Book - Manual of Human Embryology 18-3
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Keibel F. and Mall FP. Manual of Human Embryology II. (1912) J. B. Lippincott Company, Philadelphia.
- XVIII. Development of Blood, Vascular System and Spleen: Introduction | Origin of the Angioblast and Development of the Blood | Development of the Heart | The Development of the Vascular System | General | Special Development of the Blood-vessels | Origin of the Blood-vascular System | Blood-vascular System in Series of Human Embryos | Arteries | Veins | Development of the Lymphatic System | Development of the Spleen
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III. The Development of the Vascular System
By Herbert M. Evans. Johns Hopkins University, Baltimore.
2. The Special Development of the Blood-vessels
A. Origin of the Blood-vascular System
D. The Development of the Veins
IV. The Development of the Lymphatic System
V. The Development of the Spleen
1. General
We shall consider here, first, the more general questions concerning the development of the vascular system, and, secondly, the special development of the vascular system in human embryos.
In recent years studies on the vascular system of the higher vertebrates have opened up new and profitable fields and have given us a better conception of the method by which the blood-vessels grow and become transformed in the general growth of the embryo.
In the following account I shall confine myself to the history of the chief vascular trunks only, 1 for here our knowledge now stands on a firm footing, and any laws which we may discover as applicable in these instances may safely be taken as of general worth.
The two fundamental questions involved in the development of the vascular system are — 1. What is the origin of the blood-vessels in the body of the embryo? 2. What is the primitive form of the vessels in any area, and the manner of change from this to that of the adult? These two aspects of the subject thus concern themselves with the problem of the cellular antecedents of the endothelium, on the one hand, and with the principles governing the architecture of the vascular system, on the other.
To the former problem it is still impossible to give any decisive answer, but to the latter I trust the reader will see that a flood of new light has come.
1 There exist few accounts of the development of peripheral vessels, but mention mav be made of the work of Mall, Flint. Miller. Sabin, and Fnchs.
1. Human embryos, as will be mentioned further on, have contributed little information on the origin of the cells forming the vascular system, and indeed after a wealth of observations on other animals this question is still a very open one, having met with a decisive answer in no case.
In embryos of the higher vertebrates the first cells which can be identified as standing in any relation to the vascular system are in the form of localized thickenings of the extra-embryonic mesoderm 2 lying next the endoderm of the yolk-sac. These constitute the so-called vascular anlagen, and typically undergo a gradual differentiation from a nest of indifferent cells into two more definite cell types, blood-cells on the one hand and endothelium on the other. The endothelial cells enclose the former, and, continuing to divide, produce vascular sprouts and thus extend themselves into new areas. While this differentiation of the earlier anlagen is progressing, new anlagen are formed by the mesoblast, but eventually a time is reached when this latter process ceases, and subsequently in the history of the embryo all endothelium is derived from that of pre-existing vessels. That this is the case in older embryos and in the adult has been verified by many observations. It is important, then, to distinguish vessels which have arisen through the sprouting of the endothelium of other vessels in contrast to vessels whose endothelium has been contributed directly from the neighboring mesoderm. Even on the yolk-sac these latter vessels which arise in loco are not numerous, for they only occur at the site of the so-called anlagen, and the main mass of the vitelline capillary plexus arises from the extension and frequent anastomoses of these primary vessels.
It is a question now whether the early blood-vessels in the body of the embryo itself are not formed by an ingrowth of the vitelline capillaries, or whether, on the other hand, the embryonic stems, or at least a part of them, do not arise in situ from the mesoderm of the body. Both of these positions have been defended, the name of His (1900) being identified with the former idea and that of Riiekert and Mollier (1906) especially with the latter.
