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The maternal blood-vessels pass from the muscular wall of the uterus into the submucous tissue, and thence into the placenta, where they traverse the maternal portion and the basal plate of the decidua and open into the intervillous spaces. The arteries usually open on or near the septa and the veins in the intermediate areas. In addition, however, to the constituent parts already described, the chorionic part of the placenta contains some strands of maternal tissue, and in the maternal part there are portions of trophoblast.
The parts of the decidua found in the chorionic part of the placenta are a series of fibrous strands, the remains of parts of the stratum compactum which were not destroyed by the trophoblastic invasion. They are continuous externally with fibrous strands of the maternal part of the placenta, and serve to separate the placenta into a series of lobes, from 15 to 20 in number.
The portions of trophoblast met with in the maternal part of the placenta are variable pieces of plasmodium which appear to have wandered from the general mass. They may be found in any of the strata of the maternal part, and even in the submucous tissue.
At the end of pregnancy, when intra-uterine life terminates, the fused amnion. chorion and decidua capsularis are ruptured, in the region of the internal orifice of the uterus, and the amniotic fluid is expelled through the vagina. Next the fœtus is extruded, and as soon as it is born it becomes a child. After the child is born it remains attached to the placenta by the umbilical cord (Fig. 80), which is usually ligatured in two places and then divided, between the ligatures, by a medical man or an attendant. Afterwards the placenta is expelled from the uterus.
Detachment of the placenta is probably caused by contraction of the muscular substance of the uterus, and it takes place by rupture of the strands of the spongy layer of the decidua (Fig. 80). As the detached placenta is expelled the decidua vera is torn through along the line of the spongy layer, and the fused amnion and chorion læve and the inner part of the decidua vera, which are attached to the margin of the placenta and which constitute the membranes, are expelled with it. At birth the placenta weighs about 500 grm., it has a diameter of about 16 to 20 cm., and is about 3 cm. thick. Its inner surface is covered with the amnion which fused with the chorion towards the end of the second month of pregnancy. Its outer surface is rough, it is formed by the remains of the spongy layer of the decidua, and is divided into a number of areas by a series of fissures which correspond in position with the septa by which the organ is divided into lobes.
THE PRIMITIVE VASCULAR SYSTEM AND THE
As the zygote travels along the uterine tube, from the ovarian towards the uterine end, it exists either upon the yolk granules derived from the ovum or upon substances absorbed from the fluids by which it is surrounded. After it enters the uterus it must depend, for a time, upon the same sources of nutriment, but as it penetrates the decidua it is probable that the cells of the trophoblast actually devour the cells of the decidua which they invade. This source of food is only sufficient for a short period, whilst the zygote remains relatively small, and substances absorbed by its surface cells can be transmitted easily to all parts. Whilst the period exists, however, not only are the decidual tissues utilised as a food-supply, but fluids are absorbed from them and transmitted into the interior of the zygote to fill the expanding cavities of the amnion and the cœlom.
In all probability the fluids passed into the zygote contain nutritive materials which suffice for the requirements of the embryonic and non-embryonic parts of the zygote so long as both consist of comparatively thin layers of cells, but when the embryonic area increases in thickness, and begins to be moulded into the embryo, its association with adjacent fluids becomes less intimate, and as the development of its various parts progresses, a supply of food and oxygen is required which is greater than can be provided by osmosis from the adjacent fluid media. Thus an imperative necessity arises for a method of food-supply adequate to the increasing requirements upon which the continued development and growth depend.
To meet this necessity the blood vascular system is formed. The system is essentially an irrigation system. In its earliest stages it consists of a series of vessels, the blood-vessels, all of which contain a corpuscle-laden fluid, called blood. The blood is kept circulating, in the early stages, by the rhythmical contraction of the walls of the vessels, but, after a short time, parts of the vessels are developed into a muscular organ called the heart. After the heart is established the continuance of the circulation of the blood depends upon the regular contractions of the muscular substance of its walls.
The corpuscular portions of the blood and the walls of the blood-vessels are formed from the cells of the zygote, but it is obvious, in the early stages at all events, that the fluid portion of the blood must be obtained from the mother. It is necessary, therefore, both for this purpose and for the facilitation of interchanges between the foetal and maternal blood streams, that the foetal blood-vessels should be brought into close association with the maternal blood at an early period. It is for this purpose, among others, that large spaces appear in the trophoblast; that the spaces become filled with blood from maternal vessels which have been opened up by the destructive action of the trophoblast cells; and that the spaces are afterwards invaded by the chorionis villi, which carry in their interiors branches of the blood-vessels of the embryo. As soon as the intimate relationship between the chorionic villi and the maternal blood is established fluids can readily pass from the maternal to the fœtal vessels, and there can be no doubt that both food and oxygen pass from the maternal to the foetal blood through and by the agency of the trophoblastic epithelium, whilst, at the same time, waste products of foetal metabolism pass from the foetal to the maternal blood.
