McMurrich1914 Chapter 9
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McMurrich JP. The Development Of The Human Body. (1914) P. Blakiston's Son & Co., Philadelphia, Pennsylvania.
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Chapter IX The Development of the Circulatory and Lymphatic Systems
At present nothing is known as to the earliest stages of development of the circulatory system in the human embryo, but it may be supposed that they resemble in their fundamental features what has been observed in such forms as the rabbit and the chick. In both these the system originates in two separate parts, one of which, located in the embryonic mesoderm, gives rise to the heart, while the other, arising in the extra-embryonic mesoderm, forms the first blood-vessels. It will be convenient to consider these two parts separately, and the formation of the blood-vessels may be first described.
In the rabbit the extension of the mesoderm from the embryonic region, where it first appears, over the yolk-sac is a gradual process, and it is in the more peripheral portions of the layer that the bloodvessels first make their appearance. They can be distinguished before the splitting of the mesoderm has been completed, but are always developed in that portion of the layer which is most intimately associated with the yolk-sac, and consequently becomes the splanchnic layer. They belong, indeed, to the deeper portion of that layer, that nearest the endoderm of the yolk-sac, and so characteristic is their origin from this portion of the layer that it has been termed the angioblast and has been held to be derived from the endoderm independently of the mesoderm proper. The first indication of blood-vessels is the appearance in the peripheral portion of the mesoderm of cords or minute patches of spherical cells (Fig. 129, .4). These increase in size by the division and separation of the cells from one another (Fig. 129, B), a clear fluid appearing in the intervals which separate them. Soon the cells surrounding each cord arrange themselves to form an enclosing wall, and the cords, increasing in size, unite together to form a network of vessels in which float the spherical cells which may be known as mesamceboids (Minot). Viewed from the surface at this stage a portion of the vascular area of the mesoderm would have the appearance shown in Fig. 130, revealing a dense network of canals in which, at intervals, are groups of mesamaeboids adherent to the walls, constituting what have been termed the blood-islands, while in the meshes of the network unaltered mesoderm cells can be seen, forming the so-called substance-islands.
Fig. 129. - Transverse Section through the Area Vasculosa of Rabbit Embryos showing the Transformation of Mesoderm cells into the Vascular Cords.
Ec, Ectoderm; En, endoderm; Me, mesoderm. - (van der Stricht.)
At the periphery of the vascular area the vessels arrange themselves to form a sinus terminalis enclosing the entire area, and the vascularization of the splanchnic mesoderm gradually extends toward the embryo. Reaching it, the vessels penetrate the embryonic tissues and eventually come into connection with the heart, which has already differentiated and has begun to beat before the connection with the vessels is made, so that when it is made the circulation is at once established. Before, however, the vascularization reaches the embryo some of the canals begin to enlarge (Fig.
131,-4), producing arteries and veins, the rest of the network forming capillaries uniting these two sets of vessels, and, this process continuing, there are eventually differentiated a single vitelline artery and two vitelline veins (Fig. 131, B).
In the human embryo the small size of the yolk-sac permits of the extension of the vascular area over its entire surface at an early period, and this condition has already been reached in the earliest stages known and consequently no sinus terminalis such as occurs in the rabbit is visible. Otherwise the conditions are probably similar to what has been described above, the first circulation developed being associated with the yolk-sac.
It is to be noted that the capillary network of the area vasculosa consists of relatively wide anastomosing spaces whose endothelial lining rests directly upon the substance islands (Fig. 130). In certain of the embryonic organs, notably the liver, the metanephros and the heart, the network has a similar character, consisting of wide anastomosing spaces bounded by an endothelium which rests directly, or almost so, upon the parenchyma of the organ (the hepatic cylinders, the mesonephric tubules, or the cardiac muscle trabecular) (Figs. 132 and 190, B). To this form of capillary the term sinusoid has been applied (Minot), and it appears to be formed by the expansion of the wall of a previously existing blood-vessel, which thus moulds itself, as it were, over the parenchyma of the organ. The
Fig. 130. - Surface View of a Portion of the Area Vasculosa of a Chick.
The vascular network is represented by the shaded portion. Bi, Bloodisland; Si, substance-island. - (Disse.)
true capillaries, on the other hand, are more definitely tubular in form, are usually imbedded in mesenchymatous connective tissue and are developed in the same manner as the primary capillaries of the area vasculosa, by the aggregation of vasifactive cells to form cords, and the subsequent hollowing out of these. Whether these vasifactive cells are new differentiations of the embryonic mesenchyme or are budded off from the walls of existing capillaries which have grown in from extra-embryonic regions, is at present undecided. The Formation of the Blood. - The mesamceboids, which are
Fig. 131. - The Vascular Areas of Rabbit Embryos. In B the Veins are Represented by Black and the Network is Omitted. - (van Beneden and Julin.)
the first formed blood-corpuscles are all nucleated and destitute or nearly so of haemoglobin. They have been held by some observers to be the only source of the various forms of corpuscles that are found in the adult vessels, while others maintain that they give rise only to the red corpuscles, the leukocytes arising in tissues external to the blood-vessels and only secondarily making their way into them. According to this latter view the red and white corpuscles have a different origin and remain distinct throughout life.
So long as the formation of blood-vessels is taking place in the extra-embryonic mesoderm, so long are new mesamceboids being differentiated from the mesoderm. But whether the formation of blood-vessels within the embryo results from a differentiation of the embryonic mesoderm in situ, or from the actual ingrowth of vessels from the extra-embryonic regions (His), is as yet uncertain, and hence it is also uncertain whether mesamceboids are differentiated from the embryonic mesoderm or merely pass into the embryonic region from the more peripheral areas. However this may be, it is certain that they and the erythrocytes that are formed from them increase by division in the interior of the embryo, and that there are certain portions of the body in which these divisions take place most abundantly, partly, perhaps, on account of the more favorable conditions of nutrition which they present and partly because they are regions where the circulation is sluggish and permits the accumulation of erythrocytes. These regions constitute what have been termed the hematopoietic organs, and are especially noticeable in the later stages of fetal life, diminishing in number and variety about the time of birth. It must be remembered, however, that the life of individual corpuscles is comparatively short, their death and disintegration taking place continually during the entire life of the individual, so that there is a necessity for the formation of new corpuscles and for the existence of haematopoietic organs at all stages of life.
In the fetus mesamceboids in process of division may be found in the general circulation and even in the heart itself, but they are much more plentiful in places where the blood-pressure is diminished, as, for instance, in the larger capillaries of the lower limbs and in the capillaries of all the visceral organs and of the subcutaneous tissues. Certain organs, however, such as the liver, the spleen, and the bone-marrow, present especially favorable conditions for the multiplication of the blood-cells, and in these not only are the capillaries enlarged so as to afford resting-places for the corpuscles, but gaps appear in the walls of the vessels through which the blood-elements may pass and so come into intimate relations with the actual tissues is of the organs (Fig. 132). After birth the haematopoietic function of the liver ceases and that of the spleen becomes limited to the formation of white corpuscles, though the complete function may be re-established in cases of extreme anaemia. The bone-marrow, however, retains the function completely, being throughout life the seat of formation of both red and white corpuscles, the lymphatic nodes and follicles, as well as the spleen, assisting in the formation of the latter elements.
The mesamceboids early become converted into nucleated red
corpuscles or erythrocytes by the development of haemoglobin in their cytoplasm, their nuclei at the same time becoming granular. Up to a stage at which the embryo has a length of about 12 mm. these are the only form of red corpuscle in the circulation, but at this time (Minot) a new form, characterized by its smaller size and more deeply staining nucleus, makes its appearance. These erythrocytes have been termed normoblasts (Ehrlich), although they are merely transition stages leading to the formation of erythroplastids by the extrusion of their nuclei (Fig. 133). The cast-off nuclei undergo degeneration and phagocytic absorption by the leukocytes, and the masses of cytoplasm pass into the circulation, becoming more and more numerous as development proceeds, until finally they are the typical haemoglobin-containing elements in the blood and form what are properly termed the red bloodcorpuscles.
It has already (p. 224) been pointed out that discrepant views
Fig. 132. - Section of a Portion or the Liver of a Rabbit Embryo of 5 mm. e, Erythrocytes in the liver substance and in a capillary; h, hepatic cells. - (van der Stricht)
prevail as to the origin of the white blood-corpuscles. Indeed, three distinct modes of origin have been assigned to them. According to one view they have a common origin with the erythrocytes from the mesamceboids (Weidenreich), according to another they are formed from mesenchyme cells outside the cavities of the blood-vessels (Maximo w), while according to a third view the first formed leukocytes take their origin from the endodermal epithelial cells of the thymus gland (Prenant).
