Talk:Paper - The development of the aorta and aortic arches in rabbits
The Development Of The Aorta And Aortic Arches In Rabbits
John Lewis Bremer
From the Harvard Medical School, Boston
The development of the primary blood-vessels in the body of the embryo has for many years been a matter of dispute. Evans, in the German edition of the second volume of the Keibel-Mall Embryology, sums up the matter as follows: 1
Whether the first blood-vessels of the embryonic body arise by ingrowth from the yolk-sac capillaries, or whether the embryonic vesselstems, or at least a part of them, originate in situ from the mesoderm of the body, is still an open question. Both views have found their supporters ; the name of His is connected with the first mentioned idea, the names of Riickert and Mollier especially with the second.
In birds it is possible to prove that the greater part of the descending aortae develop from the mesial border of the capillary plexus which has extended in from the yolk-sac, and this is very probably true of mammals also ; but (to quote again) :
For the cranial part of the aorta, on the other hand, the results are contradictory. His describes it as arising from a further ingrowth of the same extra-embryonic capillaries which form the aorta in its more caudal portion; the capillary chain grows finally over the blind end of the pharynx, turns ventral-ward, and joins the cranial part of the heart cavity. In rebuttal, Riickert and Mollier have stated in numerous articles that the aortae arise in loco from cells of the visceral layer of the mesoderm. It is impossible at present to insist that the anlagen found on the yolk-sac are the only ones for the endothelium of the body vessels. (Keibel-Mall Entwickelungsgeschichte, vol. 2, p. 552, etc.)
1 In the American edition of this work some of the results of the present paper have been added.
In this paper and by means of the following reconstructions made from serial sections of embryos in the Harvard Embryological Collection, I hope to show clearly that the view of His and his supporters is in the main correct, that the cranial part of the aorta arises as an extension of the capillary network of the yolksac ; and also to throw more light on the development of the ventral aorta, the aortic arches, and the pulmonary artery. For the study of this question I have chosen to work primarily with the rabbit, partly because of the excellence of this material in this laboratory, and partly because the presence of the 'lateral hearts,' described by Rathke, and easily recognizable in this species even in early stages, readily marks the position of this part of the bloodvessel net, and makes interpretation of the secondary foldings much simpler.
To some extent this same material was used by Dr. F. T. Lewis in a paper on "The Intra-embryonic Blood-vessels of Rabbits from 8§ to 13 days," which, accompanied by a demonstration of sections and graphic reconstructions, was read at the meeting of the American Association of Anatomists in 1903, but never published in full. In the report of the Proceedings in The American Journal of Anatomy, vol. 3, a resume is given as follows: ' ' From the network of vessels in the splanchnopleure of the yolksac, all intra-embryonic vessels are apparently derived as offshoots. The network ends mesially in the two aortae. With the formation of the pharynx, this net is so folded as to produce dorsal and ventral aortae with the connecting first arch." It will be seen that Lewis agrees with His as to the origin of the dorsal aorta, but discards the idea that this vessel grows forward around the tip of the pharynx to join ventrally with the anterior end of the heart. A glance at figs. 1 and 3 will show that this is correct ; dorsal aorta, first arch, ventral aorta and heart anlage are all laid down almost simultaneously.
