Paper - On an instance of two subclavian arteries of the early arm bud of man and its fundamental significance

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Evans HM. On an instance of two subclavian arteries of the early arm bud of man and its fundamental significance. (1908) Anat. Rec.

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On an instance of two subclavian arteries of the early arm bud of man and its fundamental significance

Herbert McLean Evans
Herbert McLean Evans (1882—1971)


Herbert McLean Evans

From the Anatomical Laboratory of the Johns Hopkins University.

I. Introductory. (a) General considerations of the arm as a polymetameric structure. (b) Historical review of the literature on the earliest arm vessels.

11. Description of the present case.

III. Relation of the foregoing to the general principles of the development of the vascular system.

I. Introductory

The question of the morphological nature of the vertebrate limb has for many years interested anatomists, and among them, some of the greatest minds in anatomical science. Curiously enough, however, some of our most fundamental conceptions of the limb are as yet by no means clearly established or beyond dispute, 6. g. its relation to the metamerism of the body.

This problem has been attacked again and again through the skeletal, the muscular, and the nervous system, but it is only within the last year 01 so that any significant work has been done on the primitive limb vessels. The blood vessels, moreover, must claim our deepest interest from quite another point of view, namely for what light they may shed on those general principles which govern the development of the vascular system throughout the body.

Into the early arm bud grow the fibers from all those spinal nerves opposite it and whatever changes in position the limb may subsequently undergo, it always carries its nerves with it. These structures, then, form excellent guide ropes, as it were, marking not only the early position of the arm but also, with great clearness, the particular segments involved in its structure.

What we know of the close association of muscle and nerve in normal development calls for a similar participation in the developing limb of 412 The Anatomical Record.

the corresponding myotomes, and as a matter of fact, the ingrowth of the myotomes into the limb fundament had been known ever since the time of Balfour. Though undoubtedly the case in Selachians, for example, this has been seriously disputed for man and it is only very recently that new observations in fortunate stages of development make it probable that the human embryo does not differ in this respect from what has so clearly been observed elsewhere by Balfour, Kleinenberg, Mollier, Braus, Dohrn, and others.‘

With these conceptions consciously or unconsciously in mind, it is only to be expected that the vascular system of the extremities should be similarly derived, i. e. in each case from several segments. But since in the case of the arm, for instance, only a single supplying artery exists, several hypotheses have been advanced for its explanation. These may be briefly enumerated as follows:

1. The subclavian artery is in reality several segmental arteries fused into a common trunk.

2. The subclavian is a single segmental vessel. Either

(a) This is from the very beginning the only vessel supplying the arm, or

(b) This is merely the persisting one of a series of original segmental vessels to the limb.

We may dismiss the first theory with short notice. It was advanced by the English anatomists Mac-alister’ and Mackey’ on purely theoretical grounds, and discredited by the very first actual observations made on the embryonic subclavian. Hochstetter‘ showed clearly, at first in his research on the primary avian subclavian, and afterwards in the case of mammals, that this artery belongs to only one of the segmentals. In all cases it pierced the limb as an axially directed vessel and undoubtedly corresponded to the adult trunk.

  • It is not my intention to more than refer here to the very extensive literature on the origin of the limb musculature. One does not need the appearance of the ingrowth or myotomes or of large muscle buds streaming from the myotomes to establish the metameric origin of the limb musculature. Ample evidence is furnished by the observation of a few cells budding in this way. Cf. Ingalls, Archiv f. Mik. Anat., LXX, 1907.
  • Macalister. Morphology of the Arterial System in Man. Jour. Anat. and Phys., XX, 1886.
  • Mackay. “The arterial system of vertebrates homologically considered." Memoirs and Memoranda in Anatomy, Vol. 1, 1889.
  • Hochstetter. “Ueber den Ursprung der Arteria subclavia der Viigel.” Morph Jahrb., Bd. 16, S. 494, October, 1890. “Ueber die Entwicklung der A. vertebralis beim Kaninchen.” Ibld., December, 1890.

