Paper - Development of the thoracic vertebrae in man
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Development of the Thoracic Vertebrae in Man
Professor of Anatomy, the University of Wisconsin, Madison, Wisconsin.
With 7 Plates
There is a somewhat extensive literature dealing with the development of the spinal column in various vertebrates. The chief stages in its diiferentiation are fairly well determined. Special attention has been given to the early development in the lower vertebrates. The recent literature on this subject up to 1897 has been reviewed by Gaupp, 97.‘ Somewhat less attention has been devoted to the mammals. To Froriep, 86, is due a valuable account of the development of the cervical vertebrae in the cow, and to Weiss, 01, an important description of the development of the thoracic and cervical vertebra in the white rat.
We shall not attempt to enter here into a description of the early stages of diﬁerentiation in the spinal axis; that is of the period covering the formation of the chorda dorsalis and of axial and peripheral mesoblast, the diﬁerentiation of primitive segments, and the origin of the axial mesenchyme. This period in the human embryo has been well treated by Kollmann, 91, and some account of it has previously been given in this Journal (Bardeen and Lewis, 01). We shall therefore proceed at once to a consideration of vertebral dilferentiation in the axial mesenchyme.
Vertebral development in the embryo may be divided into three overlapping periods: a membranous or blastemal, a chondrogenous, and an osseogenous.’
‘Among more recent papers may be mentioned, those of Baldu, 01; Hay, 97; Kapelkin, oo; Manner, 99; Minnich, oz; Ridewood, or; and Schauinsland, o3,
’ In the text books.the ﬂrst or these periods is usually called the precartilage, prochomlral, or Vorlcnorpel stage, but the condensed tissue from which the skeletal parts are derived gives rise not only to cartilage but also to perichondrium and to ligaments. Recognizing this fact, Schomburg, go, has called the condensed-tissue stage, the mesenchymal period, and restricted the term Vorknorpel to the earlier stages of the formation of cartilage. The term “blastemal” is now-a-days commonly used to designate a mass of mesenchymal tissue from which organs are to be differentiated, and is applied to the tissue of the limb-bud before differentiation has commenced. It seems to me that it would be well to extend this term to the structures ﬂrst dinerentiated in the limb. Thus, “sclerob1astema" would mean the tissue dimerentiated from the blastema. of the leg and destined to give rise to skeletal structures; myoblastema, the time differentiated for the muscles, and dermablastema that destined for the skin.
The Blastemal Period
The division of the axial mesenchyme into segments, sclerotomes, which correspond to the myotomes and spinal ganglia, is marked at an early stage by intersegmental arteries. Schultze, 96, has shown that the segmental diiferentiation of the axial mesenchyme extends into the region dorsal to the spinal cord. Ventrally it does not, however, extend quite to the chorda dorsalis. Fig. 1, Plate I, illustrates the conditions existing in the thoracic region of man at this period.
v. Ebner, 88, found in the embryos of several vertebrates a ﬁssure which divides each sclerotome into an anterior and a posterior portion. Schultze, in 1896, showed, that in selachians and reptiles this ﬁssure is represented from the time of its formation by a diverticulum which communicates with the myocoel. In birds the diverticulum arises secondarily and later becomes fused with the myocoel, and in mammals it arises after the myocael has disappeared.
In man the fissure becomes distinct in the thoracic region at about the end of the third week of development (Fig. 2, Plate 1) .' At this period the median surface of each myotome has become converted into muscle ﬁbres (Fig. 2, M yo.) . At the same time the mesenchyme in the posterolateral region of each sclerotome has become condensed so that it appears, in a stained section, dark when compared with that of the anterior half (Fig. 2). At the lateral margin of the anterior halves of the sclerotomes -the spinal nerves extend out toward the thoracic wall (Fig. 2, Sp. N .). The division between the sclerotomes is still marked by the intersegmental arteries (Fig. 2). About the chords. dorsalis the cells of the axial mesenchyme become densely grouped into a perichordal sheath. The long axes of the cells lie parallel with the chords (Fig. 2, Pch. 8.).