In the birds it has been possible to establish beyond all doubt that most of the aorta descendens is formed from the medial margin of the vitelline capillary plexus (Vialleton 1892, His 1900, Evans 1909; see Eig. 390). The frequent early connections of this vessel with the same plexus in mammals makes it highly probable that a similar origin obtains here (Tursting, 1884). For the head portion of the aortae, on the other hand, conflicting accounts are given. His described it as arising from a continued growth of the same extra-embryonic capillaries which formed the vessel in its lower course, but which were restricted to a capillary chain growing headward, eventually turning ventrally over the blind end of the head-gut and fusing with the cephalic portion of the heart tube. On the contrary, Riiekert and Mollier have given various details of what they consider the local origin of the aorta in this locality from the mesodermal cells of the lateral plate of the mesoderm and from the splanchnic mesoderm. It is not possible then to state positively that the yolk-sac anlagen are the only source of the endothelium of the body vessels, for the earliest of these latter may themselves be primary vascular anlagen in the sense of being directly derived from neighboring mesoderm. Another possible source for the endothelium of the vessels of the head is constituted by the paired anlage of the heart (C. Rabl, 1887). 2a After the establishment of the aorta it is possible to satisfactorily deny the further local origin of any of the subsequent vessels of the embryo, since these can all be demonstrated to arise from capillary sprouts of true vascular
1 Although lying in the mesoderm, these anlagen may have actually arisen from the entoderm. This is the view taken by Ruekert (1906), who has been the last to subject the question to a special study. On the other hand, most investigators, beginning with Kolliker's early work on the rabbit (1875), have emphatically denied any entodermal participation here, and affirmed that the blood islands of mammalian embryos are to be looked upon as special localized proliferations of the mesoblast. Robinson (mouse) and Heape (mole), Janosik (marmot, pig), Fleischman (cat), Keibel (guinea-pig, man), and Van der Strieht (rabbit and bat) may be mentioned.
Fig. 390a. — Ventral view of the left side of a rabbit embryo of five somites, showing the vascular plexus from which the left heart and aortae are derived. The heart is already indicated as an especially enlarged member of the mesh. This is not yet true for the aorta; the arrows indicate two places where the aorta is as yet unconnected into a longitudinal plexus. The brackets show the position of the five somites. ( After J. L. Bremer.)
Fig. 390b. — Ventral view of posterior portion of an injected chick embryo of 20 somites, showing the formation of the lower aortae from a capillary plexus continuous with that of the yolk-sac.
endothelium, just, as is the case in every locality where the development of vessels has been carefully studied in the living animal, e.g., the tail of the living frog larva.
The various claims for a local origin of blood-vessels relatively late in the growth of the embryo have gradually been successfully disproved. It will be remembered that the appearances known to many observers as Ranvier's " vaso formative cells " were supposedly instances in which a local origin of blood-vessels occurred relatively late in the growth of the embiyo. Recently Vosmear (1898) has shown
" a Since the above was written, Bremer (1911) has demonstrated that in the head of rabbit ernbryos of five somites, the aorta is represented by a distinct plexiform angioblast coterminous with the vitelline angioblast. These facts make it highly probable that the extra embryonic endothelium has grown into the body in this region just as can be demonstrated in successive stages for the more caudal portion of the aorta of the chick.
that the vessels in question were isolated secondarily after having clearly arisen from other vessels, and Clark has observed the same phenomenon in the living frog larva (personal communication). (See also Renant, 1901, 1902.) It would seem that a mere histological analysis, even though on perfectly fixed material, would not suffice to settle the question of the delicate connection of embryonic vessels. These collapse so readily that the most perfect of the usual methods of study will not suffice to disclose them. On the other hand, injections of the vascular systems in young embryos show a wealth of capillaries and interconnections not hitherto demonstrable, and it would consequently seem unwise to overvalue any negative evidence in this respect given by uninjected embryos.
It is evident, then, that, while it is probable that the only source for the endothelium for the blood-vessels is comprised in the cells of the vascular anlagen, it is nevertheless possible to prove that this source is comprised in the endothelium of the first intra-embryonic vessels (aortae and cardinal veins), however these may have arisen. Injections of the embryo after these early stages and subsequent exploration with the microscope show no vessels unconnected with the general system, and lead us to be certain that new vessels in any area arise exclusively as offshoots from the older ones. This doctrine of the specificity of the endothelium has met many apparent confirmations in the histogenesis of new growths, for there also, as in normal development, Rabl's dictum is doubtless true, " Endothelium only from endothelium." 2. Concerning the development of the form of the vascular system, two positions have been taken, — one, that the arteries and veins grow out as single trunks to their respective territories, the other, that the first vessels in any area are capillaries usually in the form of a typical plexus from which secondarily arteries and veins arise.
It will be seen that a correct conception of the actual truth here affects vitally our ideas even of the factors concerned in development as a whole ; for it is difficult to see in the blind outgrowth of single trunks to their future territory anything but a teleological design or hereditary predestination. On the other hand, the adherents of the idea of a capillary plexus ancestry for vessels view the vascular system as functioning from the beginning, and the formation of arteries and veins as only an expression of the functional adaptation of these plexuses to a beating heart.
