Book - Developmental Anatomy 1924-3

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Arey LB. Developmental Anatomy. (1924) W.B. Saunders Company, Philadelphia.

   Developmental Anatomy 1924: 1 The Germ Cells and Fertilization | 2 Cleavage and the Origin of the Germ Layers | 3 Implantation and Fetal Membranes | 4 Age, Body Form and Growth Changes | 5 The Digestive System | 6 The Respiratory System | 7 The Mesenteries and Coelom | 8 The Urogenital System | 9 The Vascular System | 10 The Skeletal System | 11 The Muscular System | 12 The Integumentary System | 13 The Central Nervous System | 14 The Peripheral Nervous System | 15 The Sense Organs | C16 The Study of Chick Embryos | 17 The Study of Pig Embryos | Figures Leslie Arey.jpg
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Chapter III. Implantation and Fetal Membranes

The conditions under which vertebrate eggs develop vary markedly. In all vcrtel)rates below mammals the eggs are laid and develop in the surrounding medium, aided sometimes (especially in reptiles and birds) by parental protection and incubation. As a group, the mammals alone develop their young within the genital tract of the mother.

The embryos of fishes and amphibia grow rapidly to immature forms capable of independent existence. All other vertebrates are much farther advanced at Ihrth and accordingly form various organs, of use during develoi)ment only. Especially in higher mammals has the absence of yolk, and the resulting physiological dependence upon the mother, led to the greatest elaboration of these ajopendages.

Such fetal organs include the yolk sac and stalk, the allantois, amnion, and chorion. They have to do with the nutrition and respiration of the embryo, and the elimination of katabolic wastes. In higher mammals, the chorion is associated intimately with the uterine mucosa and forms with it an important organ called the placenta.

The Fetal Membranes of Reptiles and Birds

Development is similar in both classes. The chick illustrates typically the manner of membrane formation.

Amnion and Chorion

The embryo develops in the center of the blastoderm, which first lies like a disc upon the massive yolk (Fig. 6). Later, the periphery of the blastoderm, not concerned in embryo formation, expands and encloses the yolk mass. This envelope consists of somatopleure and splanchnopleure, separated by the coelom (Fig. 4). The amnion and chorion arise from the somatopleure. This double layer (ectoderm and somatic mesoderm) is thrown up into crescentic folds, just in front of and behind the embryo (Fig. 36 A). Gradually, the hood-like folds close in from all sides until they meet and fuse over the embryo ( Fig. 36 B-C ) . The inner somatopleuric layer, thus formed, is the amnion; it constitutes a protective sac, lined with ectoderm and soon filled with fluid, within which the embryo is suspended. The outer of the two somatopleuric sheets is the chorion. It lies next the shell and is separated by the extra-embryonic coelom from the enclosed embryo and its other membranes (Fig. 37).

Yolk Sac

As the embryo enlarges, its original connection with the extra-embryonic blastoderm becomes a slender stalk, uniting embryo and yolk (Fig. 37). It is designated the yolk stalk, whereas the yolk, enveloped by extra-embryonic blastoderm, is the yolk sac. Vitelline blood vessels ramify on the surface of the yolk sac and through them all the food material of the liquefied yolk is conveyed to the chick during the incubation period.

Fig. 36. Diagrams in a sagittal plane illustrating the development of the fetal membranes of most amniotes (after Gegenbaur in McMurrich). Ectoderm, mesoderm, and entoderm are represented by heavy, light, and dotted lines respectively. .1/., Allantois: Am., amniotic cavity; I 5 .yolk sac.

Fig. 37. Diagram of a five-day chick embryo and its membranes (Marshall). X 1.5.


There is an early outpouching of the ventral floor of the gut, near its hind end. This entodermal diverticulum pushes outward into the extra-embryonic coelom, carrying before it an investment of splanchnic mesoderm (Fig. 36). It forms a vesicle, known as the allantois, which develops rapidly into a large sac, connected to the hind-gut by the narrower allantoic stalk (Fig. 37). Finally, the allantois flattens and fuses with the chorion, just underlying the porous shell (Fig. 36 D). The blood vessels that ramify in the combined mesodermal wall are situated favorably for gaseous interchange, and the allantois becomes the embryonic respiratory organ. The allantoic cavity also serves in its primitive capacity as a reservoir for the excreta of the embryonic kidneys, and the wall assists in the absorption_of albumen.