Although I am now convinced that a whole series of interesting changes in the history of the arm’s vessels had occurred before the stages observed by Hochstetter, nevertheless his observation was of fundamental importance. It pointed clearly to the fact that the subclavian was to be regarded as the enlarged vessel from a single segment. But it afforded no explanation of the dissimilarity apparent in the behavior of the blood vessels when compared with any of the other segmental structures——the nerves or the myotomes. In every other respect the arm was a polymetameric structure; in its vessels, singularly enough, but one segment was represented. Not only was this unexplained, but, on the contrary, an erroneous interpretation of this as the most primitive stage observable here destroyed much of the value of this contribution. Nevertheless the observation of a stage in which the subclavian can be clearly seen to come from one of the dorsal segmental vessels has been abundantly confirmed by many workers in both mammalian and human embryology. In Man, for instance, we know that the subclavian is a lateral branch of the seventh segmental artery.

But, recognizing none of the recent work on this subject, Hochstetter‘a has maintained this as the earliest condition and this view has been reflected and emphasized in the recent article by Curt Elze" from Hoolistetter’s laboratory. On the other hand, the most convincing evidence in favor of the existence of many segmental subclavians as the more primary condition has just been given 11s by the work of Erik Muller“ and of Hans Rab]?

Professor Miiller has for several years prosecuted a series of embryological and comparative studies of the limb vessels which have apparently yielded a rich harvest from this special standpoint. He has not only described embryos in which several segmental subclavians existed, but has also been able to point out that different ones of the segmental series are chosen to become the main vessel in different species.

  • a Handbuch der vergleich. und experiment. Eutwickelungslehre der Wirbelthiere. Hertwig, 1906, IV Kapitel von Hochstetter, p. 108. “Bei den Embryonen der Amphibien und siinitlichen Amnioten verliiuft die einfache Arterie der Extremitiit urspriinglich ziemlich genau in der Achse des Extrernit§.tenstmnmels.” The Italics are mine.
  • Elze. “Beschreibung eines menschlichen Embryo von zirka 7 mm. griisster Liinge.” Anat. Hefte, L35, S. 409. December, 1907.
  • Miil1er, Erik. “Beitr-age zur Morphologie des Gefasssystems." I. “Die Armnrterien des Menschen,” Annt; Hefte, I-22, 379-574, June, 1903; II. “Die Armarterien der Sliugetiere.” Anat. Ilefte. 1-27, 73-235, December, 1908. III. “Zur Kenntnis der Fltigelarterien der Pingnine." Anat. I-Iefte, 1135, February, 1908.
  • Ram, Hans. “Die erste Anlage der Arterien der vorderen Extremltfiten bei den Viigeln. Arch. f. Mik. Anat., Bd. 69. 340-389, 1907. 414 The Anatomical Record.

Miiller has described multiple segmental subclavians in Selachians, Reptiles, and Birds, but his studies did not disclose such a condition in Mammals or, till recently, in Man. In human embryos, though, models of the early arm vessels had shown him an interesting condition of the axillary artery—his so-called plexus axillaris arteriosus. This vessel, later a single trunk, was in this stage represented by a plexus and Miiller thought he could trace in this plexus the remnants of the segmental subclavians which had helped form it. Moreover, this condition of the axillary suggested clearly to him that he had seen a place where the plexus origin of "a large trunk was proven.

Rabl, ignorant of the work of Miiller, was suprised to find three segmental subclavian in the wing bud of duck embryos. His own recent contribution to this subject has all the value of an independent discovery. Furthermore Rabl has demonstrated the capillary-plexus origin of everal of the limb vessels in the Bird.

Curt Elze‘ has recently everely criticized Miiller’s findings and interpretations. His conclusion is that Miiller has mistaken secondary formations for primary plexuses and that the arm vessels do not develop from such primary capillary plexuses. Elze is not only mistaken in some of his other arguments, but he has chiefly attacked Miiller’s work by discrediting the normal conditions of the embryos he studied.

Elze is evidently unaware that a system of delicate and easily collapsed tubes like the embryonic capillaries require some distending agent, be it the blood corpuscles or an artificial injection before we can trace with certainty all of their connections.’ Thus he has censured Miiller, who would intentionally choose such material, and cites the well-known fact that living embryos quickly fixed do not show the capillary system well filled, whereas those permitted to die more slowly, always do. He argues then, that specimens showing well-filled capillaries are consequently not trustworthy. Certain it is that just such specimens are best suited to follow carefully the finer vessels, nor do I believe that the distention of the capillaries thus produced has altered gravely the appearance of the vessel wall or neighborhood from what it may be in life.

