as fibrin, although they are presumably not the same as the true fibrin from the blood. The fibrin is much more developed upon the inner or chorionic side than upon the outer side of the reflexa. On the inner side it forms a dense network, which fuses with the degenerated ectoderm of the chorionic villi wherever the villi are in contact with the decidua. It also ramifies nearly.half-way through the decidua, the ramifications being followed easily, owing to the dark staining of the substance. Over the outside of the decidua the fibrin forms a much thinner layer or may be only indistinctly formed.

In a decidua reflexa of three months the conditions are essentially the same, except that the degeneration is further advanced and the membrane thinner. Traces of cellular structure are still more vague and the fibrin is more developed. In all parts of the membrane there appear leucocytes which are particularly numerous and conspicuous in the neighborhood of the placenta. It is natural to assume that they are concerned in the resorption of the reflexa. There is an inner thicker layer of fibrin and a thinner outer layer, which is now always present and distinct. Between these two layers is a stratum in which the remains of the cells may be seen. Occasionally there is an appearance which suggests surviving decidual cells, and, indeed, in sections taken from parts close to the placenta true decidual cells may be identified.

File:Minot1897 fig188.jpg

Fig. 188 Section of Human Decidua Reflexa at Two Months.

The origin of the chorion lseve and the disappearance of its villi have been described (page 121). The sections of the decidua reflexa will serve also to enable the student to see some of the phases of the degeneration of the villi. They are very much altered. Their ectoderm undergoes a hypertrophic degeneration and becomes hyaline tissue, which stains darkly. The degenerated ectoderm of adjacent villi fuses more or less extensively. The mesoderm of the villi shows a partial loss of its primitive cellular organization.

Decidua Vera and Chorion Laeve of the Second Stage

Pieces of the decidua vera of from six to nine months with the chorion and amnion carefully preserved in situ may be hardened in Midler's or Tellyesnicky's fluid. Blocks half an inch or less in size may be imbedded in celloidin and sections made perpendicularly to the surface, stained with alum hematoxylin and eosin, or with Heidenhain's hematoxylin and orange G, or with picro-carmine.

The decidua reflexa having been resorbed, the chorion (Fig. 189, Cho) has come into contact with the surface of the uterus, and the chorionic epithelium, c, is closely adherent to the surface of the decidua, from which the original epithelium has completely disappeared. The amnion is loosely connected with the chorion by a few strands or threads, which are represented in the figure and the nature of which is not known. Both the amnion, Am, and the chorion, Cho, being developed from the original somatopleure (compare page 78), consist of a mesodermic and an ectodermal layer. The ectoderm of the amnion is a single layer of cuboidal cells placed on the side of the membrane toward the embryo and away from the uterus. The ectoderm, c, of the chorion, on the contrary, is next the uterus. Hence it will be noticed that the mesodermic layers of the amnion and chorion are adjacent. Both membranes are quite thin. The decidua is a relatively voluminous membrane containing blood-vessels, v, which for the sake of distinctness have been filled in with black in the drawing. It also contains a series of elongated spaces, which represent sections of the glands. These spaces, gl, are present only in the inferior half of the decidua. Owing to their absence from the superior half, that portion has a more compact structure, and is, therefore, designated as the compact layer; the lower portion, being broken up and made loose in texture by the somewhat numerous gland cavities, is called the cavernous layer, the caverns, of course, corresponding to the gland spaces. The gland spaces are now very much stretched out, a condition which results simply from the general expansion of the uterus during pregnancy. In the gland spaces appear patches of epithelium still intact, and in the cavities themselves isolated cells in various phases of degeneration and disintegration, similar to the phases which may be observed in the decidua vera of one month ; but the degeneration is, on the whole, considerably more advanced than in the early stage. Around some of the larger blood-vessels there is connective tissue but slightly modified, and the original structure of the mucous membrane is more or less, but not perfectly, preserved in the deep portion of the decidua. But the majority of the cells, especially in the compact layer, have grown in size and become transformed into true decidual cells. In the ectoderm of the chorion, c, the cells lie two or three deep. They have distinct walls, a very coarsely granular protoplasm, and nuclei which stain darkly. By these characteristics they are easily distinguished from the neighboring decidual cells, to which, however, they offer a slight superficial resemblance.*

  • It should perhaps he noted that in some comparatively recent text-books the chorionic ectoderm has been described as the decidua refiexa, an error which is much to be regretted.

File:Minot1897 fig189.jpg

Fig. 189. Human Uterus about Seven Months Pregnant. Vertical Section of the Decidua Vera with the Foetal Membranes in situ.

Am, Amnion. Cho, Chorion, c, Chorionic epithelium, v, Blood-vessel, gl, Glands, muse, Muscular. x 40 diams.

The Placenta in Situ

The placenta in its natural position in the uterus follows the curvature of the uterine walls, hence its free or amniotic surface is slightly concave. Its decidual surface is strongly convex. It is thickest in the center and thins out gradually toward its edge. The uterus should be obtained in the freshest possible condition and be opened by a crucial incision on the ventral side. The embryo should then be removed, the umbilical cord cut through, care being taken to bring as little pressure as possible on the uterus or the placenta. The whole organ is then placed in the preservative, which should be either Tellyesnicky's or Midler's fluid. In view of the large size of the organ, it is very necessary to use large quantities of the preserving fluid, and this fluid must be changed several times in order to insure good histological preservation. When the hardening is completed, columns about one-half inch square may be cut out so as to pass vertically from the inner to the outer surface of the placenta, preserving the amniotic and chorionic membranes in place. The blocks are to be imbedded in celloidin and ought to remain at least three days in thin and three days in thick celloidin, so as to insure a thorough penetration of the imbedding material into the intervillous spaces. Make the sections so that they pass vertically through the placenta. Stain with hematoxylin and eosin or with picro-carmine.

