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A personal message from Dr Mark Hill (May 2020)  
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I have decided to take early retirement in September 2020. During the many years online I have received wonderful feedback from many readers, researchers and students interested in human embryology. I especially thank my research collaborators and contributors to the site. The good news is Embryology will remain online and I will continue my association with UNSW Australia. I look forward to updating and including the many exciting new discoveries in Embryology!

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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B. Description of the Vascular System Present in Early Human Embryos

Evans HM. The development of the vascular system. In Keibel F. and Mall FP. Manual of Human Embryology II. (1912) J. B. Lippincott Company, Philadelphia. pp570-708.

Herbert McLean Evans
Herbert McLean Evans (1882—1971)

By Herbert M. Evans. Johns Hopkins University, Baltimore.

Embryos Which As Yet Possess No Mesodermic Somites

It is certain that long before any vessels are present in the body of the human embryo, and at a time so early as considerably to precede the formation of any somites, typical " vascular anlagen" are found scattered over the ventral pole of the yolk-sac.

In those mammals which, like the rabbit, possess a vitelline vascular area of the limited circular form, bounded by a marginal sinus, which characterizes the lower vertebrates, it is probable that the vascular anlagen first form in a ring-like row around the borders of the future area vasculosa, as Van der Stricht has described for the rabbit. But in the primates (and presumably in all other mammals in which the yolk suffers a complete overgrowth by the area vasculosa, — e.g., carnivores and artiodactyls) the vascular anlagen are most irregularly scattered, covering at an early date the whole ventral surface and soon all the yolk-sac.

These vascular anlagen or blood islands, as in other vertebrates, appear as nodular swellings of the wall of the yolk-sac, and consist microscopically of circumscribed cell clumps lying between the mesoderm and entoderm. The cells of these clumps very early show a differentiation into centrally lying blood-cells and a row of peripheral bounding cells, — the endothelium. The best-developed, and hence earliest, of these anlagen are situated more ventrally, the younger nearer the body of the embryo, in very early stages they can be shown to be concerned in the formation of the vessels in the belly stalk (aa. umbilicales), and these vessels belonging to the placental circulation, are so exaggerated in development as to precede the appearance of vessels in the embryonic body proper. Furthermore, as Eternod discovered, when, later, the vascular trunks of the embryo proper make their appearance (the aorta? and vv. umbilicales), they are already connected with the chorionic capillaries through the precocious aa. umbilicales

  • It may perhaps be mentioned that those who, like Hubrecht (190S), consider that a considerable part of the early mesoblast has really come from the entoderm will dispute the above statement.


Fig. 401. Section of a vascular anlage in the wall of the yolk-sac in the human embryo 2 mm. long shown in Fig. 408. E., entoderm; V., vascular cells; M., usual mesoderm cells.


Fig. 402. Section of a more advanced vascular anlage from the yolk-sac of the same embryo as shown in Fig. 401. EC endothelial cell.


Fig. 403. Section of a well-developed vessel from the same yolk-sac.

and w. chorioplacentares, and so it comes about in the human embryo that an umbilical circulation exists in embryos so young that the mesoderm is as yet unsegniented into somites.

The embryos which I have been able to examine with respect to their early vessels constitute, together with that so fully described by Eternod, the following series, given in order of their probable age.

Designation of embryo Length of embryonic shield Collection Literature
Von Herff .37 mm Graf Spee Graf Spee, Arch. f. Anat. u. Phy., 1896.
Frassi, NT. 1 1 17 mm Prof. Keibel Frassi, Arch, f . mik. Anat., Bd. 70 u. 71.
Glaevecke 54 mm Graf Spee Graf Spee, Arch. Anat. u. Phy., 1889, 1896.
Eternod 1.3 mm Prof. Eternod Eternod, Anat. Anz., Bd. xv, No. 11, 12, 1898.

For further descriptions of the embryos themselves the reader is referred to Chapter TV of the present work, where this has been done by Prof. Keibel. Here we need be concerned only with remarks on what blood-vessels are present.

The embryo von Herff, the youngest, and probably only consisting of the region of the primitive streak, possesses abundant vascular anlagen scattered over the entire ventral and part of the. lateral surfaces of the yolk-sac, reaching often to the angle of junction of the yolk-sac with the embryonic shield. Whereas in general those of the anlagen whose development seems most advanced are more ventrally situated, there exist also many not widely separated from the embryonic body, in which an evident differentiation into endothelium and blood-cells has come about. In the belly stalk and chorion of this embryo there are, as Graf Spee has described, highly characteristic strands of spindle cells, which often consist of a double row of nuclei and, again, may enclose a distinct lumen. These cells appear to keep to themselves, and to constitute a single unified but widely branched tissue which grows oftenest in strands frequently anastomosing among themselves. This tendency to constitute a distinct tissue element different from the connective tissue, together with the histological appearance of longer oval nuclei and more deeply staining cytoplasm, suggests strongly that we may be dealing here with endothelium, a conviction strengthened by the typical vascular appearance given in those instances where the cells surround a distinct lumen. 15 The embryo Frassi, which now, besides the primitive streak region, shows a considerable embryonic area in front of this (the two being separated by a typical canalis neurentericus) also exhibits many evident blood-vessels. Not only have we here again an abundance of well-differentiated vascular anlagen on the ventral walls of the yolk-sac, but in many of the sections through the belly stalk vessels can be recognized (one of these being especially large), and this is also the case in the chorion.