But whatever may be their origin in later stages the leukocytes multiply by mitosis and there is a tendency for the dividing cells to collect in the lymphoid tissues, such as
the lymph nodes, tonsils, etc., to form /||\ /^s 0$\ /^p. & more or less definite groups which - ^^ v^ KjJ \Jj
have been termed germ-centers (Flem
. m , .. . . _ Fig. 133. - Stages in the
ming). The new cells when they first transformation of an Ery
pass into the circulation have a rel- throcyte into an _ Erythro
r plastid. - (van der Stricnt.)
atively large nucleus surrounded by a
small amount of cytoplasm without granules and, since they resemble the cells found in the lymphatic vessels, are termed lymphocytes (Fig. 134, a). In the circulation, however, other forms of leukocytes also occur, which are believed to have their origin from cells with much larger nuclei and more abundant cytoplasm, which occur throughout life in the bone-marrow and have been termed myelocytes. Cells of a similar type, free in the circulation, constitute what are termed the finely granular leukocytes (neutrophile cells of Ehrlich) (Fig. 134, b), but whether these and the myelocytes are derived from lymphocytes or have an independent origin is as yet undetermined. Less abundant are the coarsely granular leukocytes (eosinophile cells of Ehrlich) (Fig. 134, c), characterized by the coarseness and staining reactions of their cytoplasmic granules and by their reniform or constricted nucleus. They are probably derivatives of the finely granular type and it has been maintained by Weidenreich that their granules have been acquired by the phagocytosis of degenerated erythrocytes. Finally, a third type is formed by the polymorphonuclear or polynuclear leukocytes (basophile cells of Ehrlich) (Fig. 134, d), which are to be regarded as leukocytes in the process of degeneration and are characterized by their irregularly lobed or fragmented nuclei, as well as by their staining peculiarities.
In the fetal haematopoietic organs and in the bone-marrow of the adult large, so-called giant-cells are found, which, although they do not enter into the general circulation, are yet associated with the development of the blood-corpuscles. These giant-cells as they
Fig. 134. - Figures of the Different Forms of White Corpuscles occurring in Human Blood.
a, Lymphocytes; b, finely granular (neutrophile) leukocyte; c, coarsely granular (eosino
phile) leukocyte; d, polymorphonuclear (basophile) leukocyte. - (Weidenreich.)
occur in the bone-marrow are of two kinds which seem to be quite distinct, although both are probably formed from leukocytes. In one kind the cytoplasm contains several nuclei, wherce they have been termed polycaryocytes, and they seem to be the cells which have already been mentioned as osteoclasts (p. 158). In the other kind (Fig- I 35) tne nucleus is single, but it is large and irregular in shape, frequently appearing as if it were producing buds. These megacaryocytes appear to be phagocytic cells, having as their function the destruction of degenerated corpuscles and of the nuclei of the erythrocytes.
The blood-platelets have recently been shown by Wright to be formed from the cytoplasm of the megacaryocytes, by the constriction and separation of portions of the slender processes to which they give rise in their amoeboid movements (Fig. 135).
Fig. 135. - Megacaryocyte from a Kitten, which has Extended two pseudopodial processes through the wall of blood-vessel and is budding off blood-platelets. bp, Blood-platelets; V, blood-vessel. - (J. H. Wright.)
The Formation of the Heart. - The heart makes its appearance while the embryo is still spread out upon the surface of the yolk-sac, and arises as two separate portions which only later come into contact in the median line. On each side of the body near the margins of the embryonic area a fold of the splanchnopleure appears, projecting into the ccelomic cavity, and within this fold a very thinwalled sac is formed, probably by a splitting off of its innermost cells (Fig. 136, .4). Each fold will produce a portion of the muscular walls (myocardium) of the heart, and each sac part of its endothelium (endocardium). As the constriction of the embryo from the yolk-sac proceeds, the two folds are gradually brought nearer together (Fig. 136, B), until they meet in the mid-ventral line, when the myocardial folds and endocardial sacs fuse together (Fig. 136, C) to form a cylindrical heart lying in the mid-ventral line of the body, in front of the anterior surface of the yolk-sac and in what will later be the
230
THE FORMATION OF THE HEART
cervical region of the body. At an early stage the various veins which have already been formed, the vitellines, umbilicals, jugulars
Fig. 136. - Diagrams Illustrating the Formation or the Heart in the
Guinea-pig. The mesoderm is represented in black and the endocardium by a broken line. am, Amnion; en, endoderm; h, heart; i, digestive tract.- - (After Strahl and Carius.)
and cardinals, unite together to open into a sac-like structure, the sinus venosus, and this opens into the posterior end of the heart cylinder. The anterior end of the cylinder tapers off to form the aortic bulb, which is continued forward on the ventral surface of the pharyngeal region and carries the blood away from the heart. The blood accordingly opens into the posterior end of the heart tube and flows out from its anterior end.
The simple cylindrical form soon changes, however, the heart tube in embryos of 2.15 mm. in length having become bent upon itself into a somewhat S-shaped curve (Fig. 137). Dorsally and to the left is the end into which the sinus venosus opens, and from this
Fig. 137. - Heart of EmbrycTof 2.15 mm., from a Reconstruction.
a, Atrium; ab, aortic bulb; d, diaphragm; dc, ductus Cuvieri; /, liver; v, ventricle; vj, jugular vein; vu, umbilical vein. - (His.)
Fig. 138. - Heart of Embryo of 4.2 mm., seen from the Dorsal Surface.
DC, Ductus Cuvieri; I A , left atrium rA, right atrium; vf, jugular vein; VI, left ventricle; vu, umbilical vein. - (His.)
the heart tube ascends somewhat and then bends so as to pass at first ventrally and then caudally and to the right, where it again bends at first dorsally and then anteriorly to pass over into the aortic bulb. The portion of the curve which lies dorsally and to the left is destined to give rise to both atria, the portion which passes from right to left represents the future left ventricle, while the succeeding portion represents the right ventricle. In later stages (Fig. 138) the left ventricular portion drops downward in front of the atrial portion, assuming a more horizontal position, while the portion which represents the right ventricle is drawn forward so as to lie in the same plane as the left.
At the same time two small out-pouchings develop from the atrial part of the heart and form the first indications of the two atria. As development progresses, these increase in size to form large pouches opening into a common atrial canal (Fig. 139) which is directly continuous with the left ventricle, and as the enlargement of the pouches continues their openings into the canal enlarge,
until finally the pouches become continuous with one another, forming a single large sac, and the atrial canal becomes reduced to a short tube which is slightly invaginated into the ventricle (Fig. 140).
In the meantime the sinus venosus, which was originally an oval sac and opened into the atrial canal, has elongated transversely until it has assumed the form of a crescent whose convexity is in contact with the walls of the atria, and its opening into the heart has verged toward the right, until it is situated entirely within the area of the right atrium. As the enlargement of the atria continues, the right horn and median portion of the crescent are gradually taken up into their walls, so that the various veins which originally opened into the sinus now open directly into the right atrium by a single opening, guarded by a projecting fold which is continued upon the roof of the atrium as a muscular ridge known as the septum spurium (Fig. 140, sp). The left horn of the crescent is not taken up into the atrial wall, but remains upon its posterior surface as an elongated sac forming the coronary sinus.
The division of the now practically single atrial cavity into the
Fig. 139. - Heart of Embryo of 5 mm., Seen from in Front and slightly from Above. - (His).
permanent right and left atria begins with the formation of a falciform ridge running dorso-ventrally across the roof of the cavity. This is the atrial septum or septum primum (Fig. 140, ss), and it rapidly increases in size and thickens upon its free margin, which reaches almost to the upper border of the short atrial canal (Fig. 142). The continuity of the two atria is thus almost dissolved, but is soon re-established by the formation in the dorsal part of the septum of an opening which soon reaches a considerable size and is known as
Fig. 140. - Inner Surface of the Heart of an Embryo of 10 mm.
al, Atrio-ventricular thickening; sp, septum spurium; ss, septum primum; sv, septum
ventriculi; ve, Eustachian valve. - (His.)
the foramen ovale (Fig. 141, fo). Close to the atrial septum, and parallel with it, a second ridge appears in the roof and ventral wall of the right atrium. This septum secundum (Fig. 141, S 2 ) is of relatively slight development in the human embryo, and its free edge, arching around the ventral edge and floor of the foramen ovale, becomes continuous with the left lip of the fold which guards the opening of the sinus venosus and with this forms the annulus of Vieussens of the adult heart.
When the absorption of the sinus venosus into the wall of the right atrium has proceeded so far that the veins communicate directly with the atrium, the vena cava superior opens into it at the upper part of the dorsal wall, the vena cava inferior more laterally, and below this is the smaller opening of the coronary sinus. The upper portion of the right lip of the fold which originally surrounded the opening of the sinus venosus, together with the septum spurium, gradually disappears; the lower portion persists, however, and forms (i) the Eustachian valve (Fig. 141, Ve), guarding the opening of the inferior cava and directing the blood entering by it toward the foramen ovale, and (2) the Thebesian valve, which guards the opening of the coronary sinus. At first no Fig. 141. - Heart of Embryo veins communicate with the left atrium, OF I0.2 CM. FROM WHICH HALF , 111 c t i 1 of the Right Auricle has but on the development of the lungs and been Removed. the establishment of their vessels, the fo, Foramen ovale; pa, pul- , . .,, monary artery; S u septum pri- pulmonary veins make connection with mum; S 2 ,_ septum secundum; j t TwQ ye j ns arise from eac h l ung an( J ba, systemic aorta; V, right ven- ° tricle; vd and vcs, inferior and as they pass toward the heart they unite superior venae cavae; Ve, Eusta- ,-, i r „ j • chfan valve. . in pairs, the two vessels so formed again uniting to form a single short trunk which opens into the upper part of the atrium (Fig. 142, Vep). As is the case with the right atrium and the sinus venosus, the expansion of the left atrium brings about the absorption of the short single trunk into its walls, and, the expansion continuing, the two vessels are also absorbed, so that eventually the four primary veins open independently into the atrium.
While the atrial septa have been developing there has appeared on the dorsal wall of the atrial canal a tubercle-like thickening of the endocardium, and a similar thickening also forms on the ventral wall. These endocardial cushions increase in size and finally unite together by their tips, forming a complete partition, dividing the atrial canal into a right and left half (Fig. 142). With the upper edge of this partition the thickened lower edge of the atrial septum unites, so that the separation of the atria would be complete were it not for the foramen ovale.