Placenta at Seven Months

A section made according to the method just described is represented in figure 190. The thin amnion, Am, covers the upper (or inner) surface of the chorionic membrane, Cho. This membrane is separated from the decidua, D, by a dense forest of villi, of which innumerable sections appear. In younger placentas the distance between the chorion and the decidua is considerably less, and the number of sections of villi is smaller, but the average size of those sections larger. In the present specimen the distance between the chorion and the decidua is nearly twice as great as the diameter of the muscular coat, Mc, of the uterus. The ends of some of the villi touch, and are imbedded in, the decidual tissue. Their imbedded ends are without covering epithelium, but their connective tissue is immediately surrounded by hyaline substance which is probably degenerated epithelium. The decidua serotina is plainly divided into an upper compact , D', and a lower cavernous layer, D" . The section figured passes through an arterial vessel, ve, which makes an abrupt turn so as to discharge its blood into the intervillous spaces.

File:Minot1897 fig190.jpg

Fig. 190. Human Placbnta in situ, about Seven Months. Vertical Section. Am, Amnion. Cko, Chorion. /7, Villous trunk. ,/, Sections of villi in the substance of the £T*J^£ I decidua serotina. Jft, Muscularis. 1 >, Uterine artery, opening into the placenta ; the fetal UflcM^ are drawn black; the maternal blood-vessels white the chorionic tissue is stippled I. except for canalized fibrin, which is shaded by lines. The remnants of the gland cavities m the dec.dua are sUppled dark. X 6 diams.

The histological structure of all the parts should be carefully studied. (As regards the structure of the amnion, see page 349.)

File:Minot1897 fig191.jpg

Fig. 191. Human Placental Chorion and Amnion of the Fifth Month. Ep, Amniotic epithelium. Am, Amnion. S/r, Stroma. Fib, Fibrillar layer. Fir, Fibrin layer, cellular layer of ectoderm. Vi, Chorionic villi. X 7 1 diams.


The chorion consists of two layers, the outer ectodermic and inner mesodermic. Over the chorionic membrane proper the ectoderm offers a great variety of appearances. In some places it may be seen to have still its primitive organization, a single inner layer of distinct cells and an outer syncytial layer, more or less similar to those represented in figure 199. For the most part, however, the chorionic ectoderm has been considerably modified from its primitive condition. The inner or cellular layer exhibits irregular, thickened patches, which present every possible degree of variation as to their size. A cell patch from a somewhat younger stage is represented in figure 191 as seen with a low magnification, and another patch of the age we are studying is represented in figure 192. The patches vary in appearance; the cells are more distinct in the small patches, less so in the large patches, in which there are often parts more or less degenerated. The cell-bodies stain lightly; their nuclei are granular, not very sharply defined, and variable in size and shape. The cellular layer is always sharply defined against the mesoderm. Toward the outside the patches offer varying relations. In some cases a part of a cell patch may form the whole thickness of the ectoderm, as shown in figure 191, or the whole of a cell patch may do so. More commonly, however, the cellular patch is covered more or less completely by a special substance, which is termed canalized fibrin, and which is believed to represent the original outer syncytial layer in a degenerated condition. The fibrin is a constant, normal, and very remarkable constituent of the placenta. Its formation seems to begin always in the outer or syncytial layer of the chorionic ectoderm, but it may also spread into the cellular layer, which then becomes replaced by fibrin, so that this last alone represents the ectoderm of the chorion. The fibrin layer consists of a very refringent substance permeated by numerous channels (Fig. 192, fb). The substance has a violent affinity for carmine and hematoxylin, and is always the most deeply colored part of a section thus stained. The channels tend to run more or less parallel, to the surface of the chorion, and are connected by numerous short cross-channels. Some of the channels contain cells or nuclei. The appearances, however, are very variable; the fibrin often sends long outshoots into the cellular layers. To summarize, we may say that the ectoderm of the chorionic membrane undergoes patchwise manifold changes. It exists in three general forms : the nucleated protoplasm or syncytium, the cellular layer, and the canalized fibrin. A patch of the ectoderm may consist of any one of these modifications or any two, or of all three. But they have fixed relative positions, for when the syncytium is present, it always covers the free surface of the chorion ; when the cellular layer is present, it always lies next the mesoderm ; and when all three forms are present over the same part, the fibrin is always the middle stratum.

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Fig. 192. Human Chorion of Seven Months' Placenta. c, Cellular layer, fi, Fibrin layer, ep. Remnants of epithelial layer, mcs, Mesoderm. X 445 diams.

The mesoderm of the chorion in early stages has a homogeneous matrix, which about the ninth week begins to change its appearance. In the frondosum, in our specimen, the matrix has acquired a distinctly fibrous structure. Usually the production of fibers is much greater in the immediate neighborhood of the ectoderm, and this may go so far as to mark out a more or less distinct subectodermal fibrillar layer (Fig. 191, Fib). There appears to be no mesothelial layer upon the chorion at this stage, but it seems possible that its presence might be revealed by the application of proper special methods.