The embryo Glaevecke of Graf Spee (NT. 2) contains, besides many yolk vascular anlagen and chorionic vessels seen in the preceding stage, the first vascular cells within the body of the embryo itself. In the region of the heart these constitute a true typical endothelial anlage for that organ (Fig 404), but also further caudalward there can be recognized many cell strands clearly isolated and different in character from the endoderm and mesoderm between which they lie; they thus occupy the typical position for, and present the typical appearance of, early vascular cells 16 (Fig. 405). At first lying about half-way between the point of insertion of the yolk-sac and the mid-line of the body, they gradually shift lateralward, and before the neurenteric canal is reached occur quite exclusively only at the 15 If such an interpretation be correct we must have a remarkable growth of the endothelium in the chorionic membrane of young human embryos, for there occurs here no coincident development of blood-cells, as is typically the case in the vitelline anlagen. These cells of the chorion (Spee's so-called spindle-cells) are present in relatively great numbers and, so far as I am aware, cannot be distinguished from similar cells in still younger embryos (e.g., that recently demonstrated by Fetzer [1910] ), where we are quite unable to distinguish vascular beginnings on the yolk-sac proper. The above suggestion, however, appears to me forced on one who now takes up the study of a series of progressively slightly older stages, such as we have in the embryos here dealt with, where difficulty would be experienced in separating these cells from those which gradually are concerned in the formation of undoubted vessels.

16 Concerning the origin of these intra-embryonic vascular cells, it can only be said that the histological appearances are inconclusive, and one may often see what might be taken for a genetic connection of the cells with the intermediate mass of the mesoderm as Mollier (1906) has reported in reptilian and avian embryos. On the other hand, however, the series of sections does not permit one to exclude the strong possibility that these cells constitute a connected unit which could have invaded the embryonic body from the splanchhopleure of the yolk-sac.

lateral margins of the embryonic area and very near the insertion of the yolk-sac. In the area behind the neurenteric canal these cells apparently retreat to the upper margin of the yolk-sac proper. This parallels the condition found in this area in the younger embryo von Herff. Finally, as the allantois is given off, these vascular anlagen can be traced into vessels which in the belly stalk lie at first on either side of and soon below the allantoic diverticulum; they are consequently in the position typical for the umbilical arteries and doubtless represent the anlagen of these vessels in the belly stalk.

We turn now to the embryo described by Eternod (1898) and in which we have the earliest circulatory conditions in the human embryo. However, a considerable gap exists between the stages which we have just been considering and that depicted by Eternod, for in the latter case a system of vessels are now present coursing through the body of the embryo, — the aorta? and umbilical veins. Exactly how these first vessels are formed in man is as yet unknown. The umbilical veins, the heart, the aorta? and umbilical arteries, and, finally, the chorionic capillaries, form the simple vascular cycle here present (Fig. 406)." The Eternod embryo measures approximately 1.3 millimetres in length and has also as yet no indication of mesodermic somites. It shows an anteriorly placed heart, the short aortic end of which, doubtless representing the future bulbus aortae aud aorta ventralis, gives off the aortic arch 18 which sweeps up on each side to the primitive aorta. The aortas course dorsal to the head-gut and on either side of the notochordal plate, and at last turning down sharply into the belly stalk, run out onto the chorion without having given any branches into the tissues of the embryo. The umbilical veins (w. umbilicales primitives), which collect the blood from the extensive chorionic vessels (w. chorio-placentares), unite in i lie belly stalk into a single trunk (v. umbilicalis Lmpar) 3 but again pari as the embryo proper is reached, and course in the body wall on either side near the attachment of the amnion. Just as they begin their embryonic course, each umbilical vein receives a large tributary from the capillary plexus nf the yolk-sac, and these two tributaries anastomose with one another on the wall of the yolk-sac so that a venous ring is produced enclosing the allantois (ansa vitellina). The significance of this is unknown. This connection of the vitelline vessels with the umbilical vein gives the possibility of an early drainage of the yolk-sac in that direction were any aortic afferents traceable to the vitelline plexus. But at a time when such afferents clearly supply the vitelline plexus, the true vitelline veins are formed, so that at the time when we are first able to affirm the possibility of a complete vitelline circulation it is supplied and drained, as in all mammals, by its own system of vessels.[1]


Fig. 404. Cross section of the human embryo Glaevecke (collection of Graf Spee), taken just in front of the anterior intestinal portal, showing the vascular cells constituting the anlage of the endothelium of the heart. X 113.

17 As yet, though there are many vessels on the yolk-sac, particularly on its ventral surface, no evidence for a vitelline circulation exists, for no connections between these capillaries and the aorta? can be traced. This, the most revolutionary result of Eternod's study, appears to place man unique among mammals in the ontogenetic precedence of the umbilical over the vitelline circulation, for in the mammalia generally, as is well known, the yolk-sac circulation is always primary.