The Fetal Membranes of Mammals

Amnion and Chorion

In most mammals these membranes arise by folding, as in reptiles and birds. Some (guinea pig; hedgehog; bat; primates) form an amnion precociously in an entirely different manner. In the bat, fluid-filled clefts appear in the interior of the embryonic cell mass; these coalesce and constitute the amnion cavity (Fig. 38). Later, a layer of somatic mesoderm envelops its ectodermal roof and the structural outcome is identical with the type derived by folding. The deer and sheep show a method transitional between these extremes; the embryonic mass hollows and its roof ruptures; then the definitive amnion develops by folding. The same group that derives an amnion by dehiscence, forms a chorion from the outer trophectoderm layer of the blastodermic vesicle, to which somatic mesoderm is added (Fig. 40).

Yolk Sac

The yolk sac of monotremes resembles that of birds, but in higher forms an actual yolk mass is lacking. There are numerous early developmental variations. In the majority, the yolk-sac entoderm spreads beneath the trophectoderm shell and for a time lines it (Fig. 38) ; when the extra-embryonic mesoderm and coelom appear, the entoderm becomes clothed with the splanchnic layer and the sac is reduced in relative size. On the contrary, the yolk sac of primates is small from the first and remains as a diminutive central vesicle (Figs. 25 and 40). In rodents, carnivores, and split-hoofed mammals it early attains a large size, but ceases growth as the allantois comes to prominence. The splanchnic mesoderm of all groups bears the vitelline blood vessels. Many animals with a highly developed yolk sac effect an intimate association (through union with the chorion) with the uterine mucosa. There is thus formed a transitory yolk-sac placenta. In some marsupials and insectivores this relation persists.


Many mammals, like reptiles and birds, form an allantois by the sacculation of gut-splanchnopleure into the extra-embryonic coelom. In some it remains small, and, especially in certain marsupials, does not come in contact with the chorion. On the contrary, in carnivores and ungulates it becomes very large and lines the chorionic sac (Fig. 39). A goat embryo of two inches has an allantois two feet long.

Fig. 38. Stages of amnion formation in the bat (Van Beneden). X about 160.

Primates have a tiny, tubular allantois; it grows into and lies within the body stalk, which is a bridge of mesoderm connecting the embryo to he chorion (Fig. 40 D). Allantoic, or umbilical blood vessels accompany he allantois.

The Placenta

The egg-laying monotremes develop under the same nutritive and respiratory conditions as do reptiles and birds. The marsupials, after a brief gestation period, give birth to immature young; their chorion, therefore, remains as a smooth membrane but in close apposition with the vascular uterine mucosa. The yolk sac is large and in some forms it unites with the chorion, apparently to serve as a nutritive path from uterus to embryo.

In all higher mammals, the chorion is beset with vascular villi and there is a more or less intimate relation, which persists throughout gestation, between the uterine mucosa and the chorionic vesicle. This arrangement results in the formation of an organ, the placenta, specialized for the nutrition of the embryo and for its respiration and excretion.

Fig. 39. Diagram of the fetal membranes and allantoic placenta of a pig embryo, in median sagittal section (adapted by Prentiss).

The form and extent of the placenta vary in accordance with the final distribution of the chorionic villi. The pig and horse have villi diffusely scattered over the entire chorion. In ruminants, they occur in broadly scattered tufts, interspaced with smooth stretches of chorion. The villi of carnivores constitute a girdle-like band about the chorionic sac. In rodents, insectivores, bats, and primates, the villi are limited to a patchlike disc (Fig. 48).

There is likewise a structural series, based on the degree of fetal maternal intimacy. At the bottom of the scale stands the mere apposition of the uterine mucosa and the avillous chorion of marsupials.

Simplest of the forms with chorionic villi is the condition illustrated by the pig or horse (Fig. 39). The allantois, developing as in the chick, comes in contact and fuses with the chorion. Allantoic vessels lie in the combined mesoderm. Meanwhile, the external ectoderm of the amnion has closely applied itself to the uterine epithelium, and, when the chorionic villi appear, they fit into corresponding pits in the mucosa. Nutritive substances and oxygen from the maternal blood must pass through both layers of epithelium before entering the allantoic vessels. In the same manner, waste products from the embryo pass in the reverse direction. The allantois has therefore become important, not only as an organ of respiration and excretion, as in reptiles and birds, but also as an organ of nutrition. Through its vessels it has taken on the function belonging to the yolk sac of lower vertebrates, and the rudimentary, yolkless sac of higher mammals is now explained.