Fig. 142. - Section through a Reconstruction of the Heart of a Rabbit
Embryo of 10. i mm. Ad and Ad u Right and As, left atrium; Bw x and Bw 2 , lower ends of the ridges which divide the aortic bulb; En, endocardial cushion; En.r and En.s, thickenings of the cushion; la, interatrial and Iv, interventricular communication; S v septum primum; Sd, right and Ss, left horn of the sinus venosus; S.iv, ventricular septum; SM, opening of the sinus venosus into the atrium; Vd, right and Vs, left ventricle; Vej, jugular vein; Vep, pulmonary vein; Vvd and Vvs, right and left limbs of the valve guarding the opening of the sinus venosus. - (Born.)
While these changes have been taking place in the atrial portion of the heart, the separation of the right and left ventricles has also been progressing, and in this two distinct septa take part. From the floor of the ventricular cavity along the line of junction of the right and left portions a ridge, composed largely of muscular tissue, arises (Figs. 140 and 142), and, growing more rapidly in its dorsal than its ventral portion, it comes into contact and fuses with the dorsal part of the partition of the atrial canal. Ventrally, however, the ridge, known as the ventricular septum, fails to reach the ventral part of the partition , so that an oval foramen, situated just below the point where the aortic bulb arises, still remains between the two ventricles. This opening is finally closed by what it termed the aortic septum. This makes its appearance in the aortic bulb just at the point where the first lateral branches which give origin to the pulmonary arteries (see p. 243) arise, and is formed by the fusion of the free edges of two endocardial ridges which develop on opposite sides of the bulb. From its point of origin it gradually extends down the bulb until it reaches the ventricle, where it fuses with the free edge of the ventricular septum and so completes the separation of the two ventricles (Fig. 143). The bulb now consists of two vessels lying side by side, and owing to the position of the partition at its anterior end, one of these vessels, that which opens into the right ventricle, is continuous with the pulmonary arteries, while the other, which opens into the left ventricle, is continuous with the rest of the vessels which arise from the forward continuation of the bulb. As soon as the development of the partition is completed, two grooves, corresponding in position to the lines of attachment of the partition on the inside of the bulb, make their appearance on the outside and gradually deepen until they finally meet and divide the bulb into two separate vessels, one of which is the pulmonary aorta and the other the systemic aorta.
In the early stages of the heart's development the muscle bundles which compose the wall of the ventricle are very loosely arranged, so that the ventricle is a somewhat spongy mass of muscular tissue with a relatively small cavity. As development proceeds the bundles nearest the outer surface come closer together and form a compact layer, those on the inner surface, however, retaining their loose arrangement for a longer time (Fig. 142). The lower edge of the atrial canal becomes prolonged on the left side into one, and on the right side into two, flaps which project downward into the ventricular cavity, and an additional flap arises on each side from the lower
Fig. 143. - Diagrams of Sections through the Heart of Embryo Rabbits to Show the Mode of Division of the Ventricles and of the Atrio-ventricular Orifice.
Ao, Aorta; Ar. p, pulmonary artery; B, aortic bulb; Bw 2 and *, one of the ridges which divide the bulb; Eo, and Eu, upper and lower thickenings of the margins of the atrio-ventricular orifice; F.av.c, the original atrio-ventricular orifice; F.av.d and F.av.s, right and left atrio-ventricular orifices; Oi, interventricular communication; S.iv, ventricular septum; Vd and Vs, right and left ventricles. - (Born.)
edge of the partition of the atrial canal, so that three flaps occur in the right atrio-ventricular opening and two in the left. To the under surfaces of these flaps the loosely arranged muscular trabecular of the ventricle are attached, and muscular tissue also occurs in the flaps. This condition is transitory, however; the muscular tissue of the flaps degenerates to form a dense layer of connective tissue, and at the same time the muscular trabecular undergo a condensation. Some of them separate from the flaps, which represent the atrio-ventricular valves, and form muscle bundles which may fuse throughout their entire length with the more compact portions of the ventricular walls, or else may be attached only by their ends, forming loops; these two varieties of muscle bundles constitute the trabecule carnece of the adult heart. Other bundles
Fig. 144. - Diagrams showing the Development of the Atjriculo-ventricular Valves.
b, Muscular trabecule; cht, chordae tendinae; mk and vtk 1 , valve; pm, musculus papillaris; tc, trabeculse carneae; v, ventricle. - (From Hertwig, after Gegenbaur.)
may retain a transverse direction, passing across the ventricular cavity and forming the so-called moderator bands; while others, again, retaining their attachment to the valves, condense only at their lower ends to form the musculi papillares, their upper portions undergoing conversion into strong though slender fibrous cords, the chorda tendinece (Fig. 144).
The endocardial lining of the ventricles is at first a simple sac separated by a distinct interval from the myocardium, but when the condensation of the muscle trabecular occurs the endocardium applies itself closely to the irregular surface so formed, dipping into all the crevices between the trabeculse carneae and wrapping itself around the musculi papillares and chordae tendineae so as to form a complete lining of the inner surface of the myocardium.
The aortic and pulmonary semilunar valves make their appearance,
before the aortic bulb undergoes its longitudinal splitting, as four
tubercle-like thickenings of connective tissue situated on the inner
wall of the bulb just where it arises from the ventricle. When the
division of the bulb occurs, two of the thickenings, situated on
opposite sides, are divided, so that both the
pulmonary and systemic aorta? receive three
thickenings (Fig. 145). Later the thickenings
become hollowed out on the surfaces directed
away from the ventricles and are so converted
into the pouch-like valves of the adult.
Changes in the Heart after Birth. - The T FlG - 145- - Diagrams
/ . Illustrating the For
heart when first formed lies far forward in the mation of the Semi
neck region of the embryo, between the head £toarValves.-(G^«»and the anterior surface of the yolk-sac, and from this position it gradually recedes until it reaches its final position in the thorax. And not only does it thus change its relative position, but the direction of its axes also changes. For at an early stage the ventricles lie directly in front of (i. e., ventrad to) the atria and not below them as in the adult heart, and this primitive condition is retained until the diaphragm has reached its final position (see p. 322).
In addition to these changes in position, which are antenatal, important changes also occur in the atrial septum after birth. Throughout the entire period of fetal life the foramen ovale persists, permitting the blood returning from the placenta and entering the right atrium to pass directly across to the left atrium, thence to the left ventricle, and so out to the body through the systemic aorta (see p. 267). At birth the lungs begin to function and the placental circulation is cut off, so that the right atrium receives only venous blood and the left only arterial; a persistence of the foramen ovale beyond this period would be injurious, since it would permit of a mixture of the arterial and venous bloods, and, consequently, it closes completely soon after birth. The closure is made possible by the fact that during the growth of the heart in size the portion of the atrial septum which is between the edge of the foramen ovale and the dorsal wall of the atrium increases in width, so that the foramen is carried further and further away from the dorsal wall of the atrium and comes to be almost completely overlapped by the annulus of Vieussens (Fig. 141). This process continuing, the dorsal portion of the atrial septum finally overlaps the free edge of the annulus, and after birth the fusion of the.overlapping surfaces takes place and the foramen is completely closed.
In a large percentage (25 to 30 per cent.) of individuals the fusion of the surfaces of the septum and annulus is not complete, so that a slit-like opening persists between the two atria. This, however, does not allow of any mingling of the blood in the two cavities, since when the atria contract the pressure of the blood on both sides will force the overlapping folds together and so practically close the opening. Occasionally the growth of the dorsal portion of the septum is imperfect or is inhibited, in which case closure of the foramen ovale is impossible.
The Development of the Arterial System
It has been seen (p. 221) that the formation of the blood-vessels begins in the extraembryonic splanchnic mesoderm surrounding the yolk-sac and extends thence toward the embryo. Furthermore, it has been seen that the vessels appear as capillary networks from which definite stems are later elaborated. This seems also to be the method of formation of the vessels developed within the body of the embryo, the arterial and venous stems being first represented by a number of anastomosing capillaries, from which, by the enlargement of some and the disappearance of the others, the definite stems are formed.
The earliest known embryo that shows a blood circulation is that described by Eternod (Fig. 43). From the plexus of vessels on the yolk-sack two veins arise which unite with two other veins returniDg from the chorion by the belly-stalk and passing forward to the heart as the two umbilical veins (Fig. 146, Vu). There is as yet no vitelline vein, the chorionic circulation in the human embryo apparently taking precedence over the vitelline. From the heart a short arterial stem arises, which soon divides so as to form three
branches* passing dorsally on either side of the pharynx. The branches of each side then unite to form a paired dorsal aorta (dAr, dAs) which extends caudally and is continued into the belly-stalk and so to the chorion as the umbilical arteries (Au). There is as yet no sign of vitelline arteries passing to the yolk-sack, again an indication of the subservience of the vitelline to the chorionic circulation in the human embryo.
Fig. 146. - Diagram showing the Arrangement of the Blood-vessels in an Embryo 1.3 mm. in Length.
Au, Umbilical artery; All, allantois; Ch, chorionic villus; dAr and dAs, right and left
dorsal aortae; Vu, umbilical veins; Ys, yolk-sack. - (From Kollmann after Eternod.)