Here a few words are needed on the character of the early bloodvessels. The most recent investigations in this field have been carried on almost exclusively by careful injections of fresh embryos, which are then studied as transparent objects or are converted into sections from which reconstructions are made. This method presupposes that all vessels are injectable, and in fact the claim is made that many collapsed vessels cannot be distinguished in sections until opened and marked by the injection mass. While in no way wishing to belittle the value of this method of research, or to discourage the increase of the many beautiful and valuable preparations obtained by its use, I still feel that its limitations, as they are shown in this paper, should be pointed out. As insisted on by His in 1900, and by many other authors (His' name is used as that of the champion of this idea) , the first blood-vessels on the yolk-sac and elsewhere are solid cords or strands of cells, without lumen : or to use other words, the actual vessels are always preceded by solid growths, which secondarily become hollow and form vessels. These solid growths, for which I wish to propose the term 'angiobast cords,' usually take the form of nets, which may persist until the separate strands are hollow, as shown by the injections of Evans and others, or may, as I hope to show, disappear in part without ever becoming injectable. The two views are clearly shown by a comparison of two figures, one from His ('00, 2, fig. 91), the other from Evans ('09, figs. 1, 2, 3) both showing the caudal end of the aorta of a chick embryo; by the injection method the capillary network is revealed, while His represents a network of solid sprouts preceding the hollow vessels. In this case, since an injection of these so-called solid sprouts would give practically the same picture as would be obtained if they were not seen and so left out of the drawing (the network being similarly placed throughout) , we have no direct proof that the sprouts are not potentially hollow, or in other words merely collapsed; but in the development of the anterior part of the aorta there are nets of solid angioblast cords present at an early stage, parts of which have certainly never been shown by injections, and may therefore, for the present at least, be considered solid. Here and there in this solid network there are hollow spaces, or true vessels, unconnected at first with one another and with the lateral capillary net except by the solid angioblast cords, and therefore not to be reached by any injection mass from this lateral net; for such unconnected hollow spaces I suggest the term 'angiocysts.' Thus the angioblast cords retain certain characteristics of the blood-islands, in that they also change from solid to hollow independently; but in the angioblast cords there is no sign of the formation of blood-cells. It was the observance of these isolated spaces, which later fuse to form large vessels, that lead to the often repeated statements of Ruckert and Mollier and others that the dorsal aortae arise in situ from the cells of the mesoderm; and in truth the connection with the lateral capillary net is short lasting and sometimes extremely tenuous. Tiirstig ('84, 1) recognized the presence of solid cords leading a short distance from these hollow spaces, but did not trace their connections; others of this school have missed them entirely.
The story of the development of the primary arterial system can best be told with the aid of the figures. Fig. 1, a reconstruction of the angioblast cords of one side of a rabbit embryo of five segments, shows these cords, streaming in from the network over the yolk-sac, the cut ends where the reconstruction was discontinued showing at the right border of the figure. They have grown from right to left of the figure, occasionally anastomosing, until near the median line, which lies at the left of the figure. The shape of the meshes of this net indicates, it seems to me, the direction of this growth, and the rapidity with which it has occurred. Cephalad the net is limited by the proamnion, where no mesoderm exists; caudad the net continues beyond the portion drawn. In a few places the cords have become hollow; here and there near the median line, and especially at the right of the figure, where a considerable chain of hollow spaces extends longitudinally, near the lateral border of the embryo proper. This chain, the future lateral heart, lies beneath the coelom, and like the coelom at this stage is situated only in the anterior third of the embryonic body. At the left of the figure the net ends rather regularly, and the meshes are slightly smaller, especially toward the head. Closer examination will show that each cord extends from the lateral heart almost directly toward the median line, then suddenly spreads longitudinally, as though its further direct course were blocked by some obstruction. In all but two places, which are marked by the arrows, the longitudinal strands of this mesial net have anastomosed with others; at these two places the longitudinal network is interrupted, and we may see clearly that this part of the net, the future dorsal aorta, is an ingrowth from the lateral vessels.
Fig. 1 Rabbit, 8§ days, 5 segments, 3.4 mm., H.E.C. no. 650. Reconstruction of the angioblast of the left side, anterior two-thirds of the embryo, seen from the entodermal cavity. The median line of the body is to the left of the drawing, solid cords of angioblast come from the yolk sac on the right. Hollow vessels are indicated by the shading. Brackets show position of somites; arrows mark regions where the aortic network is as yet incomplete'; x - y indicates plane of section of fig. 2. X 150.
Fig. 2 Rabbit, 85 days, H.E.C. no. 650, section 317. Section through the plane x - y in fig. 1. Mil. dr. = medullary groove ; Som. = 5th somite; Ent. = entoderm. 11, b. c, etc. = sections of the angioblast network. X 175.
If we examine fig. 2, a section of the same embryo, we can see these cords in their relation to the germ layers. The embryo has become shrunken in preservation, so that the layers are separated slightly from one another. The angioblast cords, indicated by the small letters, are seen to adhere now to the entoderm, now to the mesoderm, a fact which made them hard to follow, but which is obviously the result of this shrinkage. Let us imagine that the layers are close together. It will be seen that the somite will then touch both the ectoderm and the entoderm; it is this that forms the obstruction to further growth of the angioblast cords toward the median line. Where the somite does not exist, as in the head region, the cords extend further, and are then halted by the closely approximated medullary groove and entoderm; this can be seen by noting the position of the somites in fig. 1. Just lateral to the somite is the thin neck of mesoderm, the nephrotomy, which coming between two thicker portions, will be held above the entoderm, to form the roof of a small longitudinal canal; it is in this canal that the angioblast cords have room to expand and become hollow (fig. 2, a). In the head region, a similar though broader canal is provided by the uplifted lateral edge of the medullary groove; and in this region the reconstruction shows that the hollow parts of the net are found more widely distributed; in fact their chain follows forward the line of the spreading edges of the medullary groove. Thus, as in the case of the vertebral arteries, a longitudinal trunk is made by the anastomosis of branches from transverse vessels.