The germs of the vascular system are a series of cells arranged in strands which constitute, collectively, the angioblast. They appear between the entodermal and the mesodermal layers of the wall of the yolk-sac, and, therefore, entirely outside the embryo; but it is not certain whether they are derived from the mesoderm or from the entoderm.
Origin of Blood Corpuscles. After a time the angioblast separates into two parts, (1) the peripheral cells of the strands which form the endothelial walls of the primitive blood-vessels, and (2) the central cells which become the primitive blood corpuscles or mesamoeboids (Minot).
The mesamoeboids are colourless cells with large nuclei and a relatively small amount of protoplasm; from them are formed, either by transformation or division, (1) the erythrocytes, which are coloured blood corpuscles, and (2) nucleated colourless corpuscles. The erythrocytes are nucleated cells with a homogeneous protoplasm which contains the substance, called hæmoglobin, upon which the yellowish-red colour of the cells depends, and from them are derived the fully developed red corpuscles.
The primitive erythrocytes, the ichthyoid cells of Minot, are transitory structures in mammals, but they are the permanent red blood cells of the ichthyopsida (fishes and amphibia). They are succeeded by the sauroid blood cells (Minot), which represent the permanent corpuscles of reptiles and birds, and which are distinguishable from the ichthyoid cells by their smaller size and more deeply-staining nuclei.
The sauroid blood cells are replaced by the blood plastids, which are young nonnucleated red corpuscles. According to some observers the blood plastids are sauroid cells which have lost their nuclei, whilst other investigators believe the blood plastids to be the nuclei of sauroid cells. Whatever their origin, they become converted into permanent red blood corpuscles by transformation from the spherical to a cup-shaped and later to a biconcave form.
The young red blood cells are therefore the ichthyoid cells, those progressively older are sauroid cells, blood plastids, and blood corpuscles.
The colourless, nucleated corpuscles-white blood corpuscles are much less numerous than the coloured corpuscles in the adult blood. They appear to be derived from the mesamoeboids, though it is possible that they are also formed by ordinary mesoderm cells, and as regards those formed from mesamoeboids it is not certain whether a mesamoeboid cell can by division produce both erythrocytes and white corpuscles, or whether it must produce one or the other. (See note 5, p. 79.)
The primitive mesamoeboids are formed in the wall of the yolk-sac, and there some of them produce erythrocytes; many, however, migrate into the embryo, where some of them take part in the formation of the walls of the embryonic blood-vessels, and others become enclosed in the liver, the lymph glands, and the bone marrow, where they become foci for the formation of blood corpuscles. During the first two months the primitive forms of red blood cells predominate. In the second month
Dorsal intersegmental branches
Common trunk formed
are present in a very FIG. 81.-SCHEMA OF CIRCULATION OF AN EMBRYO, 135 MM. LONG, WITH SIX distinct minority. SOMITES. (After Felix, modified.)
Formation of the Primitive Blood Vascular System of the Embryo.-The earliest stage of the formation of the heart and blood-vessels in the human subject are not known, but, judging by what occurs in other mammals, it is probable that the firstformed vessels appear in the splanchnic mesoderm before the embryonic area begins to fold. It is presumed that they are formed by angioblastic cells which have migrated into the embryonic area from the walls of the yolk-sac. From their seat of origin they extend towards the caudal end of the embryonic area, one on each side of the notochord, and from the caudal end of the embryonic region. they pass along the body-stalk into the chorion. (See note 5, p. 79.)
As the cephalic end of the embryonic area is folded, to enclose the fore-gut, the corresponding parts of the primitive arteries are bent into a C-shaped form. The ventral limb of the C, which lies in the dorsal wall of the pericardium and the ventral wall of the fore-gut, is the primitive ventral aorta. The bend of the C is the
FIG. 82.-SCHEMA OF VASCULAR SYSTEM OF AN EMBRYO, 2'6 MM. LONG, WITH first aortic arch, which FOURTEEN SOMITES. (Arteries after Felix, modified.)
passes along the lateral margin of the bucco-pharyngeal membrane. The dorsal limb of the is the cranial part of the primitive dorsal aorta. The primitive dorsal aorta passes posteriorly into the tail and gives off in the region of the tail fold the primitive umbilical artery, which runs along the body-stalk to the chorion.
The caudal parts of the primitive ventral aortæ are the rudiments of the heart. At first they lie, quite separate from each other, in the dorsal wall of the pericardium. they approach one another and fuse together to form a single tubula
heart. The more cranially situated parts of the primitive ventral aortæ remain separate and take part in the formation of ventral roots of the aortic arches. Before the single heart is formed other blood-vessels have appeared, which return blood from the chorion and the yolk-sac to the heart. These vessels are the primitive veins. Two veins pass from the chorion into the body-stalk, where
2nd aortic arches
1st aortic arches
Posterior cardinal veins
Anterior cardinal veins
Duct of Cuvier
they fuse together to form the vena umbilicalis impar. This divides, at the caudal end of the embryo, into the two lateral umbilical veins, which run to the heart, one along each lateral margin of the embryo. In an embryo, 1-3 nım. long (Eternod), in which the paraxial mesoderm had not yet commenced to segment into mesodermal somites, each lateral umbilical vein received, as it entered. the embryo, a large efferent vein from the yolk-sac. This condition, if regular, is very transitory. After a very short time the connexion of the vitelline veins with the caudal ends of the lateral umbilical veins is lost, and the blood is returned from the yolk-sac directly to the heart by two vitelline veins, one on each side, which run along the sides of the vitello-intestinal duct and receive the lateral umbilical veins close to the heart (Fig. 81).