Fig. 405. — A series of sections through the human embryo Glaevecke, showing the earliest intra-embryonal vascular cells and the anlage of the aa. umbilicales in the oelly stalk.

M It is certain that we are not dealing here with three aortic arches and that the picture given by Eternod is somewhat schematized, the small irregular vessels here likely being persisting strands of a capillary plexus. (See Fig. 398 of a duck.)


Fig. 406. — Lateral view of the vascular sj stem in a human embryo 1.3 mm. long, without somites. (After Eternod, from Kollmann, Handatlas der Entw. d. Mensehen. 1S97 ' »g. 512.)


Embryos Possessing from Six to Eight Somites

Unfortunately, we possess as yet no human embryos belonging to the interesting period in which the first five somites are formed, but for stages only shortly after this several excellently preserved and trustworthy specimens are now known. I base what can be said about the vascular system at this stage chiefly on the study of the following four embryos :[2]

Designation of embryo Number of somites Collection Literature
PfannenstielKroemer NT. 3 5-6 somites Keibel-Elze, 1908. Felix, 1910.
Graf Spee embryo 6-7 somites Graf Spee Graf Spee, 1887. Kollmann, 1889, 1907 (Figs. 187 and 188).
7-8 somites Prof. Mall Dandy WE. A human embryo with seven pairs of somites measuring about 2 mm in length. (1910) Amer. J Anat. 10: 85-109.
Eternod's embryo 8 somites (the 8th not completely separated) Prof. A. C. F. Eternod Eternod, 1896, 1899, 1904, 1909. Kollmann, 1907 (Figs. 183 and 184)

The most striking change which has occurred in the vascular system of these embryos is found in their possession of the first branches of the aorta. The majority of these aortic branches go to the yolk-sac (aa. vitelline primitivae), and, though at present appealing as almost frank lateral branches (Figs. 407 and 444), in later stages are shifted so as to come off more ventrally.

The primitive vitelline arteries form an irregular series of connections between the aortse and the vitelline capillary plexus which has arisen out of the early yolk anlagen. They are not, as a rule, at first segmentally arranged. Cephalad they may extend as far as the first intersegmental cleft, as Dandy (1910) first showed. In the Pfannensteil-Kroemer embryo the first of these appear opposite the third somite on the left, and in the Eternod embryo between the third and fourth somite on the right. Occurring generally so frequently as to be opposite each somite thereafter (though they occupy no constant position with regard to the somite mass), when the unsegmented mesoderm is reached they are found in far greater numbers, and eventually resolve the termination of the aorta itself into a plexus of capillary-like vessels not unlike that to be seen in injections of chick embryos.[3] With this plexus, as Felix (1910) has shown, and as I can confirm, the umbilical artery is in connection and so by these multiple roots takes its origin from the aorta. It is by means also of the farther caudal growth of this plexus that the aorta is continued caudally and the a. umbilicalis wanders caudalward through a considerable distance. These facts establish clearly that the umbilical artery is merely a modified vitelline vessel; for a considerable time its roots of origin from the aorta are indistiguishable from the row of primary vitelline arteries.


Fig. 407. Cross section of a human embryo with six somites (NT. 3), in the region caudal to the last somite, showing the delicate aorta and their vitelline branches.

Headward the vitelline plexus is connected on each side with the heart by two primitive vitelline veins, which receive the umbilical veins which have coursed in the somatopleure and then turn in sharply from their lateral position to gain the heart; in doing


Fig. 408. Dorsa view of model of human embryo possessing 7-8 somites, being the same embryo shown in Fig. 23 B (ante, p. 32). Portion of ectoderm of right neural plate is removed, showing thickness of wall and its relation to deeper structures. The three primary cerebral vesicles are indicated. (After Dandy.) All., allantois; Ch.. chorda; Coe., ccelom; Fg., fore-gut; Hg., hind-gut; Ht., heart; Mes., mesoderm; P.c, pericardial ccelom; U ., umbilical arterial sinus; V ., umbilical vein. (Mall, No. 391 J

this they traverse the bar of mesoderm which intervenes between the pericardial cavity and the yolk-sac wall and which is destined to constitute the septum transversum of His. Their course here hence resembles entirely that taken by the terminal portions of the primitive vitelline veins in other very early mammalian embryos {e.g., the rabbit), and 1 present here in lieu of a more detailed description a series of accurate tracings of their course (Fig. 409). Besides the vitelline circulation, which is thus well established


Fig. 409. A series of sections through the human embryo with 7-8 somites (shown in Fig. 408), showing the relation of the chief venous stems to the heart.