Another place where the blood-vessels have an opportunity to develop freely is beneath the coelom. At this early stage the coelom is almost exclusively represented by the amnio-cardiac vesicles, which lie cephalad to the level of the somites. Here is seen the development of the chain of spaces which is to become the lateral heart; the extension caudally of this is limited by the absence of the coelom.
Here then, in this early stage of the embryo before the pharynx has begun to be formed, we see a flat sheet of angioblast cords, forming a network, lying between entoderm and mesoderm, derived as an extension from the lateral extra-embryonic area. Its growth is limited cephalad by the absence of mesoderm in the proamnion; mesially by neural groove or somite. It occupies two areas especially where further growth is possible, namely under the nephrotome or the angle of the medullary groove, and under the amnio-cardiac vesicle.
Turning now to the next older embryo, one of six or seven segments, figs. 3 and 4, we can see that two changes have taken place ; a portion of the net has vanished, and the remaining portions include more hollow vessels, which are still, however, connected by solid angioblast cords. The persistent parts of the net are exactly those indicated in the last paragraph as occupying favorable positions: beneath the coelom lies the lateral heart, and beneath the edge of the neural groove lies the dorsal aorta. This reconstruction does not include the region of the segments. Cephalad, this raised edge, the top of which is indicated by the dotted line, makes a wide sweep laterally to form the future optic vesicles, and extends over the region of the coelom; so that a wide portion of the net may remain here, and may join with the vessels under the coelom. Thus the first aortic arch is formed, a net of vessels and cords connecting the dorsal aortic net with the anterior end of the lateral heart.
Fig. 3 Rabbit . s', days, 6 to .7 segments, 3.4 mm., H. E.< ). no. 62 1. Reconstruction of t he angioblast of the right side, anterior third of the embryo, dorsal view. ( hientation as in fig. 1. The coelom is represented as opened to show the lateral heart within. Au.il. = network to form the dorsal aorta; Ar.l = first aortic arch; Con. Art. = conus arteriosus; l"e.=venous end of lateral heart, with vitelline vein entering it. x-ij indicates plane of section of fig. 4. Broken line marks limit of medullary groove. X 200.
Fig. 4 Rabbit, 8£ days, H.E.C. no. 624, section 101. Section through plane .r - y in fig. 3. Md.Gr., medullary groove; Coe., coelom; Ao.d., three strands of network for dorsal aorta; a, isolated portion of angioblast; b, part of network; Ve., lateral heart. X 175.
Of the original net of angioblast cords between the lateral heart and the aortic net almost nothing remains. Here and there a few isolated cells can be found, not connected with the rest of the network; one of these is shown in fig. 4, others, though recognized, were left out of the reconstruction for the sake of clearness. Other cords have also been lost; of those connecting the lateral heart with the extra-embryonic angioblast only the most caudal remains, the others have either entirely disappeared or have lost their lateral connections. The cause of this latter destruction of the net is shown in fig. 4 ; the lateral heart, covered by a reflection of the mesoderm, has expanded so far into the coelom that vessels from it to the lateral net must take a curving course, and would probably be compressed between mesoderm and entoderm. This has occurred progressively from before backward, until the lateral heart gradually leaves the region of the coelom; here no such influence is brought to bear on the angioblast cords, and here they enlarge and become the vitelline veins.
Another agency at work in the further development of these blood-vessels is shown in fig. 4. The shape of the layer of entoderm indicates a longitudinal folding of this layer to form the pharynx, of which a point near l a' is to be the lateral edge, and a union of the layer near c b' with a similar point on the opposite side of the embryo is to complete the floor. This fold ends anteriorly by curving to the median line under the network of the first aortic arch. Thus the pharynx is due, in the rabbit at least, not so much to the usually described pouching forward of the entoderm as to a lateral folding of the layer, and the floor of the pharynx is completed by a union of the entoderm of the two sides, which soon fuses and forms two continuous sheets, one for the floor of the pharynx, the other for the upper wall of the archenteron.