  • Elze. Loc. cit.
  • Familiar as we all are with the impossibility of demonstrating the finer portions of the adult vascular tree save by some sort of injection, it is quite inconceivable why we should not recognize the necessity of imilar help in the more delicate and transitory vessels of the embryo which collapse with great ease. Professor Sabin informs me that a similar neglect of method has prevented ome investigators from getting a correct conception of the connections and course of the embryo’s lymphatic system.

In his general denunciation of the results of all work on embryos showing imperfections in fixation or other departures from the most perfect conditions obtainable, Elze has seen fit to instance the embryo described by Gage as a good example of the unreliability of such material. It is, strangely enough, this very same embryo which is described in the present paper and which has been fated to furnish one of the first links in the chain of evidence against Elze’s contentions. Beyond some obvious shrinkage which dehydration has produced, this embryo meets perfectly the conditions Elze has imposed. In it are mitotic figures in all stages, and furthermore it was obtained by an artificial abortion from an entirely healthy uterus. His own best specimens, though I do not doubt their normal structure, were obtained from diseased uteri.

Elze has called in question Miiller‘s recognition of the brachial artery in these young stages, because, he says, the future territory of the brachial has not yet developed. He explains this still further by outlining briefly his own conception of the development of the arm, according to which, the stages Miiller studied contained only the anlage of the hand. The arm is successively pushed out of the body wall, he says, and after the hand, we can recognize the under-arm, the elbow, the upper-arm, and finally the shoulder. These remarkable statements show that Elze is ignorant of much work on the development of the arm, for when we can recognize the various parts of the arm at all, we can recognize the differentiation of upper-arm, lower-arm, and hand, in an order just the reverse of that Elze has maintained.“

Elze’s answers to Rabl’s ‘careful study of the developing arteries in the early wing is weak. He asserts that Rabl’s conclusion that these vessels come out of a capillary plexus are not necessarily proved by his facts, a charge which will hardly be supported by those who will read carefully this excellentresearch. And in addition, says Elze, the history of the development of the Duck’s subclavia argues nothing for the conditions in Man.

  • Cf. Lewis, W. H. "The Development of the Arm in Man." Amer. Jour. Anat., I-156. If the arm were merely pushed out from the body wall by the proliferation of the cells at the arm base, as Elze maintains, then the youngest tisue is to be found there and at the arm tip the oldest. But, unfortunately, the facts prove the reverse of this, for hand muscles. for instance, are as yet a mass of embryonic mesenchyme when those of the upper arm are differentiated Professor Lewis informs me that there is no justification in the position that the limb is at any time merely the hand.

Finally, I may be permitted to add (what he has himself frankly admitted) that E.lze’s embryos are too old to furnish any evidence on the earliest limb vessels and that they have contributed nothing to our knowledge of this portion of the subject.

In quite another way, however, Elze has contributed something to the general subject, for by opposing the plexus origin of vessels he states definitely the issue between the two leading views now held on vascular development; and that Miiller’s fundamental conceptions on this point are in the main quite correct, regardless of what minor errors he may have made, I regard as certainly true and capable of demonstration in every portion of the body.

Until the appearance of his last paper in February, 1908, Miiller had never been able to find a human embryo with more than a single subclavian. His “plexus axillaris arteriosus” was the nearest approach to this. Indeed Miiller has gone so far as to label certain of the channels in this plexus as representing the persisting proximal portions of the pre-existing ‘segmental subclavians, and on this evidence, scanty though it was, declared the human arm bud originally supplied with these multiple subclavians. Whether the individual channels in the “plexus axillaris arteriosus” are constant enough in number and exact position to be referable to the segmental subclavians or whether, indeed, they may not be merely the variable bars which form the meshes of any general capillary net, must be left to future researches to decide.“ Other and far better reasons for the prediction Miiller made were to be found in more general considerations and undoubtedly a correct fundamental conception of the growth of vessels guided him to a prediction which his few facts could hardly prove to others. Having traced the arm to a stage in which the axillary artery was represented by a capillary plexus, it was reasonable to suppose that the subclavians also had passed through this stage.

  • My own work points strongly to this latter interpretation. In many injections of the limb buds of early bird embryos, I have observed a most variable condition of the capillary plexus distal to the line of first anastomosis of the multiple subclavians. Indeed, from this point on, one cannot segregate portions of the plexus as belonging to individual subclavians. Elze’s criticism of Mtiller here is quite justitled, for one mut doubt seriously the exact value of his “plexus arteriosus axillaris.”