The immense number of vascular variations in the adult, which seem to take every conceivable direction, and the occurrence of arterial and venous anastomoses, early led the Swiss anatomist Aeby (1868) to suppose that the vascular system, arteries as well as veins, existed originally in the form of a uniform mesh-work of vessels, in which, so to speak, a competition took place for supremacy, and, the victors being the only trunks remaining, we obtained the dendritic branched appearance of the adult vascular system. Occasionally the primitive net was retained, and in these instances we saw retia mirabilia, or merely anastomoses between vessels.
In the case of variations the theory was most convenient, for the hypothetical uniform net furnished the possibility for a vessel to course in practically any direction. At a loss for a better explanation, Krause (1876) adopted the Aeby hypothetical plexus to account for the vast range of blood-vessel variations which he recorded in his well known chapter in Henle's Handbuch. But until recently Aeby's ideas have met with no other favorable reception. Indeed they were strongly opposed by the careful work inaugurated by the rise of a more exact comparative anatomy, in which C. Gegenbaur and his pupils are to be mentioned. Extensive comparative investigations soon showed that there was to be observed everywhere a remarkable constancy in the number and position of the vascular trunks and their relation to other structures (muscles, nerves).
Ruge's (1883) epoch-making work in this field demonstrated clearly that when variations occurred they tended to group themselves into quite definite types, which could be explained by the over-development of. normally inconspicuous vessels, " collateral stems " or aberrants. Thus the old conception of the outgrowth of single trunks was only strengthened, for there could be considered present in anomalies only an unusually, strong outgrowth of a normally small trunk. Moreover, another authority in this field, F. Hochstetter, took occasion to denounce the Aeby-Krause idea. In his study of the developing vessels in the early limbs, Hochstetter declared he could find no instances of an indifferent condition of the vascular system anywhere in the limb buds, and that consequently " die Hypothese Baader's und Krause's als vollkommen unrichtig bezeiehnet werden muss." s (Hochstetter, 1891, p. 42.) differentiated in this locality. Early stages showed him only an indifferent network of vessels in which no predominate trunks could be distinguished, and he was able to secure successive preparations showing the gradual formation of arteries and veins from this capillary net. The elaboration of these larger supplying and draining vessels represented merely a functional adaptation of the net to the demands of the circulation and a fortuitous location with regard to the aorta? or the venous ostia of the heart determined the use and enlargement of certain channels of the net to become arteries and veins respectively.
Fig. 391. — Injection showing the profuse outgrowth of primary subclavian capillaries into the early wing bud of a_chick 60 hours old. 14th D. I. V., fourteenth dorsal intersegmental vein.
In 1894 R. Thoma published the results of a study he had been conducting on the ancestry of the vascular trunks present in the chick's yolk-sac. Thoma set himself the task of solving how it came about that arteries and veins were 3 This statement was at least partially justified by the simple conditions Hochstetter saw in the vessels of the limbs and tail of Triton, for here the vascular trunks are remarkably simple and suffer a more or less direct transformation into arteries and veins. That such simple conditions do not apply to the limbs of higher vertebrates (birds and mammals) the studies of Goppert (1910) and myself (1909) will demonstrate.
Thoma's ideas did not at first attract the notice they deserved. Nevertheless, Mall (1908) indicated that they were applicable in the development of the body's vascular system no less than in the extra-embryonic area vasculosa, and in succeeding years he and his pupils repeatedly furnished evidence that such was indeed the case. Flint's (1903) study of the submaxillary gland and more recently of the lung (1906) showed that a capillary net always existed at the periphery of the growing vascular tree, and Mall (1906) instanced the same phenomena in the division and growth of the lobules of the liver. It may be noted that Zuckerkandl (1894) had reported an exactly similar phenomenon in the development of the median artery of the arm, for the early stages in the history of this vessel were represented by a chain of capillaries accompanying the median nerve.
Fig. 392. — Injection into the early leg buds of a chick of 32 somites, showing the capillary plexus.
In 1903 E. Miiller reported the results of a study of the development of the vessels in the human arm. His reconstructions showed that in some instances what is undoubtedly a true arterial plexus may exist in the region of the axillary artery.* These appearances were quite impossible to harmouize with the old idea of the outgrowth of single vascular trunks and led Miiller consequently to dispute this prevailing notion.