This folding in a more advanced stage is shown by the shape of the blood-vessels in figs. 5 and 6, since the vessels always lie close to the entoderm. The dorsal aortae, still showing, by their frequent subdivision, signs of their origin from a network of vessels, are in the same relative position as before, dorsal to the entoderm of the pharynx. The lateral part of the network has been rolled in underneath the pharynx, whose crescentic outline is marked by the plexus of vessels which forms the first arch. By this folding or rolling in process the lateral edge of the network now lies beneath the pharynx and near the median line, and as the mesoderm makes its way between the floor of the pharynx and the roof of the archenteron, new shoots from these vessels pass toward the median line, and may even anastomose with others from the opposite side. This ventral plexus of vessels, many of which are at first solid cords, is the first indication of the ventral aorta. It is connected with the lateral heart, as can best be seen in fig. 6, by means of a slender vessel, the conus arteriosus, or bulb; and the lateral heart has, so to speak, lagged behind in the folding, so that the curve of the blood-\ r essels and of the entoderm in trarsverse sections of the embryo makes an S, the upper curve of which comprises the dorsal aorta, first arch, and ventral aortic plexus, while the lower curve includes conus arteriosus and lateral heart. The upper curve corresponds with the pharynx, the lower is below the pharynx and associated with the coelom.
Fig. 5 Rabbit, 9 days, 8 segments, 3.2mm., H.E.C. no. 621. Reconstruction of the blood-vessels of the head end of the embryo; seen as though looking forward from the anterior end of the lateral hearts, to show network for first arch. Ao.d., dorsal aortae; Con. art., conns arteriosus; IV.. lateral hearts; Ao.d., network for ventral aorta; y, extension toward median line; x, blind ends of obliterated vessels from yolk-sac. X 125.
Fig. 6 Rabbit, 9 days, 8 segments, 3.2 mm., H.E.C. no. 709. Reconstruction and lettering similar to those in tig. 5. At y the extension of the ventral aortic network lias fused with that of the other side. X 125.
Fig. 7 Rabbit, 9 days, 11 segments, 3.8 mm., H.E.C. no. 619. Reconstruction and lettering similar to th ise of fig. 5. Ar.l . Ar.2, first and second aortic arches. X 125.
If we notice the position of the lateral heart in figs. 4 to 7 we can see it is gradually rolled over and inverted, at its anterior end, so that whereas in fig. 4 it projected dorsally into the coelom and therefore was connected with the other vessels ventrally only, in fig. 6 the projection is distinctly lateral. In fig. 7, from a rabbit of eleven segments, the lagging lateral hearts have ultimately met in the median line and partly fused, but not until the pharynx floor has been completed and the ventral aortic network established, so that the inverted and fused lateral hearts lie at a distinctly lower level, connected with the ventral aortae by the ascending conus arteriosus, which itself is composed of two fused halves. The blood-vessels of this part of the embryo now lie in three tiers or levels; dorsally, the dorsal aortae, ventrally the heart, and between these two the ventral aortic plexus, joined to the former by the first arch and to the latter by the conus arteriosus.
On the recognition of this middle tier, the plexus of the ventral aortae, depends the proper understanding of the development of the rest of the aortic arches and of the pulmonary arteries. If we turn back to fig. 3, we shall see that the three levels are already indicated in the angioblast net ; the dorsal aorta, in plexus form, occupies a distinct region toward the median line, the lateral heart lies toward the right of the figure, while, connected with this by the conus arteriosus and with the dorsal aorta by the first arch, a portion of the net remains, expanded longitudinally, and lying beneath the mesial border of the coelom. This is to form the ventral aorta, and is to lie in the floor of the pharynx, while the conus is to lead from this level to the heart, which occupies a more ventral position.
It will be noticed that in fig. 3 there is an extension of the ventral aorta caudad from the conus arteriosus. This may be precocious in this case, for in the two embryos of eight segments, figs. 5 and 6, no such extensions were found, while in the embryo of eleven segments it is again present. This caudad growth of the ventral aortic net lies in the thin sheet of mesoderm between the floor of the pharynx and the pericardial cavity, and ultimately reaches the level of the pulmonary anlagen.