The condensation of tissue which distinguishes the posterior sclerotome half begins, as mentioned above, in the posterior lateral area of each sclerotome. From here the condensation extends dorsally between the medial surface of the posterior half of the corresponding myotome and spinal ganglion and gives rise to a dorsal, or neural, process (Figs. 5 and 6, Plate 2, N. P12). At the same time it proceeds ventrally along the distal margin of the corresponding myotome and gives rise to a ventral or costal process (Figs. 5 and 6, 0'. Pr.) ; and medially toward the chorda dorsalis, giving rise to a process which joins about the chorda dorsalis with a similar one, from the other side of the segment (Figs. 5 and 6, Disk). These median processes by their fusion form what has been termed by Weiss, or, a “horizontal plate.” “Primitive disk” seems to me perhaps a better term.
‘The ﬁgures on this and the following plates are based upon embryos belonging to the collection of Prof. Mail. I am greatly indebted to him for the use of these embryos. Charles R. Bardeen 165
The whole mass of condensed tissue which gives rise to the primitive dorsal, ventral and median processes has received various designations, of which that given by Froriep, 83, “primitive vertebral arc ” seems to be the most Widely accepted. Since, however, it represents much more than -a vertebral semi-arch, I have previously, 99, suggested for it the term “ scleromere”.
Figs. 8, 9 and 10, Plate III, represent wax-plate reconstructions of several scleromeres from the -thoracic region of Embryo II, length '7 mm. The outlines of the condensed tissue ‘are not so sharp in nature as it is necessary to make them in a model of this kind. It is believed, however, that the general form relations are here fairly accurately shown. In Fig. A, Plate II, of the article by Bardeen and Lewis are shown the relations of the scleromeres to other structures.
During the period of differentiation of the scleromeres the myotomes undergo a rapid development. The median surface of each myotome gradually protrudes opposite the ﬁssure of v. Ebner. The dorsal and ventral processes of each scleromere are then slowly forced into the interval between the belly of the myotome to which it belongs and the one next posterior, and thus ﬁnally they come to occupy an intersegmental position. It is not, however, correct to call the early processes of the scleromeres “ myosepta,” as some text-book writers have done. Fig. 4 shows this.
‘By the ﬁssure of v. Ebner each sclerotome is divided into two portions, of which the posterior in the higher vertebrates plays the chief role in vertebral differentiation. “ scleromere" therefore seems an appropriate designation for the condensed sclerogenous tissue of this half-segment. Goette has recently, 97, brought forward evidence in favor of the View that primarily in the digitates there were two vertebrae to each body-segment. In the higher vertebrates, during embryonic development, the posterior skeletal area of each body-segment alone develops freely. The anterior area becomes fused with the scleromere in front.
This figure represents a horizontal section passing through several spinal ganglia, myotomes and neural processes. The last may be seen extending gradually into the area opposite the myotomic septa, but they still cover the whole posterior half of the median surface of the myotome in the region of the section. The processes are connected by membranous thickenings of the mesenchyme of the anterior half of each segment. These membranes may be called interdorsal membranes. They correspond to the interdorsalia of elasmobranchs. Figs. 12, 13, 15 and 16, Idr. M., represent these membranes. A line drawn from A to B in Fig. 12 would pass through an area corresponding to that of the section represented in Fig. 4.
In the region where the neural and costal processes spring from the primitive disks membranous septa are likewise differentiated from the anterior halves of the sclerotomes. These septa serve to unite the successive disks. Each is continuous posterially with -a dense tissue which strengthens the primitive disk and anteriorly it extends into the neural and costal processes. The relations of these interdiscal membranes are shown in Figs. 3, 11, 12 and 13, Ids. m. Since at this period structural outlines are by no means sharp, the ﬁgures based upon wax-plate reconstructions must be taken as semi-diagrammatic. A line drawn from 0 to d in Fig. 12 would represent essentially the plane of the section shown in Fig. 3.
During the development of the interdiscal membranes the primitive disks become hollowed out on the posterior surface. A comparison of Fig. 2 with Fig. 3 demonstrates this. The perichordal sheath meanwhile is developed in a ventrodorsal direction so that the area between the primitive disks becomes divided into right and left halves. Figs. 11, 13 and 7 all show this. Each lateral area is ﬁlled with a lightly staining mesenchynie which is continuous ventrally and dorsally with the tissue surrounding the spinal column.