This general scheme of the ungulates is modified by an advance among its ruminant subgroup. Here the villi penetrate deeper and come in closer relation with the connective tissue about the maternal vessels by a partial destruction of the uterine epithelium. At the end of gestation the chorionic villi of the pig, horse, and ruminant are merely withdrawn, and the maternal mucosa is not lost.

In carnivores there is marked destruction of the mucosa, so that the chorionic epithelium about the maternal vessels is separated from the circulating blood by endothelium alone.

The highest type, as in rodents and primates, is characterized by a superficial erosion and destruction of the uterine mucosa, so that the chorionic villi, dangling in cavernous spaces, are bathed by the maternal blood which issues from eroded vessels (Fig. 34). In this and the preceding type, the changes are so profound and the fusions so intimate that the mucosa is largely sloughed at birth as a decidua. The chorion was important in the ungulate chiefly as it brought the allantois into close relation with the uterine wall, but in man and most unguiculates it assumes the several placental functions, and the allantois, now superseded like the yolk sac, in turn becomes rudimentary.

The Fetal Membranes of Man


In the youngest known human embryo (Miller) the embryonic mass is solid (Fig. 40 A), but its ectoderm indicates a stage preparatory to the formation of amnion clefts. As an amniotic cavity is present in the slightly older embryo described by Bryce-Teacher (Fig. 40 B), the method of origin must be by direct splitting as in the bat (p. 46). These specimens likewise lack a coelom in the precociously formed extra-embryonic mesoderm, whereas all older embryos possess somatic and splanchnic layers bounding a more or less extensive coelomic cleft (Fig. 40 C). Somatic mesoderm then covers the primitive ectodermal roof of the amniotic cavity (Fig. 40 D) ; this order of layering is identical in all amniotes.

At first there is a broad union between the amnion and the external shell of t rophectoderm (Fig. 40 C), but this becomes reduced by the continued extension of the coelomic cavity until presently it is limited to the caudal end of the embryo alone (Fig. 40 D). This narrow, mesodermal bridge, into which the allantois and its vessels grow, is the body stalk (Fig. 43).

Fig. 40. Diagrams of early human embryos (adapted by Prentiss). A, Miller (modified); B, Bryce-Teacher (modified); C, Peters; D, Spee.

Hence, from the first, the human amniotic cavity is closed. The base of the amnion is attached to the periphery of the embryonic disc, which also constitutes the floor of the cavity (Fig. 32 A). The amnion becomes a thin, pellucid, non-vascular membrane, lined with a simple epithelium (Figs. 43, 61 and 65). The amniotic cavity enlarges rapidly at the expense of the extra-embryonic coelom, and, at the end of the second month, fills the chorionic sac (Fig. 51). It then attaches loosely to the chorionic wall, thereby obliterating the extra-embryonic body cavity (Fig. 50).

Amniotic fluid fills the sac. Its immediate origin (fetal or maternal) is disputed. During the early months of pregnancy the embryo is suspended by the umbilical cord in this fluid (Fig. 51). Throughout gestation the amniotic fluid serves as a protective water cushion, equalizing pressures and preventing adherence of the amnion. At parturition, it acts as a fluid wedge to dilate the uterine cervix. The embryo is protected from maceration by a fatty skin-secretion, the vernix caseosa.

During the early stages of childbirth the membranes usually rupture, and about a liter of amniotic fluid escapes as the - waters. - If the tough amnion fails to burst, the head is delivered enveloped in it, and it is then popularly known as the - caul. -


When the amniotic fluid is excessive in volume, the condition is designated ' hydramuios. - If less than the optimal amount is present, the amnion may adhere to the embryo and cause malformations. Fibrous bands sometimes extend across the amnion cavity. As pressure increases during growth, they may cause scars and the splitting or even amputation of parts.

Fig. 41. Section of Peters - 0.19 mm. human embryo (about fifteen days). The portion of extra-embryonic ccelom shown is limited below by a strand of the magma reticulare.

Yolk Sac

The entodermal portion of the Miller embryo is solid (Fig. 40 A), but in all other early specimens it forms a small vesicle, lined with a single layer of entoderm and covered with splanchnic mesoderm (Figs. 40 B-D, 41 and 43). In embryos of 1.5 to 2.0 mm., the entodermal roof of this vesicle begins to form the fore- and hind-gut which are then connected by a slightly narrowed region to the yolk sac proper (Figs. 43 . and 44). With the growth of the head- and tail regions of the embryo there is an apparent progressive constriction of the yolk sac (Figs. 60, 61 and 64). This, however, is a deception. Both embryo and yolk sac enlarge, whereas the region of union lags in transverse development but elongates into the slender yolk stalk (Fig. 42).