Four years later H. Rabl published the results of his study of the early vessels in the wing bud of the duck, and showed clearly that many "of these could be detected in arising from parts of a capillary plexus. Rabl also established the origin of the secondary subclavian artery, which characterizes the birds, from a chain of capillaries which grows caudally from the third aortic arch to join the plexus of the wing bud.
Above all, now, the admirable study of the vessels in the developing arm of the white mouse which E. Goppert has recently published shows clearly the development cf the successive branches of the subclavian artery "auf der Grundlage eines capillaren Netzes," and I cannot doubt but that in the further growth of the vascular system, in the various regions of the body, we will be able to observe these facts again and again.
Very recently methods of injecting living embryos so that the delicate vascular system is completely filled and yet extravasations avoided, have yielded a wealth of facts on the development of the vessels. The revelations due to such preparations have enabled us to see the capillary precxirsers of some of the more fundamental vascular trunks of the body.
Thus, if injections of chick embryos are made just preceding and during the time in which the limb buds are beginning to be elevated from the body wall, it is possible to trace the earliest vascularization of the limbs. Such preparations show that a series of capillaries springs from the lateral aortic wall opposite the limb eminence, and, anastomosing together, form a typical capillary plexus in the early limb tissue. The preservation and enlargement of one of these many aortic offshoots constitute the subclavian and sciatic artery respectively (Figs. 391 and 392).
Still other large vessels in the body can be traced to a similar stage in which there exists only a simple capillary plexus, out of which the main vessel arises through the utilization of a single channel in the mesh and the coincident atrophy of the remainder. Thus, in the early stages the head of the embryo possesses as its only vessels a delicate plexus of capillaries which arise at many points from the aorta? and more caudally are connected with the vitelline vein. Eventually with the circulation through this primary head capillary plexus, arteries and veins are formed from some channels in the mesh, and it is exactly in this way that the main stems of the internal carotid artery and jugular veins are formed (Figs. 393, 394, 395).
The pulmonary arteries are represented at first by a capillary plexus which arises from the sixth aortic arches and grows caudallv on to the lung bud (Fie. 396).
In the chick it is easy to see that the gut arteries are earliest represented by a plexus of capillaries which arise from the ventral aortic wall.
With all these instances, however, of an early plexiform anlage for many vessels, we are forced to admit that some of the primary vessels of the body are not preceded by such stages, but from the very first occupy a definite position and consist of only a single endothelial tube. The most striking example of this is furnished by the dorsal segmental arteries, which, as is well known, arise from the aorta at strictly intersegmental points and are usually distinctly single vessels.
4 A fact which has more recently been confirmed for the mouse by the important researches of Goppert (1909) in the same territory. The exact significance which Miiller would attach to this axillary plexus must be disputed, as also his contention for its constant occurrence. It must be considered merely one of the instances where the circulation has for a time taken equally favored paths through a preceding capillary plexus and thus formed for us for a time several anastomosing embryonic arteries rather than a single one, which is normally the case.
Fig. 393. — Lateral view of head of an injected chick of 15 somites, showing the primary capillary plexus here. The plexus takes origin from the convexity of the first aortic arch, and is continued posteriorly as a slender capillary chain which eventually joins the main vitelline vein near the junction of the latter with the heart. This slender capillary chain has arisen at several points from the dorsal aorta on each side, and two of these points of origin are still preserved opposite the region of the hind-brain. The delicate capillary path from head to vitelline vein is destined to form the anterior cardinal vein.
The sharply limited definite positions of such great vessels as the aortae and umbilical veins are also phenomena which have been known for a long time and which seem unquestionably due to inheritance. However, even in these cases, an exacter study shows that these vessels do not develop at first as merely simple tubes. When, for instance, we turn to a consideration of the aorta, we can see clearly that in the chick the lower part of this vessel is merely the exaggerated medial margin of the vitelline capillary plexus, which has invaded the embryonic tissue (Fig. 390). This is also exactly the condition which may be seen in human embryos of corresponding age, for here also the caudal part of the aorta is only a part of a general plexus of vessels which lie in the gut wall and continue to grow caudally. The dorsalmost members of this plexus straighten out longitudinally and form the aorta dorsalis while the connections of the latter with the plexus become the primitive vitello-umbilical complex of arteries. Inasmuch as in embryos of 6 somites these conditions occur opposite the future 7th and 8th somite (i. e., cervical somite 5), it is hence apparent that all of the aorta which exists later below this level has emerged from this preceding stage. There is little doubt but that the cephalic portion of the aorta has also an identical development, although less study has been given the appropriate stages. Turstig (1884) long ago showed that this was at first not a single vessel but a narrow meshwork of vessels (Fig. 397), and that the single aortic tube came about only secondarily by the enlargement of one channel of the narrow mesh or by the fusion of several capillaries in some places. In the latter instances he described very clearly remnants of the old partition walls between the individual preceding channels in the form of cross-strands joining the ventral and dorsal aortic wall. Recently, Bremer (1911) has demonstrated the plexiform endothelial anlage of the entire cephalic portion of the aortas in rabbit embryos of five somites. Lingering remains of the several capillary channels which constitute the aorta in its earliest stages are occasionally seen in somewhat older specimens, where it is not uncommon to find the aorta splitting into two or three vessels to reunite again, a fact which I can confirm as occurring now and then in the human embryo with from 6 to 8 somites (Embryos Pfannenstiel-Kroemer, Etemod, Graf Spee).