In the villi the ectoderm differs from that of the chorionic membrane in several respects: (1) The cellular layer after the first month becomes less and less conspicuous, and after the fourth month is present only in a few isolated patches, which have been termed the cell-knots. (2) For the most part the villi remain covered by the syncytial layer, which in many places is thickened. In later stages these thickenings are small and numerous, constituting the socalled proliferation islands with many nuclei. Many of the little thickenings appear in sections of the villi, and here and there are converted into canalized fibrin. (3) The proliferation islands are converted into canalized fibrin and at the same time grow and fuse, forming larger patches, particularly on the larger stems. In this manner are produced the large areas and columns of fibrin such as appear in our section. (4) Over the tips of the villi, where they are imbedded in the decidua serotina, the epithelium apparently degenerates and becomes hyaline tissue, but without canalization. The mesoderm exists in two principal forms, adenoid tissue and fibrillar tissue around the bloodvessels. The adenoid tissue (Fig. 193) may be considered as the proper tissue of the villus. It consists of a network of protoplasmic threads, which start from nucleated masses. There are many large meshes, which are partly occupied by the very large, coarsely granular, wandering cells, /, /. The wandering cells generally are widely scattered, but sometimes are present in large numbers. They are usually interpreted as foetal leucocytes. They differ, however, by their large size and appearance strikingly from the leucocytes of the adult. We have no knowledge of their history or functions. About the capillary bloodvessels, v, the network is more finely spun. Around the larger blood-vessels the mesoderm has a distinct intercellular substance with a tendency to fibrillar differentiation in quite a wide zone around the blood-vessels. In this zone the cells become elongated or irregularly fusiform. Around the larger vessels the cells are grouped in laminae, and apparently are contractile, so that they must be looked upon as an imperfectly differentiated form of smooth muscular tissue.

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Fig. 193. Adenoid Tissue from a Villus of a Human Placenta of Four Months. /, /, /, Wandering cells, v, V, Capillary blood-vessels. J, Finer meshwork surrounding a capillary. X 35 2 diams.

Decidua Serotina at Seven Months

Specimens may be treated as described for the placenta in situ (page 329). If, however, the best results are desired, the whole of the uterus should be cut through and the placenta divided into smaller pieces from 1 to 2 cm. in diameter, so as to allow a freer penetration of the preserving fluid. Either Zenker's or Tellyesnicky's fluid is recommended. In a normal uterus about seven months pregnant we find the following relations: The serotina is about 1.5 mm. thick, and contains an enormous number of decidual cells (Fig. 194); the cavernous, and compact layers, D", are very clearly separated; the mucosa is sharply marked off from the muscularis, although scattered decidual cells have penetrated between the muscular fibers. The muscularis is about 10 mm. thick and is characterized by the presence of quite large and numerous venous thrombi, especially in the part toward the decidua. The decidua itself contains jew bloodvessels. Upon the surface of the decidua can be distinguished a special layer of denser decidual tissue, which in many places is interrupted by the ends of the chorionic villi which have penetrated it, as is well shown in the accompanying figure. The gland cavities of the spongy layer, D', are long and slit-like; they are filled for the most part with fine granular matter, which stains light blue with hematoxylin ; they also contain a little blood, and sometimes a few decidual cells. There also occur in them hyaloid concretions, — oval bodies several times larger than any of the decidual cells, and presenting a vacuolated appearance. In uteri over two months pregnant they are probably invariably present. In many places the glandular epithelium is perfectly distinct; its cells vary greatly in appearance, neighbors being often quite dissimilar; nearly all are cuboidal, but some are flattened out ; of the former, a number are small with darkly stained nuclei, but the majority of the cells are enlarged, with greatly enlarged hyaline, very refringent nuclei. There are also in many of the gland spaces isolated enlarged cells, which have detached themselves from the wall, and in some cases the detached cells nearly fill the gland cavity, very much as in figure 186.

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Fig. 194. The Human Decidua Serotina at Seven Months. The Section is Taken from near the Margin of the Placenta. Vt, Chorionic villi ; the intervillous spaces were filled with maternal blood, which is not represented in the figure. V, Cavernous layer of the decidua. D" ', Compact layer of the decidua. mc, Muscularis.

The decidual cells of the cavernous layer (Fig. 194, D') are smaller and more crowded than most of those of the compact layer. The largest cells are scattered through the compact layer, but are most numerous toward the surface. They extend around the margin of the placenta and have penetrated the chorion, in the cellular layer of which they are very numerous; the immigration imparts to the chorionic layer in question somewhat the appearance of a decidual membrane. Misled by this peculiarity, some authors have held this layer to be maternal in origin, and accordingly have described it as a " decidua subchorialis." The decidual cells exhibit great variety in their features (Fig. 195). They are nearly all oval discs, so that their outlines differ according as they are seen lying in the tissue turned one way or another; they vary greatly in size; the larger they are, the more nuclei they contain; the nuclei are usually more or less elongated; the contents of the cell granular. Some of the cells present another type, c; these are more nearly round, are clear and transparent; the nucleus is round, stains lightly, and contains relatively few and small chromatin granules; such cells are most numerous about the placental margin.

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Fig 195. Decidual Cells from the Section Represented in Fig. 194. Multinucleate cell; at u seven blood-corpuscles have been drawn in to scale as a measure of size.

The Human Placenta

Specimens of the fresh normal human placenta can be obtained without difficulty from maternity hospitals. The specimen should be thoroughly examined in the fresh state by the student and all the points in the description below verified by him. T*o make an injected specimen either the starch injection mass or the colored gelatin mass may be used accordingly as it is desired to demonstrate only the coarser or all the branches of the vessels. The injection should be made through one of the arteries of the umbilical cord. As there is almost invariably a cross-anastomosis between the two arteries close to the placenta, it is sufficient to inject one of them in order to fill the entire system of vessels. The starch mass may be injected in the cold specimen. If the gelatin mass is used, the specimen must be submerged in warm water until it is sufficiently heated to keep the gelatin mass melted during the process of injection. After the gelatin injection is completed, the placenta may be preserved in 70 per cent, alcohol, to every 100 c.c. of which 2 c.c. of hydrochloric acid have been added. After twenty-four hours the acidulated alcohol may be replaced by fresh alcohol of 70 per cent., which should be again changed after another twentyfour hours. Specimens will then keep indefinitely. Such specimens may be used either for sections of the placenta to be made from pieces imbedded in celloidin, or for the study of isolated fragments of the villi, which are pulled out of the placenta by forceps.