in these embryos, other vascular channels are beginning to be formed; these constitute the first endothelial sprouts to be sent out into the tissues of the embryo proper; arising dorsally from the aorta, they lay the anlage for the a. en rot is interna in the region of the fore- and mid-brain, whereas farther caudally they form a series of presegmental and segmental dorsal offshoots of the aorta. In all cases these tiny vessels are directed to the sides of the neural tube, which consequently, neglecting the primitive gut and yolk-sac, must be considered the first embryonic tissue to receive vessels; this occurs, in fact, before the nervous system is in the form of a tube, for it is, in the first of these embryos, a widely open furrow. Each vessel, having reached the side of the medullary furrow, divides T-like and can be traced a short distance caudally and cephalically. It is by the anastomosis of these branches that in older embryos (those of 15 somites) a longitudinal vessel is established at the sides of the hind-brain and the neural tube caudal to it {v. capitis medialis). The dorsal segmental arteries have long been known, for they occupy accurately the interspaces between the somite masses ; some four of them are already present in the Eternod embryo with 8 somites, and, since these are progressively smaller in size cephalo-caudally, their outgrowth from the aorta quite certainly proceeds in this sequence.


Fig. 410. Reconstruction of the arterial system of a human embryo with 6 somites (NT. 3), seen from the left. (Modified after W. Felix, 1910.)

Embryos Possessing From Thirteen To Fifteen Somites

Designation of embryo Number of somites Collection Literature
Bulle, NT. 5 13-14 somites Prof. Kollmann Kollmann, 1889, 1907.
Pfannenstiel III, NT. 6 14 somites Keibel F. and Elze C. Normal Plates of the Development of the Human Embryo (Homo sapiens). (1908) Vol. 8 in series by Keibel F. Normal plates of the development of vertebrates (Normentafeln zur Entwicklungsgeschichte der Wirbelthiere) Fisher, Jena., Germany. Low A. Description of a human embryo of 13-14 mesodermic somites. (1908) J Anat Physiol. 42(3): 237-51. PMID 17232769 | PMC1289161 Felix, 1910.
Graf Spee No. 52 15 somites Graf Spee

In human embryos which possess some fifteen somites we not only have an increased number of dorsal segmental arteries (eleven in the embryo with fifteen somites) and of the primitive vitelline arteries, but also another set of aortic branches which I shall designate as the primitive lateral branches of the aorta.[4] These vessels take origin from the lateral aortic wall, often from its ventro-lateral angle, and course obliquely upward and outward in the space between the somite and the intermediate mesodermic mass; here I have seen them anastomose with the cardinal vein; they are also often connected with the dorsal segmental arteries by direct cross anastomoses in the loose mesoderm of the intersomitic clefts.


Fig. 411. A series of cross sections through a human embryo with 15 somites (collection of Graf Spee, No. 52), showing the course and relations of the primitive head vein (v. capitis medialis et v. cardinalis anterior) and the relation of the chief venous stems to the heart.


Fig. 412. Reconstruction of the arterial system of a human embryo with 14 somites (NT. 6), seen from the left. (Slightly modified, after W. Felix, 1910.)

The primitive vitelline arteries in the area opposite the first five somites have atrophied, but the series of these vessels begins from here caudally to form a continuous row unrelated apparently to metamerism and finally, in the unsegmented area, giving way, as in the younger embryos, to a plexus from which the umbilical artery takes origin.

The first venous channels of the embryo proper — the w. cardinales anteriores — are found at this stage, and in the Graf von Spee embryo with 15 somites can be traced clearly from the region of the optic vesicles, cephalically, to their opening into the common vitelline and umbilical vein, caudal ly. These veins, as Grosser (1907) has shown to be probable for all vertebrates, in man also possess two different and distinct topographical relations, for in their cephalic course they lie close to the sides of the neural tube, constituting the v. capitis medialis, whereas more caudally — i.e., in the region beginning with the first mesodermic somite — they take up a lateral position between the somite and the ccelomic mesoderm where thev may be designated the true vv. eardinales


Fig. 413. Sinus venosus and septum transversum in a human embryo with 14 somites (NT. 6), viewed from above. The right half of the septum has been removed to show the sinus venosus contained therein. (Drawn from the model by Dr. Alex. Low.) anteriores. Opposite the third somite the vessel finally joins the common vitello-umbilical vein by coursing dorsal to the ccelomic cavity in this region (Fig. 411). The v. capitis medialis receives several (four) direct dorsal offshoots from the aorta, so that it really appears as a longitudinal neural anastomosis of these presegmental dorsal arteries ; it turns out rather sharply somewhat in front of the first somite to constitute the true anterior cardinal, which is again formed apparently by a laterally-situated longitudinal anastomosis of loops formed by the dorsal segmental vessels; to it also the primitive lateral branches of the aorta are joined.[5]

The vitelline veins still behave in all essentials as in the younger stages ; they receive the umbilical veins when quite lateral in position, and the common veins receive the anterior cardinals, turning in sharply to constitute the sinus reuniens (Fig. 411).

Embryo of Twenty-Three Somites

In stages which are intermediate between those which have just been described and embryos possessing limb buds, the posterior cardinal veins develop. This has already occurred in the embryo with twenty-three somites (Eobert Meyer, 300, N.T. 7), which has no indication as yet of limbs. It is probable that lateral loops of the dorsal segmental arteries are instrumental in the formaton of these veins, as is the case with the anterior cardinals.[6] At this stage the dorsal segmental vessels form in the tissue of the intersomitic clefts large well-marked vascular arches or loops, one limb of which is against the neural tube while the other joins the cardinal vein (Fig. 436). At this stage also the primitive lateral branches of the aorta form an extensive system, and at many levels we are able to find all three systems of branches occurring together and segmentally arranged.