The yolk stalk becomes incorporated in the umbilical cord (Figs. 45, 64 and 65). It loses its attachment with the gut in embryos of 7 mm. and soon degenerates. Even earlier, the yolk sac has attained its final diameter of about i cm. ; it persists and may be found at birth adherent to the amnion in the placental region (Fig. 55). The yolk sac of man is a vestige containing a coagulum but no yolk (Fig. 41). Blood vessels arise very early in its mesoderm (Figs. 43 and 44) and institute a vitelline circulation with the embryo.

Fig. 42. Yolk sac and Stalk of a 20 mm. human embryo (Prentiss). X u.


If that portion of the yolk stalk between the intestine and umbilicus remains pervious it constitutes a fecal fistula through which intestinal contents may escape.

In 2 per cent of all adults there is a persistence of the proximal end of the yolk stalk, to form a pouch, Meckel - s diverticulum of the ileum. This varies between 3 and 9 or more cm. in length and lies about 80 cm. above the colic valve. The divei'ticulum is important surgically as it sometimes telescopes into the intestinal lumen and occludes it.


Although the allantois is absent in the youngest embryos known, it nevertheless appears very early - even before the gut. In the Spee specimen, the allantois is a slender tube extending into the mesoderm of the body stalk (Fig. 43). It never becomes saccular, as in most lower amniotes. Since the human allantois arises so precociously, it does not develop as an evagination of the hind-gut into the extra-embryonic coelom; yet the body stalk, which contains the allantois, represents mesoderm into which the coelom has failed to penetrate.

Elongation extends the allantoic tube as far as the chorion (Figs. 44, 71 and 184), and, when the developing umbilical cord includes the allantois as a component, it at first is as long as the cord (Figs. 45 and 51). Soon, however, growth ceases and at birth the only remnant is a tenuous, and generally discontinuous, solid strand.

Fig. 44. Mall's 2.0 mm human embryo in median sagittal section (adapted by Prentiss). X 23.

Umbilical blood vessels accompany the allantois; these also reach the chorion and vascularize it (Figs. 51 and 184). When the chorion becomes a part of the placenta it performs all the functions of nutrition, respiration, and excretion. Like the yolk sac, the allantois is a superseded rudiment.


The human chorion is derived directly from the trophectoderm layer of the blastodermic vesicle to which is added extra-embryonic mesoderm (Fig. 40). The trophectoderm of the youngest known embryos has already given rise to an outer syncytial layer, the irophodcrm, but the mesoderm is solid. In slightly older specimens, the mesoderm is cleft by the extra-embryonic cmlom and its outer, or somatic, layer lines the chorion (Figs. 40 B-I) and 46). The chorion forms villous processess (Fig. 48). At first these are solid ectoderm, the primary villi (Fig. 40 C), but soon the chorionic mesoderm invades them as central cores (Fig. 43) and allantoic, or umbilical blood vessels ramify in their branches. Such villi are secondary, or true villi (Figs. 51 and 65). The further history of the chorion is inseparable from placental development (p. 62).

Umbilical Cord

As the embryo enlarges, its ventral, unclosed area, bounded by the edge of the amnion, becomes relatively smaller (Fig. 45 A, 5 ). For a time the amnion attaches close to the embryo, but, during the sixth week, growth of the adjoining body wall, accompanied by an elongation of the body stalk, causes the amnion to recede from the umhilicus. The tubular structure thus formed is the itmhilical cord (Fig. 45 C). It encloses both yolk stalk and allantois, and includes a portion of the coelom. Henceforth, the umbilical cord connects the embryo to that part of the chorion which constitutes the fetal half of the placenta (Figs. 51 and 55). The umbilical cord is actually an embryonic growth, and the amnion merely attaches to its distal end ( Fig. 65).

The cord is covered with ectodermal epithelium and contains, embedded in mucous tissue (jelly of Wharton): (i) the yolk stalk (and in early stages its vitelline vessels); (2) the allantois; (3) the allantoic or umbilical vessels (two arteries and a single, large vein ) . The mucous tissue, peculiar to the umbilical cord, comes from mesenchyme; it bears neither capillaries nor nerves. Between the sixth and tenth weeks, the gut extends into the coelom of the cord and forms a temporary umbilical hernia there (Fig. 96). After it is withdrawn, the cavity of the cord disappears.