The human placenta is a disc of tissue to which the umbilical cord of the child is attached by its distal end. As a result of normal labor the amnion and chorion, by which the foetus in utero is surrounded, are ruptured; the child is then expelled, but by means of the long umbilical cord remains attached to the uterus ; after an interval the placenta, with which the cord retains its connection, is loosened from the uterine wall and expelled, together with the foetal envelopes and portions of the decidual membranes (uterine mucosa) of the mother; the parts thus thrown off secondarily constitute the so-called, after-birth of obstetricians.

The placenta at full term, as thus obtained by natural expulsion, is a moist mass, containing a great deal of blood, spongy in texture, about seven inches in diameter, but very variable in size, being roughly proportionate to the bulk of the child ; usually oval, sometimes round, but not infrequently irregular in shape. One surface is smooth and covered by a pellucid membrane (the amnion), and reddish-gray in color; to this surface the umbilical cord is attached, and it shows the arteries and veins branching out irregularly from the cord over the surface of the placenta (Fig. 196). The opposite surface is rough, lacerated, and usually covered irregularly with more or less blood, which is often dark and clotted. When the blood is removed, the surface is seen to be crossed by a system of grooves which divide the placental tissue into irregular areas, each perhaps an inch or so in diameter; these areas are called cotyledons. The placenta is about 25 or 30 mm. thick, but thins out rapidly at the edges, and its tissue passes over from the margin of the placenta.

When in situ, the placenta is fastened to the walls of the uterus by its rough or cotyledonary surface; its smooth, amniotic surface faces the cavity in which the foetus lies.

A more detailed examination of the gross appearance of a placenta discharged at term leads to the following additional observations : The color is a reddish or purplish gray, varying in tint according to the condition of the blood, and mottled between the divaricating blood-vessels by patches and networks of pale yellowish or flesh color. The light pattern is produced by the tissue of the villi shining through the membrane of the chorion. These appearances are less distinct when the placenta, as is usually the case, is covered by the thin amnion.

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Fig. 196. Human Placenta at Full Term, Doubly Injected to show the Superficial Distribution of the Blood-vessels. The veins are drawn dark and lie deeper than the arteries. One-half natural size.

The amnion, however, is very easily detached as far as the insertion of the umbilical cord, to the end of which it is firmly attached, but it cannot be traced farther because on the cord itself there is no amnion. The blood-vessels run out in all directions from the end of the cord ; each vessel produces a ridge upon the placental surface, so that its course is readily followed. The arteries and veins are more easily distinguished after double injection, as is shown in figure 196.

The two kinds of vessels do not run together; the arteries lie near the surface, just above the veins ; the arteries fork repeatedly, until they are represented only by small branches and fine vessels; some of the small branches disappear quite suddenly by dipping down into the deeper-lying tissue in order to pass into the villi. The veins (Fig. 196) are considerably larger than the arteries; they branch in a similar manner, but some of the trunks disappear from the surface more abruptly than is the case with the arteries. There is the greatest possible variability in the vessels of the placenta; I have never seen two placentae with vessels alike. So far as I have observed, the insertion of the cord is always obviously eccentric ; the degree of eccentricity is variable and is easily seen to be related to the distribution of the vessels.

The insertion of the cord may even be entirely outside the placenta, which yet may otherwise be normally developed. Such insertions are called velamentous. The usual type is shown in figure 196. The arteries come down together from the cord ; they usually, but not always, anastomose by a short transverse vessel, which lies about half an inch above the surface of the placenta; it could not be shown in the figure. Very rarely, if ever, are there any arterial or venous anastomoses on the surface of the placenta. The arteries there spread out in a manner which may be described as roughly symmetrical. The veins partially follow the course of the arteries. When the cord is inserted near the margin, the symmetry of the placental vessels is greater, when the insertion is near the center, the symmetry is less, than in the figure.

The reverse or uterine surface of the placenta is rough and divided into numerous rounded oval or angular portions termed lobes or cotyledons, as stated above. These vary from half an inch to an inch and a half in diameter. The whole of this surface consists of a thin, soft, somewhat leathery investment by the decidual membrane, which dips down in various parts to form the grooves that separate the cotyledons from each other. This layer is a portion of the decidua serotina, which, as long as the parts are in situ, constitutes the boundary between the placenta and the muscular substance of the uterus, but which at the time of labor becomes split asunder, so that while a portion is carried off along with the placenta and constitutes its external membrane, the rest remains attached to the inner surface of the uterus. If a placenta is cut through, it is found to consist of a spongy mass containing a large quantity of blood and bounded by two membranes, each less than a millimeter thick; the upper one is the chorion, covered by the still thinner amnion, and greatly thickened where the vessels lie in it; the lower one is the decidual tissue, together with the ends of the villi imbedded in it (cf. especially page 335 and Fig. 194) ; it represents only a portion of the decidua, the other portion having remained upon the uterine wall. The spongy mass is found upon examination to consist of an immense number of tufts of fine rods of tissue, which are irregularly cylindrical in shape. Further examination shows that they are twigs (Fig. 204), with rounded ends and springing from branclilets which in their turn arise from branches, and so on until a large main stem is found, which starts from the chorion. This branching system is richly supplied with blood from the fcetal vessels on the surface of the placenta. The villi are interwoven so that the twigs of one branch are interlaced with those of another, and apparently separate twigs may grow together and their vessels anastomose; but on this point we are unable to speak positively. The villous twigs next the surface of the decidua penetrate that tissue a slight distance.

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Fig. 197. Human Placenta after Delivery at Full Term.' A, Vertical section through the margin : D, decidua ; </, aborted villi outside the placenta ; Clio, chorion ; Si, sinus; li, placental villi ; Fit, fibrin. B, Portion of A more highly magnified to show the decidual tissue near b : ->, blood-vessel; d, decidual cell with one nucleus; r/', decidual cell with several nuclei.