Vena umbilicalis impar
FIG. 83. SCHEMA OF VASCULAR SYSTEM OF AN EMBRYO WITH TWENTY-
7th pair of inter
In the meantime a number of branches have been developed from both the dorsal and the ventral walls of the primitive dorsal aortæ; the former are the somatic pre-segmental and intersegmental arteries, and the latter are the primitive vitelline arteries.
1st pair of intersegmental arteries
In a human embryo which has developed six distinct mesodermal somites the vitelline arteries form a plexus on the sides of the hind-gut area of the wall of the entodermal vesicle, from which the umbilical arteries appear to spring (Felix). The plexus is represented in Fig. 81 by the bulbous dilatations. The vessels which enter this plexus arise from the ventral aspects of the primitive dorsal aortæ, some distance from their caudal ends. It is probable, however, that the caudal ends of the primitive dorsal aortæ are connected with the caudal part of the plexus at the points of origin of the umbilical arteries, though the connexions are not visible in the sections of the embryo mentioned (Fig. 81). Practically the same condition is present in an embryo 1.6 mm. long possessing fourteen distinct somites, except that the main rootlets of the umbilical artery, on each side, are situated farther caudalwards than in the younger embryo, and lie in the region of the most caudal somites (Fig. 82).
1st cephalic aortic arch 2nd cephalic aortic arch 3rd cephalic aortic arch 4th cephalic aortic arch 6th cephalic aortic arch Bulbus cordis Ventricle
FIG. 84.-DIAGRAM showing stage of five aortic arches.
Further Development of the Arterial System.-When the embryo possesses twenty-three mesodermal somites, but is still devoid of limbs, the arterial system has
advanced considerably in development. Two aortic arches, on each side, now connect the cephalic end of the heart with the primitive dorsal aorta. The umbilical artery and vitelline arteries are quite separate, and each umbilical artery springs, by a number of roots which anastomose together, from the caudal part of the corresponding dorsal aorta. The vitelline arteries are still numerous, but that which rises opposite the twelfth mesodermal somite is becoming the main artery of the yolk-sac; eventually its proximal
part is transformed into the superior mesenteric artery of the foetus.
When the embryo has attained a length of 5 mm., and is about five
weeks old, it possesses about thirty-eight
mesodermal somites, and 1st arches atrophied
2nd arches atrophied
Ventral root of 3rd arch
Ventral root of 4th and 5th arches
five aortic arches are present on each side. Commencing from the cranial end, they are the first, second, third, fourth, and sixth; the fifth arch appears subsequently between the fourth and the sixth. All five arches pass to the corresponding dorsal aorta, but the three most caudal, on each side, spring from the cranial end of the heart, which is now called the aortic trunk, whilst the two most cranial rise from a common stem which constitutes their ventral roots, and which springs, also, from the aortic trunk (Fig. 84). A little later the aortic trunk gives off only two branches on each side, (1) a stem common to the first five arches, for the fifth has now appeared, and (2) the sixth arch (Fig. 85). The fifth arch is very transitory. Whilst it is present
FIG. 85.-SCHEMA OF AORTIC ARCHES OF AN EMBRYO, 9 MM. LONG. (After
Tandler, modified.) The second and third arches have atrophied and the transitory fifth has appeared.
it runs from the
tral stem, caudal to the fourth arch, to the dorsal part of the sixth arch. It soon disappears, and no traces of it are left in the adult
The portion of the common ventral stem
which lies caudal to each of the arches is called the ven、 tral root of the
FIG. 86.-SCHEMA OF PART OF THE ARTERIAL SYSTEM OF A FŒTUS SEEN FROM THE LEFT SIDE. Parts of the first and second arches, the dorsal roots of the third arches, the dorsal part of the right sixth arch, and the dorsal roots of the right fourth and fifth arches have atrophied. The position of the fifth arch is not indicated; see Fig. 84. arch, and the parts of the primitive dorsal aorta which lie caudal to the dorsal ends of the arches are called their dorsal roots.
The first two arches, on each side, disappear, and their ventral roots become the external carotid arteries of the adult. The ventral root of the third arch becomes the common carotid, whilst the third arch and the dorsal roots of the first and second arches are transformed into the internal carotid. The ventral root of the fourth arch on the right side becomes the innominate artery, and the right fourth arch forms the proximal part of the right subclavian artery. The remainder of the