If it be true that blood vessels can grow out as single trunks before dividing to form a plexus, then Miiller’s supposition was in no sense necessitated by the picture he saw. But if vascular trunks are resolved out of and hence preceded by a simple capillar_v plexus, then Mii1ler’s declaration was most plausible—for the segmental subclavians were probably true capillaries, both in delicacy of wall and in size and their anastomoses with one another had given a plexus similar in character to, and continuous with, that now remaining in the place of the axillary artery. The interpretation, then, depended entirely on the pre-existing condition here, and, in lieu of any evidence on this point, quite entirely on one’s acceptance of certain principles of blood-vessel development.

In his last paper, Miiller“ announces the observation by Keibel of a double subclavian supply to the arm bud of a 41/; mm. human embryo. Few other details are given and the condition of the embryo was such that little could be determined save that both vessels could be distinctly traced from the aorta into the extremity, in which were numerous capillaries.

An examination of Keibel’s Normcntafeln”a. which has just appeared, shows that this embryo is numbered seventeen in his series and that the subclavians present in this case came from the sixth and seventh doral segmental arteries.

This, then, comprises all that is known at present of the earliest stages of the human subclavian. The uncertainty in which the matter appears to stand makes it necessary that these meagre results be verified. Our own specimen had been observed b_v Professor Mall a considerable time before the appearance of Miiller’s last report, so that at the time I wa permitted to investigate this point more carefully, it was believed that we had the first authentic instance of a human arm bud early enough to display more than a single subclavian.

As it is, the facts here presented supplement in an interesting way those reported for Mii1ler’s embryo, for in this case, as I shall presently describe. the segmental subclavians represent the. two lower segments opposite the arm, whereas the Embrvo 17 of .\Iiiller’s possessed subclavians form the two upper segments. This report also, I trust, will establish the reliability of this as a normal finding in the human embryo.

  • MIiiller. Loo. cit.
  • ct Keibel. 1\'ormentnfeln zur Entwicklungsgeschlchte der Wirbelthiere. lieft VIII. Norm. 7.. Ent. de .\Iensc-hen. Keibel und Elze, Jena. 1908. Keibel has also noted here another embryo (No. 16) which on one side possessed two subclavians from the same two segments, 4. e., 6th and 7th.
  • bl number the segmental vessels to correspond to the nerves. Following I-lot-listettei-‘s usage these would be 5th and 6th instead of 6th and 7th.


The present case concerns the condition present in the right arm bud of a human embryo—No. 148 of Professor Mall’s collection—four and three-tenths millimeters long (neck-breech).

As has been said, the embryo gives all the appearances of well fixed and healthy tissue, though slightly shrunken by alcohol; so that any evidence it presents can not be cast out on the grounds of any pathological taint about it. It was obtained from a healthy uterus, is beautifully fixed, and is itself normal.

A detailed description of it is not necessary, as Gage“ has done this quite fully. Suflice it to say, however, that this study showed the presence of 28-29 myotomes, of which two are occipital.

The arm buds extended from the seventh to the thirteenth myotomes. They are formed of a homogeneous appearing mesenchyme in which are many capillaries. On the right side, the serial sections showed that two vessels left the aorta and entered the root of the limb. These were separated by a considerable interval and were both perfectly distinct. They occur in sections 82 and 91 respectively. They appeared to contribute to the general capillary plexus of the limb as soon as they reached its root, 1'. e. could not be traced any distance into the limb core as is the case with the axial subclavian we find in slightly older stages. The more anterior of these two subclavians can be followed in its entire course in two succeeding sections. The first section shows it extending laterally a far as the base of the arm bud and mesially to within a short distance of the right aorta. The succeeding section discloses its origin from the aorta. Its course is almost in a straight transverse line. Taking origin from the upper lateral portion of the aortic wall, it passes over and considerably above the posterior cardinal vein and the cephalic portion of the primitive coslom here present. It has just reached the tissue of the limb bud, when an alteration in its trend, together with the collapsed state of its Walls, forbids our following it accurately further. However. several capillary channels in the immediately neighboring mesenchyme apparently join it. The subclavian is everywhere a delicate endothelial tube. Its origin from the aorta is as a small conical protuberance of the aortic wall, an appearance presented by most of the early outgrowths of the aorta. The collapse of its walls prevents any accurate estimate of its calibre, but a small distended segment next its origin indicates that it does not exceed that of any ordinary capillary. (Fig. 1.)