The intervillous spaces are filled, or nearly so, with blood; they form a complex system of channels. The intervillous blood is maternal. Farre says, in his article in Todd's "Cyclopaedia" (V. Suppl., page 716), in reference to the placental decidua: "Numerous valve-like apertures are observed upon all parts of the surface; they are the orifices of the veins, which have been torn off from the uterus. A probe passed into any one of these, after taking an oblique direction, enters at once into the placental substance. Small arteries about half an inch in length are also everywhere observed imbedded in this layer. After making several sharp spiral turns they likewise suddenly open into the placenta"; and on page 719 he adds: " These venous orifices occupy three situations. The first and most numerous are scattered over the inner side of the general layer of decidua which constitutes the upper boundary of the placenta; the second form openings upon the sides of the decidual prolongations or dissepiments which separate the lobes (cotyledons) from each other; while the third lead directly into the interrupted channel in the margin, termed the circular sinus." The circular sinus (Fig. igj,Si) is merely a space at the edge of the placenta which is left comparatively free from the villi. It is not a continuous channel, but is interrupted here and there. Subsequent writers have gone but little beyond Farre's account, which has been entirely overlooked by most recent investigators, who, accordingly, have announced as new discoveries many" facts known to Farre. Under these circumstances it is interesting to direct renewed attention to Farre's masterly article.

Histology of the Human Chorion

The chorion may be preserved in Zenker's or Tellyesnicky's fluid or in Kleinenberg's picro-sulphuric acid. Pieces maybe stained- in toto with alum cochineal or borax carmine and vertical sections cut in paraffin. The sections may be advantageously counterstained with eosin or orange G.

For the general history of the chorion see page 78. As it is formed by the somatopleure, it comprises an outer ectoderm and an inner mesoderm, which latter comprises mesenchyma and mesothelium.

The ectoderm undergoes a very precocious growth producing a very large number of cells, which form the thick trophoblastic layer as described on page 342. Then follows the stage in which, by degeneration, spaces are produced in the trophoblast into which the blood of the mother enters and circulates ; and at the same time prolongations of the chorionic mesoderm extend into the trophoblast. The ectodermal cells arrange themselves as a covering for these mesodermic outgrowths and so complete a villus. The trophoblast between the developing villi entirely disappears. The ectoderm, which covers both the villi and the chorionic membrane proper, consists of two layers, an inner cellular and an outer syncytial layer. Much of the trophoblast may still remain for awhile around and beyond the tips of the villi, but it disappears rapidly, probably during the third week, so that the villi alone are left. The two-layered stage of the ectoderm is only partially preserved during the later development. Many parts of it become thinned out so as to contain only one layer of cells, while other parts thicken and degenerate. These changes may be studied in sections of older placentas (see Fig. 190).

The mesoderm of the chorion consists at first of mesenchymal cells with a homogeneous matrix and a layer of mesothelium. In later stages the mesenchymal tissue becomes partly fibrillar, and it is doubtful whether the mesothelium persists or not. During the third week we find the chorion vascular. Around the larger blood-vessels the mesoderm forms a more or less distinct coat in which the cells are somewhat more crowded together in laminae. After the perivascular coats have acquired a certain thickness the cells of their inner portions become more elongated, more regularly spindle-shaped, and more closely packed than those of the outer layer. The transition from the denser to the looser tissue is gradual. We are perhaps entitled to calk the denser, inner layer the media, and the outer, looser layer the adventitia, although neither of the layers has by any means the full histological differentiation characteristic of the like-named layers of the blood-vessels of the adult. The histogenetic changes in the chorionic frondosum go further than in the chorion laeve, which may be said to be, as it were, arrested in its development.

The Chorion with Trophoblast

When the chorionic vesicle has an internal diameter of from 3 to 6 or 7 mm., it will be found to exhibit well-developed trophoblastic layers. Such a vesicle may be hardened in Zenker's fluid, or, better, in Flemming's or Hermann's fluid, as these produce at the same time a differential color (Fig. 198). The chorionic membrane is quite thin, and consists chiefly of mesoderm, mes, with a covering of ectoderm, Ec, consisting of two layers of cells. The mesoderm extends down to form the core of the villi shown. These villi are much branched and are also covered by a layer of ectoderm. At the denser ends of the villi the ectoderm is very much thickened, forming a great mass of cells, so that the ectoderm connected with one villus is fused with that of adjacent villi, the whole constituting a large irregular mass of cells, Tro. This is the trophoblast. In many places it has already disappeared, so that there are spaces, lac, in the trophoblastic mass. On the edges of these spaces the trophoblast is undergoing degeneration, deg, and where that is occurring it is marked in the figure by the deeper staining of the degenerated material. Upon examination with a higher power (Fig. 199) it will be noted that the mesodermic cells are stained much more deeply than the matrix. They have an elongated form and run in various directions, more or less parallel to the epithelium, Ec' . Many of them are cut transversely or obliquely. Aside from the trophoblast, the ectoderm is everywhere two-layered. The inner layer is distinctly cellular, the outlines of the cells being very sharply marked and the nuclei being relatively large. In the outer layer, which is stained more darkly, there are no cell boundaries to be recognized, the structure being syncytial. The nuclei are smaller and more deeply stained than those of the inner layer. In the trophoblast we find great masses of cells somewhat similar to those of the cellular layer upon the chorionic membrane and over the surface of the villi, but they are larger and more lightly stained. They lie closely packed together; their nuclei are rounded in form, but vary considerably in size and shape. Many of them contain one or two distinct spots, which, however, are sometimes absent. On the edges of the spaces which have been formed, and sometimes apparently in the interior of the mass of trophoblast, we find bands and lines of degenerative material in which we can find nuclei, but no distinct cell boundaries. The substance between the nuclei is more or less uniformly granular in texture and stains quite deeply. The nuclei of the degenerative material vary extremely in appearance. In some cases they are small and stain rather deeply, and are then found to be present in more or less considerable numbers. Occasionally, however, the nuclei are much larger, and more rarely one sees a nucleus of exceptionally great diameter.