Fig. 394. — Lateral view of injected pig embryo of 5.7 mm. length, showing the capillary origin of the a. car. int. X 80.
Fig. 395. — Lateral view of injected pig embryo measuring 6.5 mm. The injection was made while the heart was still beating and shows the extent of the primary capillary plexus in the head. X SO.
The aortic arches are similarly formed by narrow chains of capillaries which quickly give way to the employment of a single channel in the mesh, though in some instances remains of the earlier plexiform condition persist. I figure here the first aortic arch in a young duck embryo (Fig. 39S).
It is only natural that the studies which have previously been made on the vascular system have usually revealed only the chief stems and have consequently led us to suppose that these stems grew out as such, for the usual methods of reconstruction of uninjected embryos can not hope to reveal more than the chief trunks in any area. I present, for example (Figs. 399, 400), an embryo of about the same degree of development as Elze's (1907) shown in Fig. 420. His is from an unusually good reconstruction, mine from an injected specimen. It may be well to note that in the area here figured the reconstructed figure gives the appearance of the anterior cardinal vein growing forward and dorsally to constitute the future superior sagittal sinus; but the injected embryo shows clearly that this structure is beginning to be formed by the enlargement of the medial dorsal margin of the capillary mesh here, somewhat as the lower aorta? represent the enlarged medial margin of the vitelline net.
Most important of all, then, is the fact that the injections show its that the vascular system is not merely growing in an irregular fashion to obey an impulse given by heredity, but that it constitutes a connected and functioning whole.
Fig. 396. — Pulmonary vessels in a guinea-pig embryo 21 days old. (After Fedorow, 1911.)
If now we assemble the remarks which have just been made it is evident that we may state generally that arteries and veins do not grow out as such, but that the blood-vessels tend always to be laid down in a multiple capillary anlage rather than in single trunk-like forms, and that this is true even where tUe position of the vessel is apparently predetermined by inheritance. In many areas, however (e. g., the head and the limbs), we have more typical plexuses from which, through the secondary enlargement of some channels in the mesh and the coincident atrophy of others, arterial and venous vessels develop.
These facts are in accord with what we know to be the manner of development of blood-vessels in areas which are open to direct observation in the living animal. I refer, for example, to the studies which have been made ever since the time of Schwann on the tail of larval amphibia, where the transparency of the tissue enables one to see the various structures in their growth and to prove for himself that new vessels are formed by endothelial buds and that the latter in turn form plexuses. It is only necessary to refer here to the classical observations oi' Kolliker (1S46), Remak (1850), Billroth (1856), Strieker (1865), Golubew (1S69), Arnold (1S71), Rouget (1873), Bobritzky (1SS5), Clark (1909), and others. Clark, in a piece of careful work on the tadpole, has now proved that we can thus actually watch the outgrowth and transformation of the primary plexus in an area so large that we may note that what are at one time parts of the most peripheral members of the capillary plexus are actually later used to become the arterial pathwa} r s for capillaries which have extended far more peripheralward.
Fig. 397. — Graphic reconstructions of the blood-vessels present in three stages of early rabbit embryos, showing the formation of the aorta. (.After Tiirstig, Schriften herausgegeben von der NaturforschenGesellschaft an der Universitiit Dorpat, I, 18S4.) A, B, and C, embryos possessing 3, 4, and 7 somites respectively.