THE AORTIC ARCHES
The first aortic arch has already been described as a persistent portion of the original angioblast net, folded around the anterior end of the pharynx. The cords of this net become hollow, as is shown by Evans in the drawing of the injected vessels of a duck embryo (Keibel-Mall, vol. 2, fig. 398). Later, as is seen in figs. 7 and 8, the net becomes reduced to two vessels on each side, and in the rabbit frequently breaks up into capillaries before beingreduced to a single trunk.
The second aortic arch (figs. 7 and 8) arises as a lateral extension from the plexus of the ventral aorta, frequently double on one or both sides, consisting at first of solid cords and hollow spaces, and met by much shorter outgrowths from the dorsal aorta. The potentially double character of this arch, even after it has attained a considerable development, is shown in fig. 8, x and y. While the second arch is becoming established the plexus of the ventral aorta is extending still further caudad, and again giving off lateral branches, which run between the entodermal pouches to the dorsal side of the pharynx, and again are met by shorter growths from the dorsal aorta. Thus the third and fourth arches are formed and, as a glance at fig. 8 will show, they also, from their plexiform origin, are potentially multiple on each side.
With the growth of the pharynx the conus arteriosus, which joined the ventral aortae originally cephalad to the second arch, has moved caudad, and later opens almost directly into the third and fourth arches. This condition is represented in fig. 9. Here still we see the plexus of the ventral aorta extending caudad from the ventral part of the fourth arch, as it did earlier from the second arch. Its plexus formation is easily recognized, as it lies in the thin sheet of mesoderm between the pharynx and the pericardial cavity, but it no longer crosses the median line on account of the presence of the keel-like growth of the pulmonary anlage from the ventral wall of the pharynx. From the dorsal part of the fourth arch and from the dorsal aorta sprouts arise, at first in part solid, which curve around the pharynx and, entering the same sheet of mesoderm in which the ventral aorta lies, form there, on the left side of the embryo, an anastomosing plexus. On the other side of this embryo these sprouts from the dorsal aorta are much simpler, though still double, while a lateral extension of the plexus of the ventral aorta replaces that derived, on the left, from the dorsal sprouts. In other embryos there is great variation in these vessels. There can be no doubt that these vessels from the dorsal aorta are about to join the ventral plexus and form another arch; nor can there be any question that in this case there are two channels on each side. While not wishing to go deeply into the controversy over the presence or absence of a sixth aortic arch, I may say that it seems to me that the final solution should come from a further study of the entodermal pouches, of the branches of the nerves, and of the cartilages in this region. We have seen that the four first arches are potentially double on one or both sides, and we now find that the last arch is so also, with, as is known, great variation in the points of origin and forms of anastomosis. We know that the fourth pouch is forked at the end, but so is also the third pouch. Branches of the vagus have, in a few instances, been found which seem to indicate the presence of an extra arch; but aberrant nerve branches are not unknown elsewhere. As far as the early development of these vessels is concerned, there is nothing certainly to prove the presence of an interpolated arch.
Fig. s Rabbit, LO days, 23 segments, 3.2mm., H.E.C.j-no. 940. Reconstruction (if t he lil (Hid -vessels of 1 he head end of the embryo, seen from the ventral side ; t he heart removed by cutl ing through the conus arteriosus. Lettering same as in fig. ."), and also Ar.l. Ar.2, Ar.3, Ar.4, aortic arches; E.C., external carotid arteries, cut; x and //, remains of second channel for sec 1 aortic arch. X 125.
Fig. 9 Rabbit, 10| days, 3.2 mm., H.E.C., no. 560. Reconstruction and lettering similar to those of fig. 8. P.art., extension of ventral aortic network as pulmonary arteries. X 125.
THE PULMONARY ARTERY
It will be noted that the sprouts for this last arch arise chiefly from the dorsal vessels, instead of from the ventral net. I also wish to point out that the net grows beyond the arch, before the arch has become complete. In other words this extension of the ventral aortic net forms well defined pulmonary arteries, one on each side, before the pulmonary arch exists; the pulmonary artery is in no sense a branch of the pulmonary arch, and moreover, in the strictest sense, the arch extends only from the dorsal aorta to the pulmonary artery, the ventral part of the vessel usually called the arch is really the ventral aorta. The persistent pulmonary arteries are entirely ventral; they have been joined during embryonic life by branches from the dorsal aorta, but such branches are only temporary.