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Fig. 198. Section of a Very Young Human Chorion. dcg, Degenerating ectoderm. Ec, Epithelial ectoderm, lac, Lacuna for maternal blood. Mas, Mesoderm. Tro, Trophoblast. Vi, Villi. X 5° diams.

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Fig. 199. Portion of the Preceding Figure More Highly Magnified.

deg. Degenerating ectoderm. Ec', Outer syncytial layer of ectoderm. E<", Inner cellular layer of ectoderm. mis, Mesoderm of villus. Tro, Trophoblast. V '5° diams.

Our knowledge of the human trophoblast being still very imperfect, its full history is still partly a matter of supposition. The appearances described indicate that the trophoblast undergoes a rapid degeneration, during which the cells fuse, while their protoplasm becomes a hyaline material. We must then further suppose that the degenerated substance is resorbed and disappears altogether. Finally, we must assume that the entire trophoblast does not disappear, but that enough is preserved to form the permanent covering of the villi.

It may be noted that the specimen on which the above description is based agrees essentially with the specimen described by Siegenbeek van Heukelom, which is regarded as normal.

The Chorionic Villi

The villi may be obtained in connection with the preparations of the uterus and placenta. In order to see the youngest stages of the first villi it is necessary to have the chorionic membrane of the second or early part of the third week. At this stage the trophoblast is present and the first villi are appearing (compare page 342). To study the growth and form of the villi, single villi or pieces of villi should be snipped off from the chorion at various stages. Such pieces may be examined as opaque objects in alcohol, or they may be stained and mounted as permanent preparations. To obtain injected villi it rs best to inject the placenta through one of the arteries of the umbilical cord, using as the injecting mass, gelatin colored with carmine or Prussian blue. Such injections are very easily made.

Branching of the Villi

The formation of a branch is usually initiated by an outgrowth of the ectoderm. Branches grow very rapidly; the outgrowth which forms the branch occurs with every degree of participation of the mesoderm. The two extremes are first the bud, consisting wholly of epithelium, which may become a process with a long, thin pedicle and a thickened free end remaining sometimes entirely without mesoderm. Later the mesoderm penetrates it and completes the structure. Second, a thick bud with a well-developed cord of connective tissue and having a nearly cylindrical form. Between these extremes every intermediate stage can be found. The tips of the branches are for the most part free, but some of them come in contact with the surfaces of the decidua and penetrate it for a short distance. By this means the villi of the embryo are attached to the decidua of the mother. The villi do not penetrate the glands of the uterus at any period, as was at one time supposed. The ectoderm on the tip of the villi, where it is in contact with decidual tissue, undergoes a hyaline degeneration.

The shape of the villi varies according to the part of the chorion and the age of the embryo. Over the chorion laeve there is first an arrest of development and a subsequent slow degeneration of the tissues, which lose all recognizable organization of their protoplasm, and to a large extent of their nuclei also. At the same time they alter their shape (Fig. 200), becoming more and more filamentous. By the fourth month only a few tapering threads with very few branches remain. The villi disappear almost completely from the chorion laeve, except near the edge of the placenta. The villi of the chorion jrondosum or placental region, on the contrary, make an enormous growth. At first they are short, thick-set bodies of irregular shape, as shown in figure 201. At twelve weeks their form is extremely characteristic (Fig. 202). The main stem gives off numerous branches at more or less acute angles, and these again other branches, until at last the terminal twigs are reached. The branches are extremely irregular and variable, though in general club-shaped and constricted at the base. The branches may be bigger than the trunk which bears them, or of any less size. In older stages there is a progressive change. During the fifth month we find the irregularity of shape, though still very marked, decidedly less exaggerated (Fig. 203). The branches tend to come off at more nearly right angles. One finds very numerous free ends, as of course only a small portion of the branches touch

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Fig. 200. Aborting Villus from the Human

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Fig. 201. Fragment of the Chorion of Fig. 69, Highly Magnified. Ec, Ectoderm. Mts, Mesoderm. 1'i, Villus formed wholly by ectoderm.

Chorion Laeve of the Second Month

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Fig. 202. Isolated Terminal Branch of a Villus from a Human Chorion of Twelve Weeks.

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Fig. 203. Villous Stem from a Human Placenta of the fifth month. X 9 diams.

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Fig. 204. Terminal Branches of a Villus from a Human Placenta at Full Term.

The little spots indicate proliferation islands of the covering epithelium. Magnified.

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Fig. 205. Portion of an Injected Villus from a Placenta of about Five Months. X 2I ° diams.

the decidual surface. The branches, too, are less out of proportion to the stems, less constricted at their bases, less awkward in form. The gradual changes continue until at full term, as shown by figure 204, the branches are long, slender, and less closely set as well as less subdivided than at early stages. They have nodular projections like branches arrested at the beginning of their development. There are numerous spots upon the surfaces of the villi. Microscopic examination shows that these spots are proliferation islands, as we may call them, or little thickenings of the ectoderm with crowded nuclei. Not all the villi, however, have changed to the slender form, for some still preserve the earlier, clumsier shapes. In sections of placentas of different ages the villi offer characteristic differences; for the younger the stage, the fewer the total number of branches and the larger their average size. The older the placenta, the more numerous and smaller are the branches as they appear in sections (Fig. 190).