Keibel Mall 2 414.jpg

Fig. 414. Reconstruction of the arterial system of a human embryo with 23 somites (NT. 7). (Aftei W. Felix, 1910.)

Keibel Mall 2 415.jpg

Fig. 415. Cross section of the human embryo with 23 somites, shown in Fig. 414, taken through the region of the 9th somite, showing paired aa. vitelline.

The row of vitelline arteries is by no means exclusively segmentally arranged; nevertheless there is a symmetrical disposition in that these vessels occur in pairs, so that in the region in which the two aortae primitive have fused to a single median aorta we can observe that two vessels arise from the ventral surface of the aorta and course each on its corresponding side of the gut, which possesses as yet no mesentery (Fig. 415). When later an intestinal mesentery is formed, these vessels course for a time side by side, but eventually are completely fused to a medial ventral trunk, or it is possible that one member of the pair gains the ascendency and its fellow atrophies.

In the posterior region of the body we find not only the posterior cardinal vein, but a new one, lying ventral to the former and near the ccelomic epithelium. This vein, the v. subcardinalis (F. T. Lewis, 1902), has probably arisen by sprouts from the posterior cardinal trunk, as Graefe has shown for the chick; at any rate the presence of a large number of anastomoses between these two vessels speaks strongly for this view. In the region of the mesonephros the subcardinal vein occupies a characteristic position ventral to the )Yolffian duct, but at levels above this region, where we have as yet, according to the view of Felix, only the pronephric anlage, the vein is also found, and in the same position, i.e., ventral to the chief duct, as Fig. 416 will show. There also occurs in embryos of this age another vein medial to the pronephros, and it has probably arisen as a longitudinal anastomosis binding together vascular offshoots from the posterior cardinal veins and also the primitive lateral branches of the aorta.[7]

Keibel Mall 2 416.jpg

Fig. 416. Section showing the position of the v. cardinalis posterior and the v. subcardinalis in a human embryo with 23 somites (NT. 7), taken in the region of the 12th somite.

Embryo of 4.9 mm. Length

(35 Somites, N.T. 14).

By the time the embryo reaches a length of 5 millimetres several important changes in the vascular system have occurred. The embryo described by Ingalls (1907) and shown in Figs. 417, 418 will serve to illustrate this stage.

It will be noted that four complete aortic arches are present, and that another pair — the sixth or pulmonary arches — are being formed by both ventral and dorsal endothelial sprouts. The third pair, the carotid arches, are by far the largest of the series, while the first are already very much reduced. The aortic root on each side now appears to continue toward the head beyond the location of the first arches. This, the internal carotid artery, is doubtless the trunk representing the very early capillary sprouts which the first arch sent toward the brain. It courses headward lateral to the hypophysis and bending dorsally anastomoses with a long branch — the a. vertebralis cerebralis — given off from the first of* the dorsal segmental arteries here present (in this case the hypoglossus artery). At the optic cup the internal carotid gives off the a. ophthalmica as its first branch, and somewhat beyond this a very large branch (a. cerebri ant. et med.) which courses forward between eye and brain; other smaller branches are given off to the mid-brain region, and the carotids then sweep backward to join the cerebral vertebrals, which they furnish with their main volume of blood, although later the stem of origin of the latter vessel gives it most of its blood. The first cervical dorsal segmental artery[8] anastomoses with the hypoglossus, and consequently the path is already furnished for this stem to take over the cerebral vertebral when the hypoglossus yields the current and atrophies. Excluding the hypoglossus vessel twenty-seven dorsal segmental branches arise in pairs from the aorta and sacralis media artery, — i.e., the full number of cervical, thoracic, and lumbar vessels and the first two sacral segmentals. The umbilical arteries, though they later shift to the last lumbar level, arise here opposite the third lumbar segmentals, the remaining lumbar arteries at this stage consequently arising from the a. sacralis media. The aa. umbilicales course medial to the Wolffian ducts, but at the prominent bend which they make in turning upward are in connection with capillaries lying lateral to the Wolffian duct, which ultimately gain a connection with the aortic wall and completely displace the medial roots of origin of these arteries. The subclavian artery arises from the seventh dorsal segmental pair.