I may also point out that this method of blood-vessel formation and growth has also been demonstrated in all cases where it has been carefully studied in the adult, — e. g\, in the vascularization of granulation tissue, new growths, etc.
The cause of the early appearance of vessels in a multiple capillary form is consequently to be found in the view that this represents the fundamental method of vascular growth, and that larger vessels only come into existence secondarily when the number of capillaries induces an increased supply of blood. Such an event leads to the enlargement of certain fortuitously situated capillaries into arteries and veins. The larger vessels are to be considered in the light of servants of the capillaries, for which they are but the delivering and draining pipes. Consequently the cause for the rich vascularity of a tissue cannot be sought in its possession of larger vessels, but rather in the influences which have brought about a more abundant growth of capillaries in it.
It may be noted now that, in addition to the method of capillary sprouts and plexuses, the blood-vessels in some special regions may be looked upon as arising in an essentially different way. I refer to the invasion of large venous trunks by certain tissues in such a way that the trunk becomes broken up into a great number of smaller vessels which now nourish the tissue in question. The fundamental point here is that we have capillaries interposed in a strong venous stream instead of between arteries and veins. The best examples of this are furnished by the invasion of the vitelline veins by the liver tissue, which thus breaks these vessels up into portal and hepatic sj 7 steins, and the invasion of the posterior cardinal veins by the mesonephric tubules, creating a transient renal-portal system (F. T. Lewis, 1904). The vessels formed in this way are markedly irregular and often much larger than normal capillaries. So striking in fact is the picture produced by vessels which have arisen in this way and so many are the points of difference with the usual capillary plexuses that Minot (1900) has designated them sinusoids. It will be necessary now to refer briefly to the capillary plexuses occurring in the development of the embryo and the relation of these to the tissues. It may be stated first of all that no one has been able to verify the exact conception of Aeby, according to which a homogeneous mesh of vessels pervades all the tissues of the body. This is, mdeed, almost as far from the truth as is the existence merely of isolated arterial and venous channels. All recent work has shown that definite vascular and non-vascidar areas exist in the embryo, and that the capillaries grow from a vascular area into an adjoining non-vascular one. Thus, in the beginning the entire embryonic body is non -vascular, and after the formation of the aorta? we can recognize vascular centres or areas from which the capillaries continue to spread into areas which are as yet non-vascular. But the capillaries do not spread evenly in their growth from centre to periphery, thus invading quite uniformly an ever-widening zone, but, on the contrary, are apparently governed, even from the beginning, by the nature of the tissues, some attracting them early and others relatively late. Thus, the central . nervous system is early supplied by a close capillary net ; other areas in the embryonic tissue are apparently inimical to capillary growth, and these constitute distinctly limited non-vascular nones. Of these are to be mentioned those early condensations of the mesenchyme which represent pre-muscle and pre-cartilage masses. We are not improbably dealing here with the question of a chemical stimulant or " tropism " for endothelial proliferation, and may consider some tissues as possessing marked angiotactic properties in contract with a corresponding lack of them in others. It will be recalled that some tissues — e. g., the articular cartilages and cornea — remain nonvascular in the adult.
Fig. 398. — Injection of a duck embryo possessing 13 somites, made while the heart was still beating. Viewed ventrally.
Ail the branches of an artery do not necessarily arise from the same primitive capillary plexus which gave birth to the main stem. Assuredly many branches have emerged with the parent trunk in this way, but, on the other hand, repeated instances can be given of the origin of branches from an embryonic artery after it has become an independent tube. I should assume then that the delay in the elaboration of stronger arterial coats enables the embryonic artery of more naked endothelium to respond to a stimulus and send out branches. The most fundamental example of this is furnished by the main branches of the aorta, for, although the aorta itself arises from a narrow strand of capillaries (Tiirstig), it becomes a large unbranched tube functioning for the vitelline and chorionic capillaries long before it again sprouts out branches. It is true, however, that when these branches arise, they themselves are first in the form of capillaries and often constitute a plexus — e. g., that nourishing the limb buds. All this, then, is paramount to stating that the vascular system does not grow merely at its end bed, — i. e., the capillary area — and for a time during the development of the vascular system this fact must be conceded. 6 We find, then, in the development of the embryo, that the Aeby idea of a uniform all-pervading capillary plexus anlage for the vascular system is far too crude and inexact for the facts, but that the vessels even from the beginning take definite positions and relations to the tissues, and that consequently the main vascular stems which come out of them cannot, as a matter of fact, course in every possible direction.