Injected Villi

The arteries and veins of the chorionic membrane enter the villi. After a short course in the main stalk of a villus, the vessels give rise to many branchlets, and gradually the character of the circulation changes, until in the smallest villous twigs there are capillaries only (Fig- 205). The capillaries are remarkable for their large size, and on this account have been interpreted as arteries and veins by some of the older writers. Their caliber is often sufficient for the passage of from two to six blood-corpuscles abreast. They are very variable in diameter, and also peculiar in exhibiting sudden constrictions and dilatations. In the short knob-like branches there is often only a single capillary loop, but as the branch becomes larger the number of loops increases and they form anastomoses. In branches large enough to serve as a stem, some one or two of the vessels may be enlarged. In the branches large enough to admit of it, there are two (or sometimes only one) longitudinal central vessels, the artery and vein of a superficial network of capillaries (Fig. 206). The formation of loops and the large size of the capillaries are not especially characteristic of the villi, but of the foetal blood-vessels in general.

The histology of the villi is described in the section on the placenta in situ, page 333.

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Fig. 206. Portion ok a Small Injected Villous Stem from a Placenta of about Five Months. X 105 diams.

The Structure of the Amnion

The structure of the amnion may be studied in sections, such as will be obtained by the student in connection with the sections of the chicken and pig embryos. These preparations will show the early stages. When the amnion is first formed, it consists of two layers of cells, both very thin, and with somewhat widely separated nuclei in each layer. Sometimes the nuclei lie in small groups. Between the two layers is a distinct space. The layer facing the embryo is a continuation of the embryonic ectoderm, and is more regular and better defined than the second or mesodermal layer, which is more or less irregular and sends at intervals protoplasmic processes across the space between the two layers which attach themselves to the ectoderm.

Human Amnion at Two Months

A section is shown in figure 207. The ectoderm, Ec, is still very thin, but where the nuclei are placed the layer is a little thicker. The mesoderm, on the other hand, has become quite thick, and is readily seen to be separated into two parts, a thin mesothelial layer, Msth, covering the surface of the amnion toward the chorion, and a mesenchymal layer, Mes, which makes up the greater part of the membrane. Traces of fibrillar structure in the mesenchyma are observable. No blood-vessels, lymphatics, or nerves have been found.

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Fig. 207. — Transverse Section of a Human Amnion of Two Months. F.c, Ectoderm. Mes, Mesenchymal mesoderm. Msth, Mesothelium. X 2 5° diams.

Human Amnion after the Fifth Month. — This should be studied both in sections and in surface views of the whole membrane, small pieces being mounted with the ectodermal side up. The preparation may be stained with alum hematoxylin and eosin. Sections show that the ectoderm (Fig. 208, ect) has grown somewhat in thickness. Usually the cells are cuboidal (Fig. 208, A), each with a rounded top in which is situated the more or less nearly spherical nucleus. Sometimes, however, the nuclei lie deeper down. Less frequently the epithelium is thin (Fig. 208, B), and its nuclei, which are transversely elongated, lie further apart. As regards the mesoderm, it will be noticed that there is usuallv, perhaps always, a layer of nearly homogeneous basal substance or matrix immediately underneath the ectoderm and remarkable for containing no cells. Sometimes the remaining portion of the mesoderm is broken up so as to offer a fibrillar structure (Fig. 208, A), and when that is the ease we can no longer make out a distinct mesothelial layer. At other times the more or less homogeneous matrix can be seen through the whole thickness of the amnion (Fig. 208, B), and when this is the case the mesothelium, a, can be readily identified.

In surface views the amniotic ectoderm is seen to consist of more or less regularly distributed nuclei with cell-bodies connecting with one another by intercellular bridges of protoplasm (Fig. 209). The nuclei, nu, are relatively large, rounded, and with distinct outlines. They have a more or less well-marked internuclear network with thickened nodes and a small number of deeply stained granules which are probably chromatin. Each nucleus is surrounded by a cellbody, pi, and the adjacent cell-bodies are separated from one another by clear spaces which are crossed by threads of material, pr, stretching as bridges between the neighboring cells. The protoplasm is vacuolated. The whole picture thus leads to the view that the epithelium is a sponge-work of protoplasm somewhat condensed around each nucleus. As regards the mesoderm, it is very difficult to obtain clear pictures of the cells, though the nuclei can be readily observed. They vary greatly in appearance, being sometimes fairly regular and uniform, though always far less so than the nuclei of the mesenehyma of the embryo proper. In other cases (Fig. .210) the nuclei are exceedingly irregular; some are large with a distinct network, d; others are smaller and differ but slightly from the normal. Some are very irregular, b, others slightly irregular, c, and others again strangely elongated and narrow, a. Many other forms besides those represented in figure 210 may be found. It has been suggested that these varied shapes of the nuclei indicate degenerative changes, and, in fact, many of the nuclei are actually breaking down, for in some specimens every stage between a nucleus and scattered granules can be observed, for one may find nuclei with distinct membranes, without membranes, masses of granular matter stained, clusters of granules crowded together, and, finally, other clusters more or less scattered.

Fig. 208. — Two Sections of the Human Amnion.

A, From an embryo of eiyht months; 13, at term. eel, Ectoderm, mes, Mesoderm, a, Mesothelium. X 34° diams.

Fig. 209.— Surface View of the Human Amniotic Epithelium of the Fourth Month. //, Protoplasm, pr, Intercellular processes, nu, Nucleus. X I22 S diams.

Fig. 210. — Natural Group of Nuclei from the Mesoderm of the Human Amnion of the Fifth Month. (For lettering see text.) X I22 5 diams.

The Umbilical Cord

The umbilical cord may be best preserved in Zenker's or Tellyesnicky's fluid. Transverse sections may be prepared in paraffin and stained with alum hematoxylin and eosin, or with Heidenhain's iron hematoxylin and orange G; or, if it is desired to study the development of the connective-tissue fibrilla, with Mallory's triple connective-tissue stain.