In later stages when the branchial arches have appeared the dorsally directed arteries are seen to lie in these, forming what are termed the branchial arch vessels, and later also the two dorsal
- Evans (Keibel-Mall, Human Embryology, Vol. 11, 1912) considers two of these branches to be probably plexus formations rather than definite stems, since there is evidence to indicate that only one such stem exists at such an early stage of development.
aortae fuse as far forward as the region of the eighth cervical segment to form a single trunk from which segmental branches arise.
It will be convenient to consider first the history of the vessels which pass dorsally in the branchial arches. Altogether, six of these vessels are developed, the fifth being rudimentary and transitory, and when fully formed they have an arrangement which may be understood from the diagram (Fig. 147). This arrangement represents a condition which is permanent in the lower vertebrates. In the fishes the respiration is performed by means of gills developed upon the branchial arches, and the heart is an organ which receives venous blood from the body and pumps it to the gills, in which it becomes arterialized and is then collected into the dorsal aortae, which distribute it to the body. But in terrestrial animals, with the loss of the gills and the development of the lungs as respiratory organs, the capillaries of the gills disappear and the afferent and efferent branchial vessels become continuous, the condition represented in the diagram resulting. But this condition is merely temporary in the mammalia and numerous changes occur in the arrangement of the vessels before the adult plan is realized. The first change is a disappearance of the vessel of the first arch, the ventral stem from which it arose being continued forward to form the temporal arteries, giving off near the point where the branchial vessel originally arose a branch which represents the internal maxillary artery in part, and possibly also a
Fig. 147. - Diagram Illustrating the Primary Arrangement of the Branchial Arch Vessels.
a, aorta; db, aortic bulb; ec, external carotid; ic, internal carotid; sc, subclavian; I-VI, branchial arch vessels.
second branch which represents the external maxillary (His). A little later the second branchial vessel also degenerates (Fig. 148), a branch arising from the ventral trunk near its former origin, possibly representing the future lingual artery (His), and then the portion of the dorsal trunk which intervenes between the third and fourth branchial vessels vanishes, so that the dorsal trunk anterior to the third branchial arch is cut off from its connection with the dorsal aorta and forms, together with the vessel of the third arch, the internal carotid, while the ventral trunk, anterior to the point of
Fig. 148. - Arteriat, System of an Embryo of 10 mm.
Ic, Internal carotid; P, pulmonary artery; Ve, vertebral artery; III to VI, persistent
branchial vessels. - (His.)
origin of the third vessel, becomes the external carotid, and the portion which intervenes between the third and fourth vessels becomes the common carotid (Fig. 149).
The rudimentary fifth vessel, like the first and second, disappears, but the fourth persists to form the aortic arch, there being at this stage of development two complete aortic arches. From the sixth vessel a branch arises which passes backward to the lungs, forming the pulmonary artery, and the portion of the vessel of the right side which intervenes between this and the aortic arch disappears, while the corresponding portion of the left side persists until after birth, forming the ductus arteriosus (ductus Botalli) (Fig. 149). When the longitudinal division of the aortic bulb occurs (p. 236), the septum is so arranged as to place the sixth arch in communication with the right ventricle and the remaining vessels in connection with the left ventricle, the only direct communication
between the systemic and ec pulmonary vessels being by
way of the ductus arteriosus, whose significance will be explained later (p. 267).
One other change is still necessary before the vessels acquire the arrangement which they possess during fetal life, and this consists in the disappearance of the lower portion of the right aortic arch (Fig. 149), so that the left arch alone forms the connection between the heart and the dorsal aorta. The upper part of the right aortic arch persists to form the proximal part of the right subclavian artery, the portion of the ventral trunk which unites the arch with the aortic bulb becoming the innominate artery.
From the entire length of the thoracic aorta, and in the embryo from the aortic arches, lateral branches arise corresponding to each segment and accompanying the segmental nerves. The first of these branches arises just below the point of union of the vessel of the sixth arch with the dorsal trunk and accompanies the hypoglossal nerve (Fig. 150, h), and that which accompanies the seventh
Fig. 149. - Diagram Illustrating the changes in the branchial arch vessels.
a, Aorta; da, ductus arteriosus; ec, external carotid; ic, internal carotid; pa, pulmonary artery; sc, subclavian; I- VI, aortic arch vessels.
cervical nerve arises just above the point of union of the two aortic arches (Fig. 150, s), and extends out into the limb bud, forming the subclavian artery.* Further down twelve pairs of lateral branches, arising from the thoracic portion of the aorta, represent the intercostal arteries, and still lower four pairs of lumbar arteries are formed, the fifth lumbars being represented by two large branches, the common iliacs, which seem from their size to be the continuations of the aorta rather than branches of it. The true continuation of the aorta is, however, the middle sacral artery, which represents in a degenerated form the caudal prolongation of the aorta of other mammals, and, like this, gives off lateral branches corresponding to the sacral segments.
In addition to the segmental FlG . I50 . - diagram showing the Re
lateral branches arising from nations op the Lateral Branches to
the Aortic Arches.
the aorta, Visceral branches, EC> External carotid; h, lateral branch
Which have their origin rather cacompanying the hypoglossal nerve; IC,
° internal carotid; ICo, intercostal; IM, m
from the Ventral surface, also ternal mammary; s, subclavian; v, verte
^„„,,~ TV, ~™u„mr, ~t - mm bral; I to VIII, lateral cervical branches;
OCCUr. In embryos of 5 mm. I; 2) lateral thoracic branches.
these branches are arranged in
a segmental manner in threes, a median unpaired vessel passing to the digestive tract and a pair of more lateral branches passing to the mesonephros (see p. 339) corresponding to each of the paired branches passing to the body wall (Fig. 151). As
- It must be remembered that the right subclavian of the adult is more than equivalent to the left, since it represents the fourth branchial vessel + a portion of the dorsal longitudinal trunk + the lateral segmental branch (see Fig. 142).
development proceeds the great majority of these visceral branches disappear, certain of the lateral ones persisting, however, to form the renal, internal spermatic, and hypogastric arteries of the adult, while the unpaired branches are represented only by the c celiac artery and the superior and inferior mesenteries. The superior mesenteric artery is the adult representative of the vitelline artery of the embryo and arises from the aorta by two, three or more roots, which correspond to the fifth, fourth and higher thoracic
Fig. 151. - Diagram showing the Arrangement of the Segmental Branches arising from the aorta. A, Aorta; B, lateral somatic branch; c, lateral visceral branch; D, median visceral branch; E, peritoneum.
segments. Later, all but the lowest of the roots disappear and the persisting one undergoes a downward migration in accordance with the recession of the diaphragm and viscera (see p. 322), until in embryos of 17 mm. it lies opposite the first lumbar segment. Similarly the cceliac and inferior mesenteric arteries, which when first recognizable in embryos of 9 mm. correspond with the fourth and twelfth thoracic segments respectively, also undergo a secondary downward migration, the cceliac artery in embryos of 17 mm. arising opposite the twelfth thoracic and the inferior mesenteric opposite the third lumbar segment.
The umbilical arteries of the embryo seem at first to be the direct continuations of the dorsal aortas (Fig. 146), but as development proceeds they come to arise from the aorta opposite the third lumbar segment, where they are in line with the lateral visceral segmental branches. They pass ventral to the Wolffian duct (see p. 339) and are continued out along with the allantois to the chorionic villi. Later this original stem is joined, not far from its origin, by what appears to be the lateral somatic branch of the fifth lumbar segment, whereupon the proximal part of the original umbilical vessel degenerates and the umbilical comes to arise from the somatic branch, which is the common iliac artery of adult anatomy (Fig. 152). Hence it is that this vessel in the adult gives origin both to branches such as the external iliac, the gluteal, the sciatic and the internal pudendal, which are distributed to the body walls
or their derivatives, and to others, such as the vesical, inferior haemorrhoidal and uterine, which are distributed to the pelvic viscera. At birth the portions of the umbilical arteries beyond the umbilicus are severed when the umbilical cord is cut, and their intra-embryonic portions, which have been called the hypogastric arteries, quickly undergo a reduction in size. Their proximal portions remain functional as the superior vesical arteries, carrying blood to the urinary bladder, but the portions which intervene between the
Fig. 152. - Diagram Illustrating the Development of the Umbilical Arteries.
A, Aorta; CIl, common iliac; Ell, external iliac; G, gluteal; III, internal iliac; IP, internal pudic; IV, inferior vesical; Sc, sciatic; U, umbilical; U', primary proximal portion of the umbilical; wd, Wolffian duct.
bladder and the umbilicus become reduced to solid cords, forming the obliterated hypogastric arteries of adult anatomy. f~ In its general plan, accordingly, the arterial system may be regarded as consisting of a pair of longitudinal vessels which fuse together throughout the greater portion of their length to form the dorsal aorta, from which there arise segmentary arranged lateral somatic branches and ventral and lateral visceral branches. With the exception of the aortic trunks (together with their anterior continuations, the internal carotids) and the external carotids, no longitudinal arteries exist primarily. In the adult, however, several longitudinal vessels, such as the vertebrals, internal mammary, and epigastric arteries, exist. The formation of these secondary longitudinal trunks is the result of a development between adjacent vessels of anastomoses, which become larger and more important blood-channels than the original vessels.