However, there was still a precious kernel of truth in the old idea. The vessels arise from plexuses which, if not all-pervasive, still have frequent connections with other plexuses. More important still, a functional role is played by the plexuses and the vessels supplying and draining them, and we cannot doubt but that hydro-dynamical grounds often determine which parts of an original
" The ability of an embryonic artery to sprout out capillaries is, however, eventually lost, and in late fetal life, as in the adult, capillary sprouts occur almost exclusively at the peripheral or true capillary bed. In general, then, it may be held that the origin of a vessel from any of the largest arteries assigns it to a quite early embryonic appearance. This may be the underlying cause for the fact that vasa vasorum seldom arise from the vessel which they suppfy, for by the time an arterial wall becomes elaborated enough to need a proper nourishment of its own, the main vessel may have lost the power to send out direct sprouts.
It must be remembered that even the smaller arterioles and pre-capillary vessels of the adult are highly differentiated structures in which muscle and elastic elements occur so that their inability to directly sprout capillary branches is in no contrast with the possession of this power by even the largest of the embryonic arteries, for the latter structures have a greatly simplified histological structure, differing less from the capillaries themselves.
plexus shall be converted into larger trunks. This gives the possibility of variation not only in the exact position of a single trunk but also in the territory supplied by it, for by means of its capillary union with the area of its fellow trunk it may successfully displace the latter.
Repeatedly in the history of the vascular system we find areas which are primitively supplied by many smaller vascular trunks secondarily supplied by a single large one, and this seems certainly due to the fact that the constant presence of a functioning capillary bed enables the successful artery to annex neighboring fields. Whereas in the intestine we have originally a row of vessels which go to the gut wall and yolk-sac, these later give way to three large permanent trunks (aa. coeliaca et mesenteric^), and whereas in the arm bud a row of delicate arterioles nourish the limb, soon fewer, and eventually a single artery possesses this field. This story is repeated over and over again in the vascular system from centre to periphery. It occurs first in the history of some of the main stems, as I have just indicated, but it occurs repeatedly afterwards as the more peripheral vascular tree is gradually developed. 7 All these facts now enable us to understand better many peculiarities of the adult vascular system. Above all, can we appreciate better now a reason in the frequent occurrence of vascular variations, for we see clearly the possibility for channels other than the normal ones to obtain possession of a field. Again, there sometimes occur in embryonic vessels, and more rarely in adult ones, cases of " inselbildungen " where a chief stem is for a short distance reduplicated. See, for instance, the inselbildungen at the origin of the aortic arch s in Fig. 421, or the condition of the a. hyaloidea in Fig. 430. These phenomena are difficult to explain on the basis of our old notions of the outgrowth of naked vascular stems, but appear now as cases in which an arterial stream has for a time retained two paths instead of a single one through its preceding capillary net." We cannot, however, carry this analogy further and proclaim that all instances of anastomosis and of plexiform vessels in the adult are survivals of embryonic conditions, for many of the latter are clearly secondary formations. 10 7 Witness, for example, the history of some of the arm vessels. Goppert (1909) has shown that many branches which are at one time present on the dorsal side of the chief arterial stem are later replaced by a single artery arising in their middle, the a. interossea dorsalis. (Compare his Figs. 7 and 8, Taf. viii, with Figs. 9 and 10, Taf. ix.) 8 This is doubtless due to the tendency of the aortic arches, in common with other vascular trunks, to be formed at first from true capillary vessels which are fundamentally multiple rather than single. Thus I have seen repeated instances in injections of the chick and pig where not one but two or three capillary sprouts are sent out by the dorsal aorta into one of the visceral arches, though only one of these vessels persists to constitute an aortic arch.
- 9 Backman (1909) has recently discussed the view here advanced.
- 10 It could be thought, for instance, in cases where an artery was resolved into a rete mirabile that we had a survival of the primary embryonic net here. That we may err, however, in such an interpretation is clear from the research of Tandler (1906), who showed that the retia mirabilia occurring at the base of the skull in many artiodactyls does not really represent an incomplete resolution of the primitive plexus of the a. earotis interna, but comes from a later series of capillary sprouts which arise directly from the naked carotid stem and plexify. It is, then, likely that many of the " wundernetze " which constitute the arterial channels in some mammals — e. g., edentates and pmsimians, are specialized secondary formations. The extensive subcutaneous venous plexuses of the limbs and body wall of man are also clearly secondary formations. (See beyond.)
