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appears to exist in the human subject till the embryo is three weeks old and about 2.5 mm. long. During the fourth week the vitello-intestinal duct is elongated into a relatively long narrow tube, which is lodged in the umbilical cord and the yolk-sac, which has become a relatively small vesicle, is placed between the outer surface of the amnion and the inner surface of the chorion, in the region of the placenta (Fig. 62). During the latter part of the fourth or the early part of the fifth week, when the embryo has attained a length of about 5 mm., the vitellointestinal duct separates from the intestine and commences to undergo atrophy, but remnants of it may be found in the umbilical cord up to the third month.
The yolk-sac itself persists until birth, when it is, relatively, a very minute object which lies either between the amnion and the placenta or between the amnion and the chorion læve.
At a very early period, before the paraxial mesoderm has commenced to divide into mesodermal somites, a number of arteries, the primitive vitelline arteries, are distributed to the yolk-sac from the primitive arterial trunks of the embryo, the primitive aortæ, and the blood is returned from the yolk-sac to the embryo by a pair of vitelline veins (Fig. 81).
After a time the arteries are reduced to a single pair, and after the two primitive dorsal aortæ have fused into a single trunk, the pair of vitelline arteries also becomes converted into a single trunk, which passes through the umbilical orifice along the vitello-intestinal duct to the yolk-sac (Fig. 83).
The vitelline veins also pass through the umbilical orifice on their way to the heart of the embryo, and they become connected together, in the interior of the body of the embryo, by transverse anastomoses, which are described in the account of the development of the vascular system.
After the umbilical cord is formed, the extra-embryonic parts of the vitelline veins disappear, and can no longer be traced in the cord. The same fate overtakes the extra-embryonic and a portion of the intra-embryonic part of the vitelline artery, and the remainder of the artery persists as the superior mesenteric.
The placenta is an organ developed for the purpose of providing first the embryo and later the foetus with food and oxygen, and for removing the effete products produced by the metabolic processes which take place in the growing organism. It is formed partly from the zygote and partly from the mucous membrane of the uterus of the mother.
In the placenta the blood-vessels of the embryo of the earlier stages and the foetus of the later stages and the blood of the mother are brought into close relationship with one another, so that free interchanges may readily take place between the two blood streams; and the modifications and transformations of the uterine mucous membrane and the chorion of the zygote, by which this intimate relationship is attained, constitute the phenomena of the development of the placenta.
The details of the development of the human zygote for the first ten or twelve days after the fertilisation of the ovum are not known, but the knowledge of what happens in other mammals justifies the belief that during that time the zygote is formed, in the ovarian, or the middle part of the uterine tube, by the union of a spermatozoon with the mature ovum. During the first ten to fourteen days after its formation it passes along the uterine tube, towards the uterus, whilst, at the same time, it undergoes the divisions which convert it into a morula.
The Formation of the Placenta.-Before the zygote reaches the uterus the mucous membrane which lines the cavity of that organ undergoes changes, in preparation for its reception and retention, and when the changes are completed the modified mucous membrane is known as the uterine decidua.
The changes which take place are, for the most part, hypertrophic in character; the vascularity of the mucous membrane is increased, mainly by the dilatation of its capillaries; the tubular glands of the membrane are elongated, they become
tortuous, and dilatations form in their walls a short distance from their outer closed extremities. At the same time the interglandular tissue increases in amount, and as a result of the various processes the decidua is thicker, softer, more spongy, and more vascular than the mucous membrane from which it was evolved.
Partly on account of the dilatation of the deep part of the glands and partly on account of differences in texture of the internal as contrasted with the external part of the decidua, the membrane may be looked upon as consisting of three layers. (1) An internal layer, next the cavity, the stratum compactum. (2) An intermediate layer, the stratum spongiosum, formed largely by the dilated parts of the glands. (3) An external layer, the unchanged layer, in which lie the comparatively unaltered outer ends of the glands.
When the zygote, in the morula stage, reaches the uterus, from the tenth to the fourteenth day, it acts as a parasite, it eats its way through the epithelium on the surface of the decidua, and implants itself in the stratum compactum.
The zygote may penetrate the decidua at any point of the wall of the uterine cavity, but it usually enters at some point of the dorsal or the ventral wall. The entrance generally takes place between the mouths of adjacent glands, which are pushed aside, and the zygote becomes at once surrounded by the interglandular tissue of the stratum compactum of the decidua. The aperture through which it passes may be closed by a fibrinous plug or its margins may converge rapidly and fuse together.
The portion of the decidua in which the zygote is embedded is thicker than the other parts of the membrane, and it is separated by the zygote into an internal part, the decidua capsularis, and an external
Inner mass of cells
FIG. 74. SCHEMA OF A FRONTAL SECTION OF THE UTERUS, showing
part, the decidua basalis. the various parts of the decidua and a zygote embedded in the The junction of the decidua
capsularis with the decidua
basalis is the decidua marginalis, and the remainder of the decidua, by far the larger portion, is the decidua vera.
As soon as the zygote becomes embedded in the decidua its trophoblast undergoes rapid proliferation. The superficial part of the growing trophoblast becomes converted into a mass of nucleated protoplasm, the plasmodial or syncytial layer, but the inner part remains more or less distinctly cellular.