At an early stage each of the lateral branches of the dorsal aorta gives off a twig which passes forward to anastomose with a backwardly directed twig from the next anterior lateral branch, so as to form a longitudinal chain of anastomoses along each side of the neck. In the earliest stage at present known the chain starts from the lateral branch corresponding to the first cervical (suboccipital) segment and extends forward into the skull through the foramen magnum, terminating by anastomosing with the internal carotid. To this original chain other links are added from each of the succeeding cervical lateral branches as far back as the seventh (Figs. 150 and 153). But in the meantime the recession of the heart toward the thorax has begun, with the result that the common carotid stems are elongated and the aortic arches are apparently shortened so that the subclavian arises on the left side almost opposite the point where the aorta was joined by the sixth branchial vessel. As this apparent shortening proceeds, the various lateral branches which give rise to the chain of anastomoses, with the exception of the seventh, disappear in their proximal portions and the chain becomes an independent stem, the vertebral artery, arising from the seventh lateral branch, which is the subclavian.
The recession of the heart is continued until it lies below the level of the upper intercostal arteries, and the upper two of these, together with the last cervical branch on each side, lose their connection with the dorsal aorta, and, sending off anteriorly and posteriorly
Fig. 153. - The Development of the Vertebral Artery in a Rabbit Embryo of Twelve Days.
IIIA.B to VIA.B, Branchial arch vessels; Ap, pulmonary artery. A.v.c.b and A.v.cv, cephalic and cervical portions of the vertebral artery; A.s, subclavian; C.d and C.v internal and external carotid ; ISp.G, spinal ganglion. - (Hochstetter.)
anastomosing twigs, develop a short longitudinal stem, the superior intercostal, which opens into the subclavian.
The intercostals and their abdominal representatives, the lumbars and iliacs, also give rise to longitudinal anastomosing twigs near their ventral ends (Fig. 154), and these increasing in size give rise to the internal mammary and inferior epigastric arteries, which together form continuous stems extending from the subclavian to the external iliacs in the ventral abdominal walls. The superficial epigastrics and other secondary longitudinal vessels are formed in a similar manner.
The Development of the Arteries of the Limbs
The earliest stages in the development of the limb arteries are unknown in man, but it has been found that in the mouse the primary supply of the anterior limb bud is from five branches arising from the sides of the aorta. These anastomose to form a plexus from which later a single stem, the subclavian artery, is elaborated, occupying the position of the seventh cervical segmental vessel, the remaining branches of the plexus having disappeared. The common iliac artery similarly represents the fifth lumbar segmental artery, but whether or not it also is elaborated from a plexus is as yet unknown.
Fig. 154, -Embryo of 13 mm. showing the Mode of Development of the Internal Mammary and Deep Epigastric Arteries. - (Mall.)
The later history of the limb arteries is also but imperfectly known and one must rely largely upon the facts of comparative anatomy and on the anomalies that occur in the adult for indications of what the development is likely to be. The comparative evidence indicates the existence of several stages in the development of the limb vessels, and so far as embryological observations go they confirm the conclusions drawn from this source, although the various stages show apparently a great amount of overlapping owing to a concentration of the developmental stages. In the simplest arrangement the subclavian is continued as a single trunk along the axis of the limb as far as the carpus, where it divides into digital branches for the fingers. In its course through the forearm it lies in the interval between the radius and ulna, resting on the interosseous membrane, and in this part of its course it may be termed the arteria interossea. In the second stage a new artery accompanying the median nerve appears, arising from the main stem or brachial artery a little below the elbow-joint. This may be termed the arteria mediana, and as it develops the arteria interossea gradually diminishes in size, becoming finally the small volar interosseous artery of the adult (Fig. 155), and the median, uniting with its lower end, takes from it the digital branches and becomes the principal stem of the forearm.
A third stage is then ushered in by the appearance of a branch from the brachial which forms the arteria ulnaris, and this, passing down the ulnar side of the forearm, unites at the wrist with the median to form a superficial palmar arch from which the digital branches arise. A fourth stage is marked by the diminution of the median artery until it finally appears to be ,a small branch of the interosseous, and at the same time there develops from the brachial, at about the middle of the upper arm, what is known as the arteria radialis superficial (Fig. 155, rs). This extends down the radial side of the forearm, following the course of the radial nerve, and at the wrist passes upon the dorsal surface of the hand to form the dorsal digital arteries of the thumb and index finger. At first this artery takes no part in the formation of the palmar arches, but later it gives rise to the superficial volar branch, which usually unites with the superficial arch, while from its dorsal portion a perforating branch develops which passes between the first and second meta
Fig. 155. - Diagrams showing an Early and a Late Stage in the Development of the Arteries of the Arm.
b, Brachial; i, interosseous; m, median; r, radial; rs, superficial radial; u, ulnar.
carpal bones and unites with a deep branch of the ulnar to form the deep arch. The fifth or adult stage is reached by the development from the brachial below the elbow of a branch (Fig. 155, r) which passes downward and outward to unite with the superficial radial, whereupon the upper portion of that artery degenerates until it is represented only by a branch to the biceps muscle (Schwalbe), while the lower portion persists as the adult radial.
The various anomalies seen in the arteries of the forearm are, as a rule, due to the more or less complete persistence of one or other of the stages described above, what is described, for instance, as the high branching of the brachial being the persistence of the superficial radial.
In the leg there is a noticeable difference in the arrangement of the arteries from what occurs in the arm, in that the principal artery of the thigh, the femoral, does not accompany the principal nerve, the sciatic. This difference is apparently secondary, but, as in the case of the upper limb, it is necessary to rely largely on the facts of comparative anatomy and on anomalies which occur in the human body for an idea of the probable development of the arteries of the lower limb. It has already been seen that the common iliac artery is to be regarded as a lateral branch of the dorsal aorta, and in the simplest condition of the limb arteries its continuation, the anterior division of the hypogastric, passes down the leg as a well-developed sciatic artery as far as the ankle (Fig. 156,5). At the knee it occupies the position of the popliteal of adult anatomy, and below the knee gives off a branch corresponding to the anterior tibial (at) which, passing forward to the extensor surface of the leg, quickly loses itself in the extensor muscles. The main artery continues downward on the interosseous membrane, and some distance above the ankle divides into a strong anterior and a weaker posterior branch; the former perforates the membrane and is continued down the extensor surface of the leg to form the lower part of the anterior tibial and the dorsalis pedis arteries, while the latter, passing upon the plantar surface of the foot, is lost in the plantar muscles. At this stage the external iliac is a secondary branch of the common iliac, being but poorly developed and not extending as far as the knee.
In the second stage the external iliac artery increases in size until it equals the sciatic, and it now penetrates the adductor magnus muscle and unites with the popliteal portion of the sciatic. Before doing this, however, it gives off a strong branch (sa) which accompanies the long saphenous nerve down the inner side of the leg, and, passing behind the internal malleolus, extends upon the plantar surface of the foot, where it gives rise to the digital branches. From this arrangement the adult condition may be derived by the continued increase in size of the external iliac and its continuation, the femoral (/), accompanied by a reduction of the upper portion of the sciatic and its separation from its popliteal portion (p) to form the inferior gluteal artery of the adult. The continuation of the popli
Fig. 156. - Diagrams Illustrating Stages in the Development of the Arteries of the Leg.
at, Anterior tibial; dp, dorsalis pedis;/, femoral; p, popliteal; pe, peroneal pt, posterior tibial; s, sciatic (inferior gluteal); sa, saphenous.
teal down the leg is the peroneal artery (pe) and the upper perforating branch of this unites with the lower one to form a continuous anterior tibial, the lower connection of which with the peroneal persists in part as the anterior peroneal artery. A new branch arises from the upper part of the peroneal and passes down the back of the leg to unite with the lower part of the arteria saphena, forming the posterior tibial artery (pt), and the upper part of the saphenous becomes much reduced, persisting as the superficial branch of the art. genu suprema and a rudimentary chain of anastomoses which accompany the long saphenous nerve.
The Development of the Venous System
The earliest veins to develop are those which accompany the first-formed arteries, the umbilicals, but it will be more convenient to consider first the veins which carry the blood from the body of the embryo back to the heart. These make their appearance, while the heart is still in the pharyngeal region, as two pairs of longitudinal trunks, the anterior and posterior cardinal veins, into which lateral branches, arranged more or less segmentally, open. The anterior cardinals appear somewhat earlier than the posterior and form the internal jugular veins of adult anatomy. Each vein extends forward from the heart at the side of the notochord and is continued on the under surface of the brain, lying medial to the roots of the cranial nerves. Later sprouts arising from the vein form loops around the nerve roots and the portion of the loops formed by the original vein then disappear, so that the vessel now lies lateral to the nerve roots, except in the case of the trigeminus, where the original vessel persists to form the cavernous sinus. From the vena capitis lateralis so formed three veins, an anterior, a middle and a posterior cerebral, pass to the brain, the anterior cerebral together with the ophthalmic vein opening into the anterior end of the cavernous sinus, the middle cerebral into the posterior extremity of the same sinus and the posterior cerebral into the vena capitis lateralis behind the ear vesicle (Fig. 157). The branches of the anterior cerebral vein extending over the cerebral hemisphere unite with their fellows of the opposite side to form a longitudinal trunk, the superior sagittal sinus, lying between the two cerebral hemispheres. At first this sinus drains by way of the anterior cerebral vein (Fig. 158, A), but as the cerebral hemispheres increase in size it is gradually carried backward and makes connections first with the middle cerebral and later with the posterior cerebral vein (Fig. 158, B and C), each of these becoming in turn the principal drainage of the sinus. The connections which join the veins to the sinus become the proximal portion of the transverse sinus, the posterior cerebral vein itself becoming the distal portion, the middle cerebral vein becomes the superior petrosal sinus, while the anterior cerebral vein persists as the middle cerebral vein of adult anatomy
Fig. 157. - Reconstruction of the Head of a Human Embryo of 9 mm. showing the Cerebral Veins. acv, Anterior cerebral vein; au, auditory vesicle; cs, cavernous sinus; fa, facial nerve; mcv, middle cerebral vein; pcv, posterior cerebral vein; tr, trigeminal nerve; vcl, lateral cerebral vein. - (Mall.)