Most interesting are the ventral branches of the aorta, for these no longer form a uniform row of vessels, but reflect a beginning differentiation of the gut. Consequently there are retained, besides many smaller ones, three chief branches, to correspond to the stomach-pancreas region, the vitelline-duct region, and the colon respectively (a. cceliaca, a. omplialomesent., et a. mesent. inf.). The middle of these three stems, which is also by far the largest, since it drains yolk-sac as well as gut, takes origin from

Keibel Mall 2 417.jpg

Fig. 417. Reconstruction of the arterial system in a human embryo 4.9 mm. long, lateral view. After Ingalls, Arch. f. mik. Anat., Bd. 70, p. 530, 1907.) A.c, a. cceliaca; A.c. a., a. cerebralis ant.; A.c.s., a. caudalis sin.; Ag., optic vesicle; Al., allantois; A.m.i., a. mes.inf.; A. o., a. ophthalmica; A. omphal., a. omphalomesenterica (with three roots); A. p., a. pulmonalis; A.s., a. subclavia; A.u.s., a. umb. sin.; A. v., a.vertebralis; B. 1,2,8,4,6, aortic arches; C.a.l, first cervical artery; Cd., caudal intestine; D.p., dorsal pancreas; D.i., ductus vitello-intestinalis; Gb., gall-bladder; Ha. , hypoglossus artery; lb., inselbildung; La., lunganlage; M., stomach; Oes., oesophagus; T. a., truncus arteriosus; UK., lower jaw; V., questionable union between the a. vertebralis and a. car. int. (N. T. 14. ) the aorta by four distinct roots. It will be noticed that all these branches are much above their location in the adult, as can be seen by comparing them with the dorsal segmentals opposite, and it is not indeed until the embryo attains a length of from 16 to 20 millimetres that their definitive position is reached.

Keibel Mall 2 418.jpg

Fig. 418. Reconstruction of the venous system of the embryo shown in Fig. 417. A., fibres of the accessorius; C.l, first cervical segment; D.C.8., ductus Cuvieri sin.; F., facialis; G., glossopharyngeus; L.l, first lumbar segment; 0., ear vesicle; 0.1, first occipital segment; S-l, first sacral segment; T., trigeminus; T.l, first thoracic segment; V., union of the left umbilical vein with the liver circulation; V.c.a., v. card, ant.; V.c. p., v. card, post.; V. i., v. ischiadica; V.u.s., v. umb. sin.; V.u. 8.*, remains of the original circulation to the sinus venosus; X., linguo-facial vein.

Irregularly arising, lateral branches of the aorta go to the mesonephros.

In contrast to the earliest stages, the venous system of the embryo proper is now well developed, and one sees the well known fundamental pattern of the two cardinal veins on each side uniting to form the ductus Cuvieri which then joins the umbilical. The anterior cardinal can be seen beginning in two strong efferents in the head region, the first of which doubtless represents the ophthalmic vein. Passing medial to the ganglion of the fifth nerve, the main vein next receives a tributary from the hypophysis region, and continues caudally on the lateral side of the acusticofacial ganglion, the auditory vesicle, and the ganglion of the glossopharyngeus, but medial to the vagus ganglion ; just before reaching the latter nerve, it receives a prominent tributary from the dorsal region (v. cerebri post.. Mall), although smaller tributaries have joined it all along its previous course. Before joining the ductus Cuvieri, several venules run into it, which, from their position and correspondence with the veins of other mammalian embryos, can be recognized as the segmental veins belonging to the first cervical and the several occipital segments. The ductus Cuvieri receives on each side a slender venule, which drains the capillary plexus in the first visceral arch. This vessel crosses from the latter • into the ventral body wall (here constituted by the membrana reuniens over the front of the heart) and runs in this to open into the ductus (v. linguo- facialis, F. T. Lewis, 1909).

The posterior cardinal vein begins at about the level of the third lumbar segment, and courses in the tissue dorsolateral to the Wolffian body. It receives the dorsal segmental veins, which do not become appreciable structures until about the level of the fifth thoracic segment. The seventh cervical segmental vein receives the vessel from the arm bud (the subclavian vein), although this afterward shifts up to the anterior cardinal vein. The v. subcard inalis can first be recognized at about the level of the seventh thoracic somite, and empties into the posterior cardinals at the level of the sixth cervical one. They drain the Wolffian body at this stage, but later acquire greater significance inasmuch as they are incorporated in the formation of the inferior vena cava. The umbilical veins are already sending their main mass of blood into the liver, but with their old connections with the sinus venosus still evident. This uppermost and superficially lying part of the umbilical vein receives tributaries from the arm buds, and this source of blood delays their atrophy (Evans, 1909). The vessels from the lateral body wall also drain into the v. umbilicalis along all of its course until the liver is reached, so that the vein forms at this time an important drainage channel for the entire lateral body wall. The vitelline veins empty their blood directly into the liver sinusoids, the blood from the left omphalomesenteric vein being collected by a short trunk which enters the left horn of the sinus venosus (r. hep. sinistra) ; but the right and larger one possesses a wide passage through this organ to the right horn of the sinus venosus. The two vitelline veins are anastomosed on the ventral, then on the dorsal, and again on the ventral sides of the duodenum, forming thus two venous rings around the gut (His).

Embryo of 7 mm Length

(40 Somites, X.T. 28).