The mature cord is about 1 .5 cm. in diameter and attains an average length of 50 cm. Its insertion is usually near the center of the placenta (Fig. 56), but may be marginal or even on the adjoining membranes. A spiral twist appears (Fig. 53), just how is not known, and the blood vessels sometimes curl in masses which cause external bulgings, designated - false knots. - True knots are known also. The cord may wind about the neck or extremities of a fetus and induce atrophy or even amputation.

Fig. 45. Diagrams of the development of the human umbilical cord (DeLee). a.c., Amniotic cavity; exc., extra-embryonic coelom.

Implantation and Early Mucosal Relations During the events of cleavage and the formation of a morula and blastodermic vesicle, the ciliated lining of the uterine tube steadily transports the ovum downward. Early in this period of migration and development, the ovum loses its corona radiata cells and pellucid membrane. In about eight days it probably reaches the uterus, having attained a stage something like Fig. 40 A, although the vesicle is only about 0.2 mm in diameter. It is evident, therefore, that the foregoing sections of this chapter describe changes which occur largely after implantation, rather than before it.

Fig. 46. Section through a human embryo of 0.19 mm., embedded in the uterine mucosa (semidiagrammatic after Peters), am., Amniotic cavity; hS., body stalk; eel., ectoderm of embryo; ent., entoderm; ?nes., mesoderm; yS., yolk sac.

Implantation comprises the process by which the embryonic vesicle becomes embedded in the uterine mucosa. Actual observations on the human ovum are lacking, but from careful studies on the earliest specimens, and from more complete observations on other mammals, the course of events is reasonably certain.

The ovum penetrates the mucosa as would a parasite, the trophoderm supposedly producing an enzyme which digests away the maternal tissues until the embryo is entirely embedded. The Peters specimen, shown in Fig. 46, is well established and the chorionic vesicle has an internal diameter of more than a millimeter. Its point of entrance is marked by the customary fibrin clot which soon disappears, and the defect is repaired.

Continued rapid growth of the embryo necessitates a correspondingly progressive erosion of the maternal tissues. This causes extravasations of blood which collect in large vacuoles in the invading trophoderm and form blood lacunae (Fig. 46). The lacunce break up the trophoderm into solid cords, composed of both the inner cellular and outer syncytial layers. These constitute the primary villi. It is the syncytial layer that is active in the destruction of the uterine tissues, and probably also in the absorption of blood and tissue products (emhryotroph) for the early nutrition of the embryo.

Next, there are changes leading to the definitive [hemotrophic) type of nutrition. Chorionic mesoderm extends into the primary villi, and branching secondary or true villi result (Figs. 43 and 47). During the development of villi the blood lacunas in the original trophoderm shell expand, run together, and produce intervillous spaces which surround the villi and bathe their epithelium (Fig. 47). The formerly Spongy trophoderm is now reduced to a continuous layer covering the outer surfaces of the villi and chorion. Branches of the umbilical vessels develop in the mesoderm of the chorion and villi (Fig. 51). The mesodermal core of each villus and its branches is then covered by a two-layered epithelium; an inner, ectodermal layer (of Langhans) with distinctly outlined cuboidal cells, and an outer, syncytial trophoderm layer (Figs. 47 and 53 A). The epithelium also forms solid columns of cells which anchor the ends of certain villi to the uterine wall (Fig. 47),

Fig. 47. Diagram of the early development of chorionic villi and placenta (after Peters).

In the vessels of the chorionic villi, the chorionic circulation of the embryo is established. The blood vessels of the uterus open into the intervillous blood s]iaces, and here the maternal blood circulates and bathes the syncytial trophoderm of the villi (Figs. 47 and 54). The transfer of nutritive substances and oxygen to the fetal blood takes place through the walls of the chorionic villi, whereas fetal wastes pass in the reverse direction. The tro])hoderm, like endothelium, prevents the coagulation of maternal blood. According to Mall, it also forms a wall which dams or ulugs the maternal blood vessels as soon as eroded, and, with the decidua (p. 62), limits the flow of blood into the intervillous spaces.

Villi at first cover the entire surface of the chorion (Fig. 40 D). As the embryo enlarges, the villi next the uterine cavity become both compresSed and remote from the Idood supply (Fig. 51). During the fourth week these villi atrophy and disappear (Fig. 48). This leaves a smooth surface, called the chorion Laeve. The villi adjacent to the uterine wall persist as the chorion frondosum and become the fetal part of the placenta (Fig. 49),

Arey1924 fig048.jpg

Fig. 48. Human chorionic vesicles of five and seven weeks (De Lee). The chorion laeve and chorion frondosum are apparent. Natural size.