(Fig. 158, C). Additional sprouts from the terminal portion of the superior sagittal sinus give rise to the straight and inferior sagittal sinuses, and, after the disappearacne of the vena capitis lateralis, a new stem develops between the cavernous and transverse sinuses, passing medial to the ear vesicle, and forms the inferior petrosal sinus (Fig. 158, C). This joins the transverse sinus at the jugular foramen and from this junction onward the anterior cardinal vein may now be termed the internal jugular vein.
Passing backward from the jugular foramen the internal jugular veins unite with the posterior cardinals to form on each side a common trunk, the ductus Cuvieri, and then passing transversely toward the median line open into the sides of the sinus venosus. So long as the heart retains its original position in the pharyngeal region the jugular
Fig. 158. - Diagrams showing the Arrangement of the Cerebral Veins in Embryos of (A) the Fifth Week, (B) the Beginning of the Third Month and in (C) an Older Fetus.
acv, Anterior cerebral vein; cs, cavernous sinus; Us, inferior sagittal sinus; Inf. Pet., inferior petrosal sinus; Is, transverse sinus; ov, ophthalmic vein; sis, superior sagittal sinus; sps, spheno-parietal sinus; sr, straight sinus; 55, middle cerebral vein (Sylvian); sup. pet, superior petrosal sinus; th, torcular Herophili; v, trigeminal nerve; vca, anterior cerebral vein; vol. lateral cerebral vein; vcm, middle cerebral vein; vcp, posterior cerebral vein; vg, vein of Galen; vj, internal jugular. - (Mall.)
is a short trunk receiving lateral veins only from the uppermost segments of the neck and from the occipital segments, the remaining segmental veins opening into the inferior cardinals. As the heart recedes, however, the jugulars become more and more elongated and the cervical lateral veins shift their communication from the cardinals to the jugulars, until, when the subclavians have thus shifted, the jugulars become much larger than the cardinals. When the sinus venosus is absorbed into the wall of the right auricle, the course of the left Cuvierian duct becomes a little longer than that of the right, and from the left jugular, at the point where it is joined by the left subclavian, a branch arises which extends obliquely across to join the right jugular, forming the left innominate vein. When this is established, the connection between the left jugular and Cuvierian duct is dissolved, the blood from the left side of the head and neck and from the left subclavian vein passing over to empty
Fig. 159. - Diagrams showing the Development of the Superior Vena Cava. a, Azygos vein; cs, coronary sinus; ej, external jugular; h, hepatic vein; ij, internal jugular; inr and inl, right and left innominate veins; s, subclavian; vci and vcs, inferior and superior venae cava?.
into the right jugular, whose lower end, togethei with the right Cuvierian duct, thus becomes the superior vena cava. The left Cuvierian duct persists, forming with the left horn of the sinus venosus the coronary sinus (Fig. 159).
The external jugular vein develops somewhat later than the internal. The facial vein, which primarily forms the principal affluent of this stem, passes at first into the skull along with the fifth nerve and communicates with the internal jugular system, but later this original communication is broken and the facial vein, uniting with other superficial veins, passes over the jaw and extends down the neck as the external jugular. Later still the facial anastomoses with the ophthalmic at the inner angle of the eye and also makes connections with the internal jugular just after it has crossed the jaw, and so the adult condition is acquired.
It is interesting to note that in many of the lower mammals the external jugular becomes of much greater importance than the internal, the latter in some forms, indeed, eventually disappearing and the blood from the interior of the skull emptying by means of anastomoses which have developed into the external jugular system. In man the primitive condition is retained, but indications of a transference of the intracranial blood to the external jugular are seen in the emissary veins.
The posterior cardinal veins, or, as they may more simply be termed, the cardinals, extend backward from their union with the jugulars along the sides of the vertebral column, receiving veins from the mesentery and also from the various lateral segmental veins of the neck and trunk regions, with the exception of that of the first cervical segment which opens into the jugular. Later, however, as already described (p. 258), the cervical veins shift to the jugulars, as do also the first and second thoracic (intercostal) veins, but the remaining intercostals, together with the lumbars and sacrals, continue to open into the cardinals. In addition, the cardinals receive in early stages the veins from the primitive kidneys (meson ephros), which are exceptionally large in the human embryo, but when they are replaced later on by the permanent kidneys (metanephros) their afferent veins undergo a reduction in number and size, and this, together with the shifting of the upper lateral veins, produces a marked diminution in the size of the cardinals. The changes by which they acquire their final arrangement are, however, so intimately associated with the development of the inferior vena cava that their description may be conveniently postponed until the history of the vitelline and umbilical veins has been presented.
The vitelline veins are two in number, a right and a left, and pass in along the yolk-stalk until they reach the embryonic intestine, along the sides of which they pass forward to unite with the corresponding umbilical veins. These are represented in the bellystalk by a single venous trunk which, when it reaches the body of the embryo, divides into two stems which pass forward, one on each side of the umbilicus, and thence on each side of the median line of the ventral abdominal wall, to form with the corresponding vitelline veins common trunks which open into the ductus Cuvieri. As the liver develops it comes into intimate relation with the vitelline veins, which receive numerous branches from its substance and, indeed, seem to break up into a network (Fig. 160, A) traversing the liver
DC,
DC,
DC
Vus
Vom.s
Vud.
DC
D.K4
^drus
Vorn.s.
Kl/J.
Vamd. Vb.7ns.
-Diagrams Illustrating the Transformations of the Vitelline and Umbilical Veins.
D.C, Ductus Cuvieri; D.V.A, ductus venosus; V.o.m.d and V.o.m.s, right and left vitelline veins; V.u.d and V.u.s, right and left umbilical veins. - (Hochstetter.)
substance and uniting again to form two stems which represent the original continuations of the vitellines. From the point where the common trunk formed by the right vitelline and umbilical veins opens into the Cuvierian duct a new vein develops, passing downward and to the left to unite with the left vitelline; this is the ductus venosus (Fig. 160, B, D.V.A). In the meantime three cross-connections have developed between the two vitelline veins, two of which pass ventral and the other dorsal to the intestine, so that the latter is surrounded by two venous loops (Fig. 161, A), and a connection is developed between each umbilical vein and the corresponding vitelline (Fig. 160, B), that of the left side being the larger and uniting with the vitelline just where it is joined by the ductus venosus so as to seem to be the continuation of this vessel (Fig. 160, C). When these connections are complete, the upper portions of the umbilical veins degenerate (Fig. 161), and now the right side of the lower of the two vitelline loops which surround the intestine disappears, as does also that portion of the left side of the upper loop which intervenes
Fig. 161. - A, The Venous Trunks of an Embryo of 5 mm. seen from the Ventral Surface; B, Diagram Illustrating the Transformation to the Adult Condition.
Vcd and Vcs, Right and left superior venae cavae; Vj, jugular vein; V.om, vitelline vein; Vp, vena porta; Vu, umbilical vein (lower part); Vu', umbilical vein (upper part); Vud and Vus, right and left umbilical veins (lower parts). - (His.)
between the middle cross-connection and the ductus venosus, and so there is formed from the vitelline veins the vena porta.
While these changes have been progressing the right umbilical vein, originally the larger of the two (Fig. 160, A and B, V.u.d), has become very much reduced in size and, losing its connection with the left vein at the umbilicus, forms a vein of the ventral abdominal wall in which the blood now flows from above downward. The left umbilical now forms the only route for the return of blood from the placenta, and appears to be the direct continuation of the ductus venosus (Fig. 161, C), into which open the hepatic veins, returning the blood distributed by the portal vein to the substance of the liver. Returning now to the posterior cardinal veins, it has been found that in the rabbit the branches which come to them from the mesentery anastomose longitudinally to form a vessel lying parallel and slightly ventral to each cardinal. These may be termed the sub
Fig. 162. - Diagrams Illustrating the Development or the Inferior Vena Cava. The cardinal veins and ductus venosus are black, the subcardinal system blue, and the supracardinal yellow, cs, coronary sinus; dv, ductus venosus; il, iliac vein; r, renal; s, internal spermatic; scl, subclavian; sr, suprarenal; va, azygos; vha, hemiazygos; vi, innominate; vj, internal jugular.
cardinal veins (Lewis), and in their earliest condition they open at either end into the corresponding cardinal, with which they are also united by numerous cross-branches. Later, in rabbits of 8.8 mm., these cross-branches begin to disappear and give place to a large cross-branch situated immediately below the origin of the superior mesenteric artery, and at the same point a cross-branch between the two subcardinals also develops. The portion of the right subcardial which is anterior to the cross-connection now rapidly enlarges and unites with the ductus venosus about where the hepatic veins open into that vessel (Fig. 162, A), and the portion of each posterior cardinal immediately above the entrance of the renal veins degenerates, so that all the blood received by the posterior portions of the cardinals is returned to the heart by way of the right subcardinal, its cross-connections, and the upper part of the ductus venosus.
When this is accomplished the lower portions of the subcardinals disappear, while the portions above the large cross-connection persist, greatly diminished in size, as the suprarenal veins (Fig. 162, B).