The vascular system present in an embryo measuring 7 millimetres begins to be complex enough to demand detailed descriptions for many areas, so that with a brief presentation of the chief features here, we may leave this account of the early vascular system as a whole and turn to the explicit history of the various vessels. The main blood-vessels in an embryo of this length (7 mm.) are well known to us through the papers of Mall (1891),

Keibel Mall 2 419.jpg

Fig. 419. Profile reconstruction showing the arterial system of the head and neck in a human embryo 7 mm. long. (N.T. 28.) fAfter Elze, Anat, Hefte, Bd. 35, Heft 106, Taf. 16, Fig. 3.) S, S, 4, 6, and 6 AB, aortic arches; A. bas., a. basilaris; A. car. ext., a. carotis externa; A. p., a. pulmonalis; A.v.c, a. vertebralis cerebralis; Obi., ear vesicle.

Piper (1900), and Elze (1907) ; the latter's figures are here reproduced and his description largely followed (Figs. 419 and 420). Three complete aortic arches exist, the third, fourth, and sixth. The first pair, already very weak in the preceding embryo (4.9 mm.), are now entirely atrophied, but both dorsal and ventral end pieces of the second arch are recognizable, the dorsal remnant, in fact, being destined to constitute the trunk of origin of the stapedial artery (Tandler, 1902). The upper end of the sixth arches sends out, between these and the fourth pair, a small vascular fragment, which perhaps represents the persisting upper end of a previous transitory fifth arch. The aorta ventralis in the area of the first and second arches now constitutes the a. carotis externa which reaches into the region of the upper jaw. The internal carotid artery, after giving off the dorsal rudiment of the second arch, courses headward to give off in the region of the hypophysis a branch which courses toward the brain beneath the optic cup, then above this, a small branch to the latter (a. ophthalmica), and dorsal to the eye, a large branch which apparently supplies the main portion of the fore- and 'tween-brain (a. cerebri ant. et med.). After giving off other branches to the lateral side of the 'tweenand mid-brain, the artery ends in its ramus communicans posterior which appears to continue the main trunk into the basilar artery. The a. vertebralis cerebralis now arises from the first cervical segmental artery, and the preceding hypoglossus vessel has entirely disappeared. The stem of the first cervical vessel, however, has been shifted cranially until it is now opposite the sixth aortic arches, whereas earlier even the hypoglossus artery arose relatively further caudad. The two cerebral vertebral vessels unite in the mid-ventral plane to form the a. basilaris, which extends from the area opposite the vagus nerve to the vicinity of the oculomotor nerve, where it splits into the two posterior communicating rami which connect it with the internal carotids. Both the cerebral and the basilar arteries send off many branches to the hind-brain, some of the branches of the former anastomosing dorsal to the emerging fascicles of the twelfth nerve, so that these fibres appear to go through arterial fenestras. The full number of cervical, thoracic, and lumbar segmental arteries exist and all but the last sacral. The umbilical arteries come off the aorta at the level of the last lumbar segments, and now course lateral to the Wolffian ducts and send each a small branch into the posterior limb (a. ischiadica). The ventral branches of the aorta have been reduced to three main trunks: the a. cceliaca arises opposite the fourth thoracic artery; the a. omphalomesenterica is two-rooted, its upper root coming from a level slightly above the fifth thoracic artery, its lower one opposite the sixth; while the a. mesenterica inferior arises opposite the second lumbar vessel.[9]

The veins show several important changes. The proportionately great growth of the head gives a great drainage area for the anterior cardinal vein, which is consequently much increased in size. Its foremost tributary is the paired primitive sinus sagittalis superior on the top of the fore-brain. These are the only tributaries to reach the mid-dorsal plane, with the exception of a very short partly paired one on the roof of the mid-brain.[10] Several tributaries from the first two visceral arches reach the main trunk, while behind the ear vesicle the posterior cerebral (Mall) vein has grown to great proportions. The anterior cardinal, proceeding caudally, surrounds the vagus by a venous ring and then goes under the hypoglossus to join the posterior cardinal trunk. The ling no -facial vein (F. T. Lewis) is now no longer a tributary of the ductus Cuvieri, but joins the cardinalis anterior.

Keibel Mall 2 420.jpg

Fig. 420. Profile reconstruction of the same embryo, showing general arterial and venous system. (After Elze, Taf. 15, Fig 2.) X 26.5. A. coe„ a. cceliaca; A. i., a. iliaca; A. s., a. subclavia; A., a. omphalomesenterica; Atr. s., atrium sinistrum; A. u. s., a. umbilicalis sin.; D. At., ductus venosus Arantii; V. card, a., v. cardinalis anterior; V.cap.l., v. capitis lateralis; V.extr., extremity vein; V. hep.r.s., v. hepatica revehens sinistra; V.o.m., v. omphalomesenterica;, v. omenti minoris; V.mes.s., v. mesenterica superior; V.u.s., v. umbilicalis sin.; V.subc, v. subcardinalis; D.C'uv.s., ductus Cuvieri sin.; 7, 20, 25, 29 SA., segmental arteries; Sin. ven., sinus venosus.