But it is to the general conception of the developing vascular system as a connected and functioning whole, which recent studies and especially injections have given us, that a better notion of the formation of variations will accrue. The fact that the arterial current has formed its path from the capillaries, and with the shiftings of growth may form new ones through this mesh, is of the greatest significance. Thus, a chief vessel may channel a new way through the capillary paths connecting two of its branches, the old stem atrophying, and so come to acquire new relations, for instance, to neighboring nerves, as Goppert has recently proved. (See Goppert, 1909, pp. 376—379.) Vascular variations, however, do not occur in an infinite number of ways, because the developing arteries and in fact even the capillary plexuses have definite relations to the tissues. But even with these relations, the tendency to a lingering plexiform type in the main stem and the constant occurrence of the capillary mesh, any part of which may, as it were, be called into service, — all this gives sufficient choice in the selection of a permanent channel to cause the usual variations which are so frequent in the adult.
Exactly what hydrodynamical factors are concerned in the development of arteries and veins from the primary indifferent net which we have seen to exist, are not yet well known. 11 Knower's experiments certainly demonstrate that normal vascular development is dependent on the heart beat.
With our present ideas on the mechanical advantages enjoyed by a wellestablished channel, it might appear all the more remarkable that prominent embryonic vessels are not oftener retained, for example the median artery as the chief stem in the lower arm. But it is probable that continued studies on the manner of vascular development will only strengthen the conviction that the eventual dominance of secondary channels is due to the utilization of an actually better path by the blood when we consider the entire territory to be supplied. The path which the blood takes is dependent from the beginning on the demands of the tissues. Strong growth, which, so to say, sucks the blood in another direction, must play a prominent role in development, and so it may come about that a straight path is actually exchanged for a circuitous one. But this indeed is the whole course of vascular growth, for longer and longer paths are chosen by the developing arterial tree, and we are forced back to the conclusion that the growth and demands of the peripheral or capillary portion of the system exercise a determining influence on the architecture of its main stems, both in embryo, fetus, and adult.
Comparative. — In accordance with a similarity in the general anatomy of vertebrate embryos, we find also a remarkable agreement in the plan of their chief vessels. They furnish us with the opportunity of comparing accurately the vessels 1 The minor differences in the angles at which vessels arise may greatly favor or hinder their acquisition of a large part of the current in a contest of trunks supplying the same field. (Hess, 1903.) We also know, of course, that two distinct types of vessels are differentiated according as they stand in relation with the supplying or draining system, for in the former case we have always small independent thick- walled vessels and in the latter a greater number of large, anastomosing, and thin-walled ones. The measurements made by Mall and his pupils indicate that arterial blood is delivered to the capillaries in the various organs through a much smaller-calibred system than the veins must possess to drain it. Nevertheless an actual role of the circulation in adapting the architecture of the vessels needs to be investigated. (Since this was written Oppel, 1910, has published his extensive discussion of this phase of the subject, and the reader is referred to it.) of various vertebrates. This has been possible chiefly through the mass of splendid comparative researches which we owe to Hochstetter. The reader should consult, for this stand-point, Hochstetter's various researches and his more general pre* tat ions. He will see there that we now possess for many vessels a fundamental vertebrate plan. The first and most brilliant example of such homologies was furnished us by the work done on the homologies of the aortic arches and the vi derived from them. Rathke's (1843) work on the arches in the mammalia is a classic. Late Changes. — Finally, we may remark that the history of the development of the vascular system hardly ends with the establishment of the chief trunks, since the position of many embryonic vessels is far removed from their adult one. A remarkable shifting or wandering process must consequently take place. The studies of W. His gave us a classical example of this in the caudal displacement of the heart and of the great vessels in connection with it, and Mall first called our attention to the cervical position of the intestinal vessels which later shift into the abdomen. These great changes are usually accomplished by the time the human embryo is twenty millimetres in length and finally other, less momentous displacements occur."
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Keibel F. and Mall FP. Manual of Human Embryology II. (1912) J. B. Lippincott Company, Philadelphia.
- XVIII. Development of Blood, Vascular System and Spleen: Introduction | Origin of the Angioblast and Development of the Blood | Development of the Heart | The Development of the Vascular System | General | Special Development of the Blood-vessels | Origin of the Blood-vascular System | Blood-vascular System in Series of Human Embryos | Arteries | Veins | Development of the Lymphatic System | Development of the Spleen
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