In the early stages the veins which drain the posterior abdominal walls empty into the posterior cardinals, and later they form, in the region of the kidney on each side, a longitudinal anastomosis which opens at either extremity into the posterior cardinal. The ureter thus becomes surrounded by a venous ring, the dorsal limb of which is formed by the new longitudinal anastomosis, which has been termed the supracardinal vein (McClure and Huntington), while the ventral limb is formed by a portion of the posterior cardinal (Fig. 162, B). Still later the ventral limb of the loop disappears and the dorsal supracardinal limb replaces a portion of the more primitive posterior cardinal. An anastomosis now develops between the right and left cardinals at the point where the iliac veins open into them (Fig. 162, B), and the portion of the left cardinal which intervenes between this anastomosis and the entrance of the internal spermatic vein disappears, the remainder of it, as far forward as the renal vein, persisting as the upper part of the left internal spermatic vein, which thus comes to open into the renal vein instead of into the vena cava as does the corresponding vein of the right side of the body (Fig. 162, C, s). The renal veins originally open into the cardinals at the point where these are joined by the large crossconnection, and when the lower part of the left cardinal disappears, this cross-connection forms the proximal part of the left renal vein, which consequently receives the left suprarenal (Fig. 162, C).
The observations upon which the above description is based have been made upon the rabbit, but it seems probable from the partial observations that have been made that similar changes occur also in the human embryo. It will be noted from what has been said that the inferior vena cava is a composite vessel, consisting of at least four elements: (1) the proximal part of the ductus venosus; (2) the anterior part of the right subcardinal; (3) the right supracardinal; and (4) the posterior part of the right cardinal.
The complicated development of the inferior vena cava naturally gives rise to numerous anomalies of the vein due to inhibitions of its development. These anomalies affect especially the post-renal portion, a persistence of both cardinals (interpreting the conditions in the terms of what occurs in the rabbit) giving rise to a double post-renal cava, or a persistence of the left cardinal and the disappearance of the right to a vena cava situated on the left side of the vertebral column and crossing to the right by way of the left renal vein. So, too, the occurrence of accessory renal veins passing dorsal to the ureter is explicable on the supposition that they represent portions of the supracardinal system of veins.
It has already been noted that the portions of the posterior cardinals immediately anterior to the entrance of the renal veins disappear. The upper part of the right vein persists, however, and becomes the vena azygos of the adult, while the upper portion of the left vein sends a cross-branch over to unite with the azygos and then separates from the coronary sinus to form the vena hemiazygos. At least this is what is described as occurring in the rabbit. In the cat, however, only the very uppermost portion of the right posterior cardinal persists and the greater portion of the azygos and perhaps the entire hemiazygos vein is formed from the prerenal portions of the supracardinal veins, the right one joining on to the small persisting upper portion of the right posterior cardinal, while the crossconnection between the hemiazygos and azygos represents one of the originally numerous cross-connections between the supracardinals.
The ascending lumbar veins, frequently described as the commencements of the azygos veins, are in reality secondary formations developed by the anastomoses of anteriorly and posteriorly directed branches of the lumbar veins,
The Development of the veins of the Limbs
The development of the limb veins of the human embryo requires further investigation, but from a comparison of what is known with what has been observed in rabbit embryos it may be presumed that the changes which take place are somewhat as follows : In the anterior extremity the blood brought to the limb is collected by a vein which passes distally along the radial border of the limb bud, around its distal border, and proximally along its ulnar border to open into the anterior cardinal vein; this is the primary ulnar vein. Later a second vein grows out from the external jugular along the radial border of the limb, representing the cephalic vein of the adult, and on its appearance the digital veins, which were formed from the primary ulnar vein, become connected with it, and the distal portion of the primary ulnar vein disappears. Its proximal portion persists, however, to form the basilic vein, from which the brachial vein and its continuation, the ulnar vein, are developed, while the radial vein develops as an outgrowth from the cephalic, which at an early stage secures an opening into the axillary vein, its original communication with the external jugular forming the jugulo-cephalic vein.
In the lower limb a primary fibular vein, exactly comparable to the primary ulnar of the arm, surrounds the distal border of the limbbud and passes up its fibular border to open with the posterior cardinal vein. The further development in the lower limb differs considerably, however, from that of the upper limb. From the primary fibular vein an anterior tibial vein grows out, which receives the digital branches from the toes, and from the posterior cardinal, anterior to the point where the primary fibular opens into it, a vein grows down the tibial side of the leg, forming the long saphenous vein. From this the femoralvein is formed and from it the posterior tibial vein is continued down the leg. An anastomosis is formed between the femoral and the primary fibular veins at the level of the knee and the proximal portion of the latter vein then becomes greatly reduced, while its distal portion possibly persists as the small saphenous vein (Hochstetter).
The Pulmonary Veins
The development of the pulmonary veins has already been described in connection with the development of the heart (see p. 234).
The Fetal Circulation
During fetal life while the placenta is the sole organ in which occur the changes in the blood on which the
Fig. 163. - The Fetal Circulation. ao, Aorta; a.pu., pulmonary artery; au, umbilical artery; da, ductus arteriosus; dv, ductus venosus; int, intestine; vci and vcs, inferior and superior vena cava; vh, hepatic vein; vp, vena portas; v.pu, pulmonary vein; vu, umbilical vein. - (From Kollmann.)
nutrition of the embryo depends, the course of the blood is necessarily somewhat different from what obtains in the child after birth. Taking the placenta as the starting-point, the blood passes along the umbilical vein to enter the body of the fetus at the umbilicus, whence it passes forward in the free edge of the ventral mesentery (see p. 321) until it reaches the liver. Here, owing to the anastomoses between the umbilical and vitelline veins, a portion of the blood traverses the substance of the liver to open by the hepatic veins into the inferior vena cava, while the remainder passes on through the ductus venosus to the cava, the united streams opening into the right atrium. This blood, whose purity is only slightly reduced by mixture with the blood returning from the inferior vena cava, is prevented from passing into the right ventricle by the Eustachian valve, which directs it to the foramen ovale, and through this it passes into the left atrium, thence to the left ventricle, and so out by the systemic aorta.
The blood which has been sent to the head, neck, and upper extremities is returned by the superior vena cava also into the right atrium, but this descending stream opens into the atrium to the right of the annulus of Vieussens (see Fig. 141) and passes directly to the right ventricle without mingling to any great extent with the blood returning by way of the inferior cava. From the right ventricle this blood passes out by the pulmonary artery; but the lungs at this period are collapsed and in no condition to receive any great amount of blood, and so the stream passes by way of the ductus arteriosus into the systemic aorta, meeting there the placental blood just below the point where the left subclavian artery is given off. From this point onward the aorta contains only mixed blood, and this is distributed to the walls of the thorax and abdomen and to the lungs and abdominal viscera, the greater part of it, however, passing off in the hypogastric arteries and so out again to the placenta.
This is the generally accepted account of the fetal circulation and it is based upon the idea that the foramen ovale is practically a connection between the inferior vena cava and the left atrium. If it be correct the right ventricle receives only the blood returning to the heart by the vena cava superior, while the left receives all that returns by the inferior vena cava together with what returns by the pulmonary veins. One would, therefore, expect that the capacity and pressure of the right ventricle would in the fetus be less than those of the left. Pohlman, who has recently investigated the question in embryo pigs, finds, on the contrary, that the capacities and pressures of the two ventricles are equal and maintains that the foramen ovale is actually a connection between the two atria. That is to say, he holds that there is an actual mingling of the blood from the two venae cava? in the right atrium, whence the mixed blood passes to the right ventricle, a certain amount of it, however, passing through the foramen ovale and so to the left ventricle to equalize the deficiency that would otherwise exist in that chamber owing to the small amount of blood returning by the pulmonary veins. According to this view there would be no difference in the quality of the blood distributed to different portions of the body, such as is provided for by the current theory; all the blood leaving the heart would be mixed blood and in favor of this view is the fact that starch granules injected into either the superior or the inferior vena cava in living pig embryos were in all cases recovered from both sides of the heart.
At birth the lungs at once assume their functions, and on the cutting of the umbilical cord all communication with the placenta ceases. Shortly after birth the foramen ovale closes more or less perfectly, and the ductus arteriosus diminishes in size as the pulmonary arteries increase and becomes eventually converted into a fibrous cord. The hypogastric arteries diminish greatly, and after they have passed the bladder are also reduced to fibrous cords, a fate likewise shared by the umbilical vein, which becomes converted into the round ligament of the liver.
The Development of the Lymphatic System. - Concerning the development of the lymphatic system two discordant views exist, one (Sabin, Lewis) regarding it in its entirety as a direct development from the venous system, while the other (Huntington, McClure) recognizes for it a dual origin, a portion being derived directly from the venous system and the rest from a series of mesenchymal spaces developing in relation to veins but quite unconnected with them.
The portion of the system concerning which harmony prevails is that which forms the connection with the venous system in the adult and constitutes what in the embryo is termed the jugular lymph sac. In the early stages of development a capillary network extends along the line of the jugular veins, communicating with them at various points. In embryos of 10 mm. a portion of this network, on either side of the body, becomes completely separated from the jugular and gives rise to a number of closed cavities lined with endothelium and situated in the neighborhood of the junction of the primary ulnar and cephalic veins with the jugular. In
Fig. 164. - Diagrams showing the Arrangement of the Lymphatic Vessels in Pig Embryos of (4) 20 mm. and (B) 40 mm. ACV, Jugular vein; ADR, suprarenal body; ALU, jugular lymph sac; Ao, aorta Arm D, deep lymphatics to the arm; D, diaphragm; Du, branches to duodenum FV, femoral vein; H, branches to heart; K, kidney; LegD, deep lymphatics to leg Lu, branches to lung; MP, branches to mesenteric plexus; CE, branch to oesophagus PCV, cardinal vein: PLH, posterior lymph sac; RC, cisterna chyli; RLD, right lymphatic duct; ScV, subclavian vein; SV, sciatic vein; St, branches to stomach; TD, thoracic duct; WB. Wolffian body. - (Sdbin.)
later stages these cavities enlarge and unite to form a large sac, the jugular lymph sac (Fig. 164, ALU), and this, still later, makes a new connection with the jugular, the opening being guarded by a valve. This communication becomes the adult communication of the thoracic duct or right lymphatic duct with the venous system, but the sac itself, as development proceeds, becomes divided into smaller portions and gives rise to a number of lymph nodes.