The Decidual Membranes

Two sets of important changes take place normally in the uterine mucosa. One of these is periodic, between puberty and the menopause, and is the cause of menstruation. It is comparable to the oestrus cycle in lower animals, and may also be regarded as preparatory to the second set of changes which appear only in pregnancy and give rise to the decidual membranes and placenta.


The periodic changes that accompany the phenomenon of menstruation form a cycle which occupies twenty-eight days. This period is divisible into four phases ;

  1. Tumefaction (six days). The uterine mucosa thickens both because of vascular congestion and cellular multiplication. Blood escapes from the enlarged capillaries and forms subepithelial masses. The uterine glands elongate and their deeper portions especially are convoluted and dilated with secretion. The mucosa thus shows a superficial, compact layer and a deep, spongy layer.
  2. Menstruation proper (four days). The superficial blood vessels rupture and add to the blood and glandular discharge which is escaping into the uterine cavity. The surface epithelium and a portion of the underlying tissue may or may not be desquamated.
  3. Restoration (five days). The vascular engorgement disappears. Extravasated blood corpuscles are resorbed or cast off. The epithelium, glands, and capillaries are repaired.
  4. Intermenstruum (thirteen days). An interval of rest. Since ovulation occurs most often postmenstruum, Grosser believes that the embryo reaches the uterus during the premenstrual stage. The congestion and loosening of the uterine tissue at this time would seemingly favor the implantation of the embryo, and the glandular secretion might afford nutriment for its growth until implantation occurred. The first phase of menstruation, according to this view, prepares the uterine mucosa for the reception of the embryo. If pregnancy supervenes, it soon inhibits any further premenstrual changes so that menstruation does not occur. Menstruation proper would then represent an over-ripe condition of the mucosa and the abortion of an unfertilized ovum.

The Deciduae

The intimate fusions between fetal and maternal tissues necessitate an extensive sloughing of the uterine lining at birth.

Fig. 50. Vertical section through the decidua vera of about seven months, with the attached membranes in situ (Schaper in Lewis and Stohr). X 30.

The mucosa of the pregnant uterus is, therefore, designated the decidua. Its preparation and continuance during gestation, and the long deferred loss and repair at parturition, only exaggerate the events of an ordinary menstrual cycle. The two processes show undoubted fundamental similarities.

The chorionic vesicle lies embedded in part of the uterine wall only (Fig. 49). This allows three regions to be recognized : (i) a portion not in direct contact with the ovum, the decidua vera; (2) a portion which constitutes a superficial covering or arching dome, the decidua capsularis; (3) a portion underlying the embryo and between it and the muscularis, the decidua basalis.

Decidua Vera

The premenstrual, superficial compact layer and deep spongy layer are still further emphasized in pregnancy (Fig. 50). The compact layer contains the straight but dilated segments of the uterine glands. Its surface epithelium disappears by the end of the third month. The spongy layer is characterized by the greatly enlarged and tortuous portions of the glands of pregnancy.

A prominent constituent are the decidual cells that occur chiefly in the stratum compactum (Fig. 50). They are modified stroma cells, frequently multinucleate, which become about 50^ in diameter. Athough diagnostic of pregnancy their function is in doubt. Many degenerate during the later months.

Fig. 51. Diagrammatic section through a pregnant uterus of two months (Thomson). c, Uterine tube; c', mucous plug in cervi.x; dv, decidua vera; dr, decidua capsularis; ds, decidua basalis; ch, chorion (its longer villi constitute the chorion frondosum) ; am, amnion; u, umbilical cord; al, allantois; y,y', yolk sac and stalk.

During the first two months of gestation the long axes of the glands are vertical. Later, as the decidua is stretched and compressed, owing to the growth of the fetus, the glands are broadened and shortened, and their cavities become elongated clefts parallel to each other and to the surface of the decidua (Fig. 50). Similarly, the gland cells stretch, and flatten until they resemble endothelium. The decidua vera attains a maximum thickness of about i cm., but in the latter half of pregnancy pressure causes it to thin and lose much of its early vascularity. The cervix uteri does not form a decidua; its glands secrete a mucous plug which closes the uterus until the beginning of labor (Fig. 51).

Decidua Capsularis

In the earlier stages of development, glands and lilood vessels occur in its substance and the surface epithelium is continuous with that of the decidua vera (Fig. 49). As the chorion expands, the capsularis grows thin and atrophic. During the fourth month it comes into contact with the decidua vera, with which it fuses, thereliy obliterating the uterine cavity (Figs. 51 and 55). Soon after, the capsularis degenerates and disappears. This allows the chorion Ireve to become adherent to the decidua vera (Fig. 50).