Here I must add a few words in regard to some recent statements on the development of the pulmonary arteries. Evans ('09, fig. 21) gives a figure of the injected pulmonary arteries of a pig embryo of 12 mm., and in the German edition of the KeibelMall Embryology he copies (fig. 396) a figure of Fedorow showing the pulmonary arteries of a guinea-pig embryo of twenty-one days. In both cases the vessels form a narrow plexus in front of the trachea, and Evans 2 concludes that here, as in other growing vessels, the main trunks are preceded by a capillary net. That he is correct in the main, that the pulmonary arteries do arise as the extension of the net of the ventral aorta, we have just seen; but these plexuses of the pulmonary arteries across the median line are of later occurrence, after much mesoderm has grown in between the trachea and the pericardial cavity. It is unfortunate that the pig and the guinea-pig were chosen as illustrations, for, as I have shown in previous papers ('02, '09), in these two species only, so far as I am aware, is such a secondary net found. After the formation of this net one of the connections with the pulmonary arches is lost, so that in these two embryos the trunk of the pulmonary arteries seems very long before the right and left branches are given off. In those two papers I spoke of the pulmonary arteries as branches of the pulmonary arches, a statement which I am now very glad to correct.
2 Dr. Evans has very readily and kindly acknowledged his error to me; Fedorow did not make the misinterpretation.
In the rabbit, the dorsal aorta, the first aortic arch, the conus arteriosus and the lateral heart are all parts of an original network of angioblast cords derived from the extraembryonic plexus of blood-vessels. Those portions of this network which are mechanically favored in their position persist ; the other portions disappear. The favored portions lie (1) under the coelom, (2) under the nephrotome, or (3) under the raised edge of the medullary groove. The connection between first arch and lateral heart is permitted by the lateral expansion of the medullary groove which extends over the coelom.
This net of angioblast cords is folded in the formation of the pharynx, so that its lateral edge, anterior to the lateral heart, becomes the ventral aorta. In this folding the lateral heart is retarded, and thus comes to lie on a lower plane, more ventral. The connection between the two planes is by means of the conus arteriosus.
From the net of the ventral aorta a plexus grows mesially and caudally, still forming the ventral aorta. Lateral growths from this pass around the pharynx, often in plexus form, and make the second, third and fourth aortic arches. The conus arteriosus is moved caudad to a position opposite the fourth arch.
A further extension of the plexus of the ventral aorta, situated between the floor of the pharynx and the dorsal wall of the pericardial cavity, but prevented from crossing the median line by the presence of the median pharyngeal outgrowth to form the trachea, extends to the lungs as the pulmonary arteries, which are later joined by vessels springing from the dorsal aortae. These vessels, which may be double and plexiform, constitute the fifth (and sixth) arch.
Although dealing in this paper with the development as found in rabbit embryos, I have examined various other species, as chick, pig, sheep, etc., and feel satisfied that in all essential points the story of the development of these primary vessels in other vertebrates will be found similar to that here described.
Bremer, John L. 1902 On the origin of the pulmonary arteries in mammals. Am. Jour. Anat., vol. 1, p. 137, and Anat. Rec, vol. 3, p. 334.
Evans, H. M. 1909 In Keibel-Mall Entwickelungsgeschichte, vol. 2, p. 551, etc. Anat. Rec, vol. 3, p. 498.
Fedorow, V. 1910 Ueber die Entwickelung der Lungenvene. Anat. Hefte. Bd. 40, S. 529.
His, W. 1900 Lecithoblast und Angioblast der Wirbelthiere. Abh. d. math.phys. Klasse d. Kgl. Sachs. Ges. d. Wiss. Bd. 26, no. 4. Leipzig.
RtJCKERT u. Mollier 1906 Die Entstehung der Gefasse und des Blutes bei Wirbelthieren. Handb. d. vergl. u. expt. Entw. d. Wirbelthiere, herausg. von O. Hertwig. Bd. 1, S. 1019.
Turstig, J. 1884 Untersuchungen iiber die Entwickelung der primitiven Aorten. Schrift., herausg. v. d. Naturf.-Ges. bei. d. Univ. Dorpat. Bd. 1.