A similar pair of lymph sacs also develop in relation to the sciatic vein, but their exact mode of origin is uncertain. In embryos of 20 mm. venous plexuses, similar to the jugular plexuses of younger stages, are found accompanying the sciatic veins, and a little later there are found in the same region a pair of posterior or sciatic lymph sacs (Fig. 164, PLH), which, like the jugular sacs, later give rise to a series of lymph nodes. At about the same stage of development & retroperitoneal sac (Fig. 165, Lsr) is also formed in the root of the mesentery cranial to the origin of the superior mesenteric artery, and this, too, later gives rise to a plexus of lymphatic vessels in connection with which the mesenteric lymphatic nodes develop. This last sac is much more pronounced in the pig embryo than in man, and in that form it has been found to have its origin from a capillary network that separates from the renal veins (Baetjer).
There are thus formed five sacs, all of which are associated with the formation of groups of lymphatic nodes, and in the case of one pair at least it is agreed that they are directly developed from venous capillaries. It is in connection with the remaining sac and especially with the formation of the thoracic duct and the peripheral lymphatics that the want of harmony referred to above occurs. The first portion of the thoracic duct to appear is the cisterna chyli, which is found in embryos of 23 mm. in the region of the third and fourth lumbar segments, in close proximity to the vena cava (Fig. 165, Cc). After its appearance the rest of the thoracic duct develops quickly, it being completely formed in embryos of 30 mm., and it is interesting to note that at this stage the duct is paired in its caudal portion, two trunks passing forward from the cisterna chyli, the right one passing behind the aorta and uniting with the left after it has entered the thorax.
The mode of origin of the duct has not yet been made out in human embryos. In the pig and rabbit isolated spaces lined with endothelium occur along the course of the duct, but without communicating with it, and the fact that some of these showed connection with the neighboring azygos veins gave basis for the view that they were the remains of a venous capillary plexus from which the duct had developed. It is possible, however, that the duct is formed
Fig. 165. - Diagram of the Posterior Portion of the Body of a Human Embryo of 23 mm., showing the Relations of the Retroperitoneal Lymph Sac and the Cisterna Chyli to the Veins.
Am, Superior mesenteric artery; Ao, aorta; Cc, cisterna chyli; Isr, retroperitoneal lymph sac; S, suprarenal body; Va, vena azygos; Vci, vena cava inferior; vl u first lumbar vertebra; vs u first sacral vertebra. - (After Sabin.)
by the union of outgrowths from the cisterna chyli and jugular sac, in which case it would also be a derivative of the venous system, provided that the cisterna chyli is formed in the same way as the jugular sac. Huntington and McClure, however, maintain that it is formed by the fusion of spaces appearing in the mesenchyme immediately external to the intima of degenerating veins; hence the spaces are termed extraintimal spaces. These at first have no endothelial lining and they are never in connection with the lumina of the veins. They are perfectly independent structures and any connections they may«nake with the venous system are entirely secondary. This mode of origin from extraintimal spaces is not confined to the thoracic duct, according to the authors mentioned, but is the method of development of all parts of the lymphatic system, with the exception of the jugular sacs. According to the supporters of the direct venous origin the peripheral lymphatic stems develop, like blood-vessels, as outgrowths from the stems already present.
Lymph nodes nave not been observed in human embryos until toward the end of the third month of development, but ' ! .<l'-V\LY. they appear in pig embryos of 3 cm. X^Hi^. Their unit of structure is a blood-vessel, breaking up at its termination into a leash of capillaries, around which a condensation of lymphocytes occurs in the mesenchyme. A structure of this kind forms what is termed a lymphoid follicle and may exist, even in this simple condition, in the adult. More frequently, however, there are associated with the follicle lymphatic vessels, or rather the follicle develops in a network of lymphatic vessels, which, become an investment of the follicle and form with it a simple lymph node (Fig. 166). This condition is, however, in many cases but transitory, the artery branching and collections of lymphoid tissue forming around each of the branches, so that a series of follicles are formed, which, together with the surrounding lymphatic vessels, become enclosed by a connective-tissue capsule to
Fig. 166. - Diagram of a Primary Lymph Node of an Embryo Pig of 8 cm. a, Artery; aid, afferent lymph duct; eld, efferent lymph duct; /, follicle. - (Sabin.)
form a compound lymph node. Later trabecular of connective tissue extend from the capsule toward the center of the node, between the follicles, the lymphatic network gives rise to peripheral and central lymph sinuses, and the follicles, each with its arterial branch, constitute the peripheral nodules and the medullary cords, the portions of these immediately surrounding the leash of capillaries into which
Fig. 167. - Developing H^emolymph Node.
be, central blood-vessel; bh, blood-vessel at hilus; ps, peripheral blood sinus. - (Sabin from Morris' Human Anatomy.)
the artery dissolves, constituting the so-called germ centers in which multiplication of the lymphocytes occurs.
In various portions of the body, but especially along the root of
the mesentery, what are termed hcemolymph nodes occur. In these
the lymph sinus is replaced by a blood sinus, but with this exception
their structure resembles that of an ordinary lymph node, a simple one consisting of a follicle, composed of adenoid tissue with a central blood-vessel, and a peripheral blood sinus (Fig. 167).
The Development of the Spleen
Recent studies (Mall) have shown that the spleen may well be regarded as possessing a structure comparable to that of the lymph nodes, the pulp being more or less distinctly divided by trabecular into areas termed pulp cords, the axis of each of which is occupied by a twig of the splenic artery. The spleen, therefore, seems to fall into the same category of organs as the lymph and hsemolymph nodes, differing from these chiefly in the absence of sinuses. It has generally been regarded as a development of the mesenchyme situated between the two layers of the mesogastrium. To this view, however, recent observers have taken exception, holding that the ultimate origin of the organ is in part or entirely from the ccelomic epithelium of the left layer of the mesogastrium. The first indication of the spleen has been observed in embryos of the fifth week as a slight elevation on the left (dorsal) surface of the mesogastrium, due to a local thickening and vascularization of the mesenchyme, accompanied by a thickening of the ccelomic epithelium which covers the elevation. The mesenchyme thickening presents no differences from the neighboring mesenchyme, but the epithelium is not distinctly separated from it over its entire surface, as it is elsewhere in the mesentery. In later stages, which have been observed in detail in pig and other amniote embryos, cells separate from the deeper layers of the epithelium (Fig. 168) and pass into the mesenchyme thickening, whose tissue soon assumes a different appearance from the surrounding mesenchyme by its cells being much crowded. This migration soon' Ceases, however, and in embryos of forty-two days the ccelomic epithelium covering the thickening is reduced to a simple layer of cells.
The later stages of development consist of an enlargement of the thickening and its gradual constriction from the surface of the mesogastrium, until it is finally united to it only by a narrow band through which the large splenic vessels gain access to the organ The cells differentiate themselves into trabecular and pulp cords special collections of lymphoid cells around the branches of the splenic artery forming the Malpighian corpuscles.
It has already been pointed out (p. 225) that during embryonic life the spleen is an important haematopoietic organ, both red and white corpuscles undergoing active formation within its substance. The Malpighian corpuscles are collections of lymphocytes in which multiplication takes place, and while nothing is as yet known as to the fate of the cells which are contributed to the spleen from the ccelomic epithelium, since they quickly come to resemble the mesenchyme cells with which they are associated, yet the growing number of observations indicating an epithelial origin for lymphocytes suggests the possibility that the cells in question may be responsible for the first leukocytes of the spleen.
Fig. 168. - Section through the Left Layer of the Mesogastrium of a Chick Embryo of Ninety-three Hours, Showing the Origin of the Spleen.
ep, Ccelomic epithelium; ms, mesenchyme. - (Tonkoff.)
The Coccygeal or Luschka's Ganglion
In embryos of about 15 cm. there is to be found on the ventral surface of the apex of the coccyx a small oval group of polygonal cells, clearly separated from the surrounding tissue by a mesenchymal capsule. Later, connective-tissue trabecular make their way into the mass, which thus becomes divided into lobules, and, at the same time, a rich vascular supply, derived principally from branches of the middle sacral artery, penetrates the body, which thus assumes the adult condition in which it presents a general resemblance to a group of lymph follicles.
It has generally been supposed that the coccygeal ganglion was in part derived from the sympathetic nervous system and belonged to the same group of organs as the suprarenal bodies. The most recent work on its development (Stoerk) tends, however, to disprove this view, and the ganglion seems accordingly to find its place among the lymphoid organs.
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McMurrich JP. The Development Of The Human Body. (1914) P. Blakiston's Son & Co., Philadelphia, Pennsylvania.
Cite this page: Hill, M.A. (2024, April 19) Embryology McMurrich1914 Chapter 9. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/McMurrich1914_Chapter_9
- © Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G