Fig. 17, Plate V, represents a sagittal section cut slightly obliquely through an embryo 12 mm long. In the region where the chorda (Oh. d.) is cut, the primitive disks may be seen united by a fairly dense tissue, the perichordal septum (Sptm.). Posterior to this region the section passes to one side of the chief axis of the embryo. The intervertebral disks may here be seen separated by a lighter tissue and in the more posterior portion of the section, which passes still more lateral to the chordal region, the tissue between the disks is seen to be continuous with that surrounding the spinal column. In this region the interdiscal membrane (Ids. m.) is seen anterior, the primitive disk posterior to the ﬁssure of v. Ebner (F. v. E.).
Meanwhile the ventral processes of the thoracic vertebrae extend well into the thoracic wall, giving rise to primitive ribs, illustrated in Fig. B, Plate II, in the article of Bardeen and Lewis, 01.
Development proceeds rapidly. In Embryo CIX 109, length 11 mm, age about five weeks (Figs. 14-16), the conditions are as follows: The neural processes are somewhat better developed than those of the preceding stage, but otherwise are similar in character. The costal processes are considerably farther developed (Bardeen and Lewis, 01, Plate V, Fig. E). At the angle between the neural and costal processes opposim where they join the primitive disks a transverse process, but slightly indicated at the preceding stage, is now fairly clearly marked. Each primitive disk has become further hollowed out at its posterior surface, owing, in all probability, to the conversion of its tissue into that of the area between the disks. The interdiscal membrane (Ids. m.), on the other hand, has become thicker and has extended anteriorly and posteriorly about the area between the disks so that this has become completely enclosed (Figs. 14, 15 and 16). The tissue of each segment immediately anterior to the primitive disk has become greatly thickened and the line between it and the disk indistinct.
The area between each two primitive disks is still divided by the perichordal septum (Fig. 7). Each half represents the anlage of .a chendrogenous center of the vertebral body. Formation of cartilage has not, however, begun. The thickening of the ventral margin of the primitive disk at this stage represents the “hypochordal Spange,” which Froriep has shown to play an important part in the development of the vertebra of birds and of the atlas in mammals. It has merely a transitory existence in the thoracic region of man.
To sum up brieﬂy, we may say that during the blastemal period each scleromere becomes divided into two portions, an anterior and a posterior, characterized by a much greater condensation of tissue in the posterior. From this condensed tissue arises a primitive vertebra of Remak, or scleromere, with dorsal (neural) and ventral (costal) processes and a disk uniting them to the mesenchyme condensed about the chorda dorsalis. From the tissue of the anterior half of each sclerotome arise membranes which serve to unite the dorsal processes of the scleromeres, interdorsal membranes, and to cover in the areas between the successive disks, interdiscal membranes. The primitive disks become hollowed out posteriorly by a loosening up of their tissue and strengthened anteriorly by a condensation of the tissue immediately bounding the ﬁssure of v. Ebner. The area between each two disks is bilaterally divided by a membrane springing from the perichordal sheath. The formation of a cartilagenous keleton now begins.
The tissue relations during this period have been carefully studied in representatives of most of the chief groups of vertebrates. The form of the early structures has been less accurately determined because most investigators have avoided the somewhat laborious methods of plastic reconstruction.
On each side of the blastemal vertebra three primary centers of chondriﬁcation appear at about the same time, one for neural process, one for the costal process and one for the vertebral body. Fig. 7, Plate II, shows these centers as they appear in a cross section at an early period. Figs. 25, 26 and 27, Plate VI, show the early cartilages of an embryo slightly older, CXLIV, length 14 mm., age 5% weeks.