Decidua Basalis

During the first four months of pregnancy this portion of the mucosa resembles the decidua vera in structure (Fig. 49). Both compact and spongy layers are represented, although there are superficial erosions and blood extravasations caused by the activity of the chorionic trophoderm. The decidua basalis does not share in the degeneration common to the other deciduae but persists until birth as a component of the nutritional organ termed the placenta (Figs. 52 and 55). The decidua is said to help in preventing excessive hemorrhage during the earlier part of pregnancy by acting as a dam between the chorionic villi and the eroded uterus (p. 58).

The Placenta

The placenta has a double origin. The chorion frondosum is the fetal portion and the decidua basalis is the maternal contribution (Fig. 49). The area of persistent frondosum villi is somewhat circular in form, so that the placenta becomes disc-shaped (Fig. 56). Near the middle of its fetal surface is attached the umbilical cord ; the surface itself is covered by glistening amnion that has fused with the subjacent chorion (Fig. 52).

The Placenta Fetalis

The villi of this portion of the chorion form profusely branched, tree-like structures which lie in the intervillous spaces (Figs. 52 and 54). The ends of some of the villi are attached to the wall of the decidua basalis and are known as anchoring villi, in contrast to the floating free villi. In the connective-tissue core of each villus are commonly two arteries and two veins (branches of the umbilical vessels), cells like lymphocytes, and special ceils of Hofbauer apparently phagocytic in function. Lymphatics are also present. The epithelium of the villi is at first composed of a layer of trophectoderm, with the outlines of its cuboidal cells sharply defined (Fig. 53 A). This layer (of Langhans ) forms and is covered by a syncytium, the trophoderm. In the later months of pregnancy, as the villi grow, the trophectoderm is used up in forming the syncytium, so that at term the trophoderm is the only continuous epithelial layer of the villi (Fig. 53 B). About the margin of the placenta the trophectoderm persists as the closing ring, which is continuous with the epithelium of the chorion Iaeve.

The Placenta Materna

This, like the decidua vera, is differentiated into a basal plate, which is the remains of the compact layer and forms the floor of the intervillous spaces, and into a deep spongy layer (Figs. 52 and 54 ) 1 he basal plate is composed of a connective-tissue stroma, containing decidual cells, canalized fibrin, and persisting portions of the epithelium of the villi. The - canalized fibrin - (Fig. 47) forms chiefly by a fibrinoid necrosis of the mucosa, but the fibrin of the maternal blood and the chorionic trophoderm also participate (Mall, 1915). Septa extend from the basal plate into the intervillous spaces but do not unite with the chorion frondosum (Grosser). Near term, these constitute the septa placentae (Fig. 54) which incompletely divide the placenta into lobules, or cotyledons (Fig. 56 B).

Fig. 53. Transverse sections of chorionic villi (Schaper in Lewis and Stohr). A, At the fourth week; B, C, at the end of pregnancy.

The maternal arteries and veins pass through the basal plate, taking a sinuous course and opening into the intervillous spaces (Fig. 54). Near their entrance they proceed obliquety and lose all but their endothelial layers. The original openings of the vessels into the intervillous spaces were formed during the implantation of the ovum when their walls were eroded by the invading trophoderm of the villi (Fig. 47). As the placenta increases in size, the vessels grow larger. The ends of the villi frequently are sucked into the veins and interfere with the placental circulation.

Arey1924 fig054.jpg

Fig. 54. Scheme of placental circulation (Kollmann). Arrows indicate the blood flow in the intervillous spaces.

At the periphery of the placenta is an enlarged intervillous space that varies in extent but never circumscribes the placenta completely. This space is the marginal sinus through which blood is carried away from the placenta by the maternal veins (Fig. 55). The blood of the mother and fetus does not mix, although the epithelial cells of the villi are instrumental in transferring nutritive substances to the blood of the fetus and in eliminating wastes from the fetal circulation into the maternal blood stream of the intervillous spaces.

Mall (1915) states that there is little evidence of an actual intervillous circulation; the decidua and trophoderm are active in preventing this (pp. 58 and 62). Some authorities hold that the intervillous circulation is peculiar to the second half of pregnancy. In summary, Mall regards the entire question as still open.