  • Gage, S. P. “A Three Weeks’ Human Embryo.” Amer. Jour. Anat., IV, pp. 409-445. 1905.

A few sections beyond the above, the corresponding segmental vein appears and can be followed from the ventro-lateral aspect of the spinal cord to the post-cardinal trunk.

Flg. 1. Camera lucida tracing of the subclavian artery of the seventh cervical segment in human embryo No. 148. C., spinal cord; Ch., Chorda; Ao., right aorta; Co., cmlom; P.C.V.. post-cardinal vein.

The second subclavian present here does not diflfer essentially from the more anterior one. However, it is less collapsed and penetrates the blastema of the arm bud a slight distance, showing there a considerable dilatation. (Fig. 2.)

One of the points of greatest interest in the foregoing, was the deter~ mination of the exact position of these vessels and their relation to the primitive segments. The sections were cut obliquely; consequently it was impossible to take the particular spinal ganglion appearing in a section 420 The Anatomical Record.

which showed a subclavian as representing the level of origin of the vessel in question. Fortunately, however, two careful models of the embryo had been made, in one of which the relation of the plane of section to the embryo had been most painstakingly determined. Employing an accurate profile of the reconstruction with the lines of section indicated, it was easy to draw in those parallel lines representing the sections which interested us. The section disclosing the first subclavian had also shown the ventral root of the eighth cervical nerve and, employing my figure, I

Fig. 2. Camera luclda tracing of the subclavian artery of the first thoracic segment. Lettering as in Fig. 1.

found that such a section must have cut the aorta at the preceding intersegmental point. There was high probability, then, that the first vessel could have corresponded only to the seventh segmental one; and when the segmental vein, referred to above, was located near it, there appeared adequate evidence for definitely determining it as at this location.

The second subclavian, by the same evidence, fell at the second lower inter-segmental point and hence is doubtless to be regarded as opposite the first thoracic segmental vessel.

Fig. 3 shows a reconstruction of these vessels and their relation to the myotomes. I regret that we can not speak more definitely of their relation to the dorsal, so-called “segmental” vessels, not that their position could thus be determined any more positively than it already has been by reference to the myotomes, but for quite another reason. In the Birds, where we now have the most complete account of the earliest limb vessels, the subclavian arteries are primitively independent lateral offshoots of the aorta and only later come to be included as branches of the dorsal segmentals. It is not unlikely that our case represents this primary condition in Man. In Miiller’s embryo, which is slightly older, the subclavians are already definitely branches of the corresponding dorsal vessels.

FIG. 3. Reconstruction of position and course of segmental subclavian arteries present in this case. 7th I.S.. seventh cervical iutersegmental space; 1st I.S., first thoracic intersegmental space.


It has already been emphasized that the dispute on the method of development of the subclavian has established clearly the two modern views on the development of the vascular system. According to the one, blood vessels grow out to their territory as definite stems from the very beginning; whereas, according to the other view, they are always preceded by a capillary plexus the conversion of which into the later stem or trunk involves the selection of one path through the mesh and the atrophy of all or most of the others.

Many years ago, as Miiller has shown, Aeby“ and his pupil Baader“ put forth the hypothesis of a general embryonic net from which all arteries and veins arose. They were led to this view by a careful study of variation in the vascular trunks of the adult. The variations could be explained on no other basis so well as by the theory of a pre-existing uniform net or plexus. N o more direct evidence than that afforded by the grown body did they attempt to seek, and an exploration of developing blood vessels to throw light on this point, waited almost twenty years, when Rage“ thought he could test the point by examining the limb vessels in a twenty-five millimeter human embryo. There, of course, the subclavian artery was present as a distinct trunk, giving off its customary branches, all of which was so clear that Ruge saw no need of regarding the theory further or of testing it in earlier stages! On this evidence he took issue with the idea of a capillary mesh anlage for the vessels and stated his belief that “bereits an allen Embryonen die grossen Gefiisse der Extremitat als solche ausgebildet waren.”

Most of the voluminous researches since Ruge have told us nothing on this fundamental point. One brilliant exception must be made in the work of Thoma." Choosing a flat and expanding area of blood-vessel development, the area vasculosa of the chick, he concentrated his attention on the very points we have been discussing and established beyond doubt that both arteries and veins in any area are a later elaboration from a pre-existing capillary net. Thoma went further, as is well known, and showed that this elaboration or transformation was merely an inevitable result of definitely acting hydrodynamic laws which need not be referred to here.