The posterior cardinal vein, receiving the same number of dorsal segmental tributaries as is sent out from the aorta and sacralis media artery, drains also the marginal vein of the leg bud and along its entire length receives efferents from the Wolffian body; but the axillary vein now reaches the ductus Cuvieri and will soon indeed be a branch of the anterior cardinal trunk. The subcardinal veins (not illustrated) exist from the level of the tenth thoracic segment caudally and are in frequent communication with the corresponding posterior cardinal. The umbilical veins can no longer be traced to the ductus Cuvieri on either side, and the superficial portion of the primitive vein is only represented by several small stems draining from the body wall into the main trunk just before it plunges into the liver. The left umbilical is by far the larger of the two, and the same is true for the vitelline trunks; the right vitelline indeed has atrophied practically completely, and its previous large pathway through the liver has given way to many sinusoidal paths, whose supplying or portal stem may be called the ramus arcuatus while the corresponding draining or hepatic venule is the v. hepatica dextra. The v. hepatica sinistra still opens independently into the sinus venosus. The main mass of the umbilical blood takes a direct path through the liver in the ductus venosus (Arantii), which receives several small veins from the caudal end of the stomach (Magenvene, Hochstetter, 1893; v. omenti minoris, Broman, 1903). From the liver capillaries a slender vessel grows out into the caval mesentery, the anlage of the v. cava inferior.


  1. Eternod has pointed out that in the Selenka specimen of Hylobates the vitelline plexus is also shown in connection with the chorionic vessels by means of two stems which surround the allantoic tube in reaching the belly stalk, and in the latter they fuse to a single large vessel (v. umbilicus impar) which can be traced into the chorionic vascular tree. The ansa vitellina may consequently be a characteristic primate structure, but it is impossible in the light of present knowledge to assign to it any significance. It is quite possible that these vessels merely represent a persisting connection of the vitelline and chorionic vessels indicating a primitive common anlage.
  2. The manuscripts of Dandy and of Felix were generously placed at my disposal by their authors and were of much service during the study of the embryos concerned.
  3. Felix (1910) is unable to follow the aorta throughout its entire extent in the first of the above embryos (NT. 3). indicating his belief that it is not present in some places (see his account, p. 603). I am not able to agree #with his account, nor with his statement concerning the intestinal vessels (p. 606) — " cranialwarts offnen sich seine Gefasse teilweise frei in die primare Leibeshohle " !
  4. Grafe (1905) was, I believe, the first to see these vessels, describing them in the posterior portion of a chick embryo of about sixty hours; but their cephalic extension was shown by Williams (1910), who described them in the first two intersegmental clefts.
  5. This history of the formation of the anterior cardinal vein in man is thus identical with that which has been previously found in the chick (Evans, 1909), in which latter embryo Williams has recently described the same phenomena in a careful account of the region about the second somite. It is consequently probably significant that the picture furnished by the endothelial cells constituting the first dorsal segmental vessel in the Eternod embryo shows a marked lateral wandering of the endothelium (Fig. 439). This probably should be accounted the first staee in the formation of the anterior cardinal in man.
  6. This method of formation of the posterior cardinal veins appears fundamental. Raffaele (1892) and Hoffman (1893) described it for selachian embryos and Grafe (1905) and the writer have indicated it in the case of the chick.
  7. One finds in this embryo pictures which very much resemble that given by Grafe in his table 11, Fig. 7, for a chick of 71 hours.
  8. I refer to the segmental artery cranial to Hochstetter's "first cervical artery," naming that artery the first cervical which courses with the first cervical nei've, as do Mall. Tandler, and Broman.
  9. A ventral vessel is seen arising from the sacralis media beyond the level of the fourth sacral segmental vessels and supplying the end of the gut where it goes over into the cloaca. Branches of this type from the sacralis media are seen in injections of other mammalian embryos, namely the pig, where they are more numerous and reach further headward.
  10. Considerable interest should attach to these mid-dorsal veins of the midbrain, inasmuch as Grosser (1901, p. 322) has demonstrated a pair of veins in this locality in bat embryos where they are retained, in fact, in the adult as the v. longitudinales meseneephali, apparently homologous with this vein in reptiles (Grosser and Brezina, 1895) . Grosser attributes the disappearance of these veins in other rnammals to the great overgrowth of the cerebral hemispheres, which, as is well known, are notably small in the Chiroptera. He also calls attention to the fact that Salvi (1897) probably saw the same structure, if we may judge from the descriptions in his paper on the development of the meninges in Cavia and Lepus. Attention may here be called to the fact that the primary head capillary plexus in pig embryos halts in its spread along two parallel mid-dorsal lines in the mid-brain as well as fore-brain, and, just as the medial margin of the former plexus comes to constitute the primitive paired sinus sagittalis superior, so also in some embryos an exactly similar condition transitorily exists on the top of the mid-brain. So that from the careful description oil Grosser and the less satisfactory references of Salvi this evidence may now be added to indicate quite clearly, I think, that a condition resembling the reptilian v. longitndinalis meseneephali exists transitorily in all mammalian embryos, including man.

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A personal message from Dr Mark Hill (May 2020)  
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I have decided to take early retirement in September 2020. During the many years online I have received wonderful feedback from many readers, researchers and students interested in human embryology. I especially thank my research collaborators and contributors to the site. The good news is Embryology will remain online and I will continue my association with UNSW Australia. I look forward to updating and including the many exciting new discoveries in Embryology!

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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