Before birth, the placenta is concave on its amniotic surface, its curvature corresponding to that of the uterus (Fig. 55). At term, the duration of which is taken as ten lunar months, the muscular contractions of the uterus, termed - pains, - bring about a dilation of the cervix uteri, the rupture of the amnion and chorion lasve, and cause the extrusion of the child. With the rupture of the membranes the amniotic liquor is expelled, but the fetal membranes remain behind, attached to the deciduae. The pains of labor begin the detachment of the decidual membranes, the plane of their separation lying in the spongy layer of the decidua basalis and decidua vera, where there are only thin-walled partitions between the enlarged glands (Figs. 50 and 52). Following the birth of the child, the tension of the umbilical cord and the - after pains - which diminish the size of the uterus normally complete the separation of the decidual membranes from the wall of the uterus. The uterine contractions serve also to diminish the size of the ruptured placental vessels and prevent extensive hemorrhage. From the persisting portions of the spongy layer and from the epithelium of the glands are regenerated' the tunica propria, glands, and epithelium of the uterine mucosa.

Arey1924 fig055.jpg

Fig. 55. Section of the uterus, illustrating the relation of an advanced fetus to the placenta and membranes (Ahlfeld).

Arey1924 fig056.jpg

Fig. 56. Mature placenta (Heisler). A, Entire fetal surface with membranes attached to its periphery; B, detail of maternal surface showing cotyledons.


The decidual membranes, and the structures attached to them when expelled, constitute the - after birth. - The placenta is disc-shaped, about 17 cm. in diameter, 2 cm. thick, and weighs 500 gm. It is usually everted so that its amniotic surface is convex, its maternal surface concave (Fig. 56). The placenta is composed of the amnion, chorion frondosum (chorionic villi with intervillous spaces divided incompletely by the septa into cotyledons), and includes on the maternal side the basal plate and a portion of the spongy layer of the decidua basalis (Fig. 52). Near the center is attached the umbilical cord, and at its margins the placenta is continuous with the decidua vera and the remains of the chorion Iseve and decidua capsularis. The amnion lines all the deciduae (Fig. 55).

Gross Changes in the Uterus

During pregnancy the uterus enlarges enormously, due chiefly to the hypertrophy of its muscle fibers, and the fundus reaches the level of the xiphoid process. After birth, it undergoes rapid involution; at the end of one week it has lost one-half its weight, and in the eighth week the return is complete. The mucosa is regenerated in two or three weeks from the remains of the spongy layer (Fig. 52).

Position of the Placenta and Its Variations

The position of the placenta is determined by the point at which the embryo is implanted. In most cases it is situated on either the dorsal or ventral wall of the uterus. Occasionally it is lateral in position, and, very rarely, it is located near the cervix and covers the internal os uteri, constituting a placenta prcBvia. A partially or wholly duplicated placenta, or accessory {succenturiate) placentas may be formed from persistent patches of villi on the chorion laeve.

Ectopic Pregnancy

If the ovum becomes implanted and develops elsewhere than in the uterus, the condition is known as an extra-uterine, or ectopic pregnancy. The commonest site is the uterine tube, tubal pregnancy. Attachment to the peritoneum, abdominal pregnancy, and the development of an unexpelled ovum within the ruptured follicle, ovarian pregnancy, are known also.

Plural Pregnancy

Twins occur once in 85 births; triplets, once in 7000; quadruplets, once in 750,000. Each member of ordinary double-ovum - twins - (p. 42) has its own amnion, chorion, and umbilical cord. The placenta and decidua capsularis are also individual, except in those cases where the original proximity of implantation leads to secondary fusions. Single-ovum, identical twins comprise only 15 per cent of the entire twin group; the chorion, placenta, and decidua capsularis are necessarily common, but the cord and usually the amnion are double.

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   Developmental Anatomy 1924: 1 The Germ Cells and Fertilization | 2 Cleavage and the Origin of the Germ Layers | 3 Implantation and Fetal Membranes | 4 Age, Body Form and Growth Changes | 5 The Digestive System | 6 The Respiratory System | 7 The Mesenteries and Coelom | 8 The Urogenital System | 9 The Vascular System | 10 The Skeletal System | 11 The Muscular System | 12 The Integumentary System | 13 The Central Nervous System | 14 The Peripheral Nervous System | 15 The Sense Organs | C16 The Study of Chick Embryos | 17 The Study of Pig Embryos | Figures Leslie Arey.jpg


Arey LB. Developmental Anatomy. (1924) W.B. Saunders Company, Philadelphia.

Cite this page: Hill, M.A. (2024, April 25) Embryology Book - Developmental Anatomy 1924-3. Retrieved from

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