Most of the workers since Thoma have either ignored this important work or have possibly fancied that facts gained from the chick’s yolk vessels could not be seen to apply in the case of the trunks in the body of the embryo. However, from this laboratory from time to time, earnest attention has been called to Thoma’s great contribution and at least the capillary anlage for the vessels has not been doubted to apply in the growth of the vascular system of any area or organ of the embryo.

  • Aeby. “Der Ban des menschllchen Kiirpers,” 1871.
  • Baader. “Ueber die Varletltten der Armarterien des Menschen." Inaug. Dlss., Bern, 1866.
  • Ruse, G. “Beitrlige zur Geflisslehre des Menschen.” Morph. Jahrb., Bd. 9,329, 1884.
  • Thoma, R. “Untersuchungen fiber die Histogenese und Histomechanik des Geflisssystems,” 1893. The Anatomical Record. 423

Perhaps the clearest expression of this is to be found in Professor Mall’s “A Study of the Structural Unit of the Liver,” Amer. J our. Anat., Vol. V, No. 3, p. 251: “The first and guiding blood vessel is the capillary, which grows in all directions, forming a plexus. Secondary changes make arteries and veins of them.” Convincing proof that this is true will probably only come when a method of investigation is used which will give us without doubt the whole picture of all the vessels present in the various parts of the embryo, including the entire capillary bed and even its endothelial sprouts. Such a method is the method of injection.

However, without this aid, attentive examination of sections have revealed much to Miiller and to Rabl. Their contributions mark the first established instances of the application of the capillary plexus theory for the growth of the body’s vessels. ‘But their interpretations will doubtless be questioned until the successful employment of the method suggested above brings the telling proof.

It is only necessary to call attention to one other point which may be used by the opposing school. They may admit, as Elze does, that segmental subclavians may occur, but see in this rather support for their own ideas. How is it, they may ask, if this is a simple capillary plexus, that the first mesh is rigidly governed by the somite’s width?

Till recently, we could have only answered here that these vessels are capillaries as far as their size, the character of their walls, and their anastomoses are concerned. That they do not arise at more frequent intervals from the aortic wall—intervals more the width of the usual capillary mesh—is doubtless owing to the influence of the metameric organization of this portion of the embryo, an influence governing the outgrowth of all of the dorsal capillaries of the aorta and an influence, they may be reminded, whose fundamental significance has not yet been altogether explained. If vessels grow at all, it appears they must grow at segmental points, i. e. between the myotomes. But if the vessels which grow there are capillaries, behaving from these points on as capillaries do, then they ofier no contradiction to our theory.

I have discussed this merely to support the evidence which Rabl and Miiller have acquired by a body of new facts which have come out of a long series of vascular injections of young bird embryos. I may thus he pardoned for announcing here so briefly what I have described in necessary detail elsewhere.“

  • Evans. “On the Earliest Bloodvessels in the Anterior Limb Bud of Birds and their Relation to the Primary Subclavian." Amer. Jour. Anat., Vol. VIII, 1908.

Injections will give us at a glance more knowledge than we can hope to laboriously acquire by reconstructions. By employing the method of injection in the case of the arm bad, it is now possible to recognize, preceding the stage of segmental subclavians, a more profuse outgrowth of capillaries from the lateral aortic wall than we had ever suspected. Indeed, in one case, I was able to count eleven of these delicate vessels streaming into the arm blastema where but three could have been expected had they risen merely at intersegmental points. In other words, then, when the arm tissue is first supplied by vessels, and it is supplied very early—it is supplied merely by capillaries growing from multiple points and anastomosing to form a typical and simple plexus.

Thus we have followed down to this most primitive condition one of the largest trunks in the entire body——the subclavian artery—and one can hardly doubt but that the methods of its formation can be seen again in its growth —that all of its branches down to the digital are successively preceded in their development by zones of true capillary nets, out of which, in each instance, the stem in question is ultimately derived.

We must now expect to see the discovery of more vessels in still earlier human arm buds and in the mammals as Well. But that such observations will be abundant or easy is negatived by what we know of the delicate and extremely transitory character of such vessels in those cases where they have already been demonstrated.

In conclusion, I wish to express my many thanks to Professor Mall.

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