A general description of the umbilical cord has been given, pages 109 to in, and there two sections (Fig. 5 1 j are represented showing the structures which appear in sections of the umbilical cord. At full term some of these structures are still present but somewhat modified (Fig. 211), while others have been partly or wholly obliterated. As contrasted with the early stages, we find that the coelom is entirely obliterated, that the yolk-stalk has usually been completely resorbed, and that only traces of the allantois can be seen, Y . The bloodvessels have grown ; there are two arteries, .4 , A', and a single vein, V. Around each of these is a well-developed muscular coat produced by differentiation of the surrounding mesenchymal cells, which have assumed an elongated form and contractile function. It will be remembered that the allantois in man is primitively a very narrow tubular diverticulum which extends originally nearly to the chorion (compare Fig. 71). As the umbilical cord lengthens the allantois fails to lengthen equally. During the second month it increases very little in diameter. After the second month it appears in sections as a small group of epithelioid cells (Fig. 212) with distinct walls, irregularly granular contents, and round nuclei ; the group may or may not show a remnant of the original central cavity. Around the cells, ent, there is a slight condensation of the connective tissue, mes, to form, as it were, an envelope.

Fig. 211. — Cross-section of a Human Umbilical Cord at Term. A, ./', Umbilical arteries much contracted. V, Umbilical vein. }', Remnant of allantois. X' 2c " ain s.

Fig. 212. — Section of the Allantois from a Human Umbilical Cord of Three Months. enl, Allantoic entoderm, mes, Mesoderm. X 34° diams.

Fig. 213. — Connective Tissue from thb Umbilical Cord of a Human Embryo of 21 mm. Stained wiiii Alum Cochineal and Eosin. x 540 diams.

11. Nucleus. /, Protoplasmic network.

Fig. 214. — Connective Tissue from the Umbilical Cord of a Human Embryo of three Months, Stained with Alum Cochineal and Eosin. X 5' 1 diams.

The mesoderm varies in appearance according to the age of the specimen. Its growth and differentiation are rapid. During the second month it consists merely of numerous cells (Fig. 213) imbedded in a clear substance. The cells form a complex network of which the filaments and meshes are extremely variable in size. The nuclei are oval, granular, and do not always have accumulations of protoplasm about them forming main cell-bodies. (Compare description of first stage of the mesenchyma, page 65.) By the end of the third month the cells have assumed nearly their definite form (Fig. 214). Their protoplasm is increased in amount and forms a large body around each nucleus. The network has become simpler and coarser, the meshes bigger, and the filaments fewer and thicker. In the matrix are numerous connective-tissue fibrillae, not yet disposed in bundles. In older cords there is an obvious increase in the number of fibrillae and they form wavy bundles. In the cord of yet older stages the matrix also contains mucin which may be stained by alum hematoxylin. In such cords so stained the blotch of color appears in the intercellular spaces.

Fig. 215. — Ectoderm of an Umbilical Cord of a Human Embryo of Three Months. Ec, Ectoderm, mes. Mesoderm, r, Mesenchymal cell, a, Outer layer of ectoderm, d, Inner layer of ectoderm. X 545 diams.

The ectoderm is at first a single layer of cells, as it is also over the body of the embryo, and as it remains permanently over the amnion. At three months we find the ectoderm to be two-layered, corresponding to the second stage of the epidermis of the embryo. In still older stages there is slight increase in the number of layers of the ectoderm, but it never passes much beyond the stage of the embryonic epidermis at the fourth month. Figure 215 is from a cord at three months. The outer layer, a, of ectodermal cells is granular and stains much more darkly than the inner layer, b, in which also cell bundles are more distinct.

The Structure of the Human Yolk-sac

The human yolk-sac may be preserved in Zenker's or Tellyesnicky's fluid, stained in toto with alum cochineal, imbedded in paraffin, and cut in transverse sections. Yolk-sacs of the second month are preferable for study.

The general history of the yolk-sac is described on pages 85 and 89. It becomes a pear-shaped vesicle which in man attains its maximum diameter about the end of the fourth week. It then measures from 7 to 1 1 mm. From its pointed end runs the long stalk by which it is connected with the intestine. In very early stages the stalk is hollow and its cavity is lined by entoderm. But this condition is soon obliterated, the stalk becoming solid and the entoderm disappearing. In this condition we found the yolk-stalk in an embryo of 2 1 mm. (Fig. 51, A). The sac itself remains hollow (Fig. 216). It has a lining of entodermal cells, En, and a thicker layer of mesoderm, mes, containing blood-vessels, v. The network of the vessels imparts a characteristic appearance to the external or mesodermic surface of the yolk-sac. In the earliest stages observed the entoderm consisted of a single layer of cuboidal cells.

Transverse Section of a Yolk-sac of about Two Months. — The contents of the fresh yolksac are fluid, but coagulate when the organ is hardened. In the coagulum are found some stained bodies which are supposed to be yolk material. The entoderm has undergone proliferation and thickening. These cells are more or less irregular and disposed in two or three layers. Many of the superficial cells are stained deeply and have small nuclei, while the deeper lying cells are larger, more lightly stained, and have larger nuclei and more distinct cell boundaries. The mesoderm consists chiefly of somewhat crowded mesenchymal cells, the nuclei of which are smaller than the entodermal cells, and a well-marked layer of mesothelium, which forms the external covering of the yolk-sac. In the mesoderm appear relatively large blood-vessels, which are usually found filled with blood-corpuscles. The blood-vessels have distinct endothelial walls and lie in the part of the mesoderm toward the mesothelium, so that they are separated somewhat from the entoderm and seem often to lie immediately underneath the mesothelium. They are so large that each vessel causes a protuberance upon the yolk-sac.

Fig. 216. — Section of the Yolk-sac of a Very Young Human Embryo.

En, Entoderm. mes, Mesoderm. a, Blood-vessel. — {After Fr. A'eibel.)

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