The plasmodial portion of the trophoblast invades and destroys the surrounding maternal tissue, and at the same time spaces appear in its substance. As the plasmodium destroys the walls of the dilated maternal blood-vessels, channels are made through which the maternal blood flows into the spaces in the plasmodium, and thus maternal blood begins to circulate in the trophoblast of the zygote.
In the meantime the extra-embryonic coelom has appeared in the primary mesoderm of the zygote, and the outer layer of the mesoderm has associated itself with the trophoblast to form the chorion.
The spaces in the plasmodium enlarge rapidly after the maternal blood
begins to circulate within them and the plasmodium becomes divided into
three series of parts. (1) The parts which lie between adjacent blood spaces, the primary chorionic villi. (2) The parts which lie in contact with the mesoderm of the chorion, and which form with the mesoderm the chorion plate. (3) The parts which cover the maternal tissues and form the outer boundaries of the blood spaces, the basal layer. The blood spaces themselves are called the intervillous spaces (Figs. 76, 79).
After a time each primary villus differenti
Unchanged part of
gland Dilated part of gland Cavity of uterus
The first-formed villi are non-vascular, but by the time the secondary villi have developed the umbilical arteries have grown through the body-stalk (allantoic stalk) into the mesoderm of the chorion, and branches from them enter the mesodermal cores of the villi, which thus become vascular.
When the second
ary villi are fully developed each consists of a vascular mesodermal core continuous with the mesoderm of the chorion. The mesodermal core is covered
FIG. 75.-SCHEMA OF A SECTION OF A PREGNANT UTERUS AFTER THE FORMATION
ates into a cellular core and plasmodial periphery, and thereafter the villi are invaded by the mesoderm of the chorion and are thus converted into secondary villi (Fig. 76).
Cavity of entoderm sac Extra-embryonic
cœlom Muscular wall of uterus
Pericardium Buccopharyngeal membrane Yolk-sac
FIG. 76. SCHEMA OF A FRONTAL SECTION OF A PREGNANT UTERUS AT THE
Note extension of amnion
as contrasted with stage shown in Fig. 75.
by a layer of cellular trophoblast, Langhan's layer, which lies neshe uterine cavity, and a layer of plasmodium external to the cellular layer. The the uterus is of each villus is continuous with the chorion plate of the intervilonth, and as formed by the chorion, and the distal extremity is connected with the pus at its basal layer of the trophoblast, which forms the outer boundary of the intervillous spaces and which is fused with the maternal decidual tissue.
After a time branches are projected from the sides of the secondary villi into the intervillous spaces. In this way two sets of secondary villi are differentiated, (1) the anchoring villi (Fig. 79), which cross from the chorion to the
Temporarily herniated smallintestine
Uterine tube Unchanged part of uterine gland Dilated part of uterine gland
FIG. 77.-SCHEMA OF A SECTION OF A PREGNANT UTERUS AFTER THE FORMATION OF THE UMBILICAL CORD. Note that the expanding amnion has almost obliterated the extra-embryonic coelom which lies between it and the chorion.
basal layer of trophoblast and are attached to the latter by cell columns, which are the remains of the primary villi which have not been penetrated by the foetal mesoderm, and (2) free or absorbing villi (Fig. 76), which extend from the sides of the original secondary villi into the blood, in the intervillous spaces.
Whilst the trophoblastic invasion of the compact layer of the decidua is proceeding, not only are the interglandular elements of the decidua destroyed, but the walls of the glands also, and, as a consequence, some of the glands in the decidua basalis open for a time into the intervillous spaces, and become filled with blood which passes from the spaces into the gland cavities. In many cases, however, before the glands are destroyed their walls are converted into solid strands of cells, and thus the cavities of their more external undestroyed portions are converted into closed spaces.
In the early stages the trophoblast is differentiated in a similar manner over
chorion plate closing the intervillous spaces internally; (2) the villi; (3) the intervillous spaces; and (4) the basal layer of the trophoblast, which closes the intervillous spaces externally, and is perforated by the maternal vessels passing to and from the spaces.
The maternal portion of the completed placenta consists from within outwards of (1) the basal layer of the decidua; (2) the remains of the spongy layer of the decidua; and (3) the unchanged layer.
FIG. 80. SCHEMA OF PREGNANT UTERUS IMMEDIATELY AFTER BIRTH OF THE CHILD, showing commencing separation of the placenta. Part of the umbilical cord is shown in section and part in surface view. The blue streaks in the former part indicate the position occupied by the vitello-intestinal duct in earlier stages.
The basal layer of the decidua is the remains of the compact part of the decidua basalis of earlier stages. It is fused internally with the basal plate of the trophoblast, and is continuous externally with the spongy layer. The spongy layer consists of a series of cleft-like spaces. These spaces are the compressed remains of the earlier dilated portions of the glands of the stratum spongiosum, from which the epithelial lining has, to a great extent, disappeared. The spongy layer is continuous externally with the unchanged layer, in which lie the unaltered outer parts of the glands and the intervening interglandular tissue.