The cartilages of the vertebral body develop by a transformation of the tissue lying between the primitive vertebral disks and surrounded by the interdiscal membrane. A considerable part of this tissue is derived from the posterior surface of each primitive disk. At ﬁrst the cartilage of the left side is separated from that of the right by the perichordal septum. Soon this is broken through and the two anlages of cartilage become united about the chorda. In the thoracic region this union seems to take place at about the same time dorsally that it does ventrally. A sagittal section of an embryo at this stage is shown in Fig. 18. The chorda dorsalis is surrounded by a perichordal sheath. The latter is encircled by dense intervertebral disks which alternate with light cartilagenous rings. The latter are surrounded by perichondrium which is less condensed than the tissue of the disks, but more so than that of the bodies and about the same as that of the perichordal sheath. Ventrally and dorsally a longitudinal ligament has been differentiated from the surrounding mesenchyme.
It is probable that the disks seen in this section are formed in part from the primitive disks, in part from the posterior layer of the anterior sclerotome halves; in other words, that each is formed about the rudiment of the ﬁssure of v. Ebner. Compare Figs. 17 and 18. The tissue is concentrically arranged in a way somewhat resembling that of the intervertebral disks of the adult.
The perichordal tissue rapidly decreases in thickness. At the same time the cartilage of the vertebral bodies grows also at the expense of the intervertebral disks (Figs. 19, 20, 21 and 22). According to Schultze, 96, the cartilages of the bodies ﬁnally fuse to form a continuous cartilagenous column. This does not seem to be the case in man. In all of the embryos belonging to the collection of Prof. Mall some membranous tissue may be seen separating completely the successive bodies, Charles R. Bardeen 169
but in embryos between 20 and 40 mm. in length this membrane in the vicinity of the chords dorsalis is very thin. At the periphery of the disks the annulus ﬁbrosus is meanwhile differentiated more and more into a condition resembling the adult (Figs. 17-23, Plate V).
The chorda dorsalis at the period shown in Fig. 18 is of about the same size at the level of the disks and between them, but as the bodies increase in size at the expense of the disks the chordal canal becomes enlarged in the intervertebral areas and constricted at the center of the bodies (Figs. 19, 20 and 21). The chordva loses its continuity and the chordal cells become clumped in the vicinity of the disks (Figs. 21 and 22) and ﬁnally spread out there in the form of a ﬁat disk (Fig. 23). At this last period the perichondrium of. the bodies is again becoming well marked and the portion of each intervertebral disk in the vicinity of the chorda dorsalis is better developed than during the stages immediately preceding. The chordal canal long remains in the vertebral body (Figs. 23 and 24).
The cartilage of the bodies in Embryo CXLIV(Fig. 18) is of an early embryonic hyaline type. At a slightly later stage (Fig. 19) two regions may be distinguished, a central and a peripheral. The central cartilage is denser than that of the preceding stage, while the peripheral cartilage resembles it. Gradually the cartilage at the center of the body undergoes further changes. The cells enlarge and become sharply set oﬂf against the intercellular substance (Figs. 22, 23 and 24), and ﬁnally an invasion of blood vessels takes place, chieﬂy from the posterior surface (Fig. 23). These changes in the cartilage, represented also in Fig. 41, Plate VII, are preliminary to ossiﬁcation.
Deposit of calcium salts and actual ossiﬁcation begins in the distal thoracic and proximal lumbar vertebrae of embryos about 5 to '7 cm. long and three months of age. Fig. 42 shows a center of ossiﬁcation in an embryo of '70 mm.
During the development of the vertebral bodies changes have been active in the neural cartilages. At the period represented in Fig. 7, Plate II, the neural cartilage is «a small, ﬂat body situated in the dorsal process of the scleromere; from this as a center, pedicular, transverse, anterior (superior) and posterior (inferior) articular, and laminar processes are rapidly developed. This structural differentiation is best followed in the ﬁgures representing the models (Figs. 25-36). The pediculazr processes are at ﬁrst slender rods (Fig. 26), each of which grows out towards and ﬁnally fuses with its corresponding vertebral body. Froriep has shown (83) that in the chick this process forms a more essential element of the body than in mammals. In the atlas it forms a lateral half of the 170 Development of Thoracic Vertebrw in Man
ventral arch, but in the thoracic region of mammals it fuses with the antero-lateral portion of the corresponding vertebral body. After its junction with this the pedicle increases in size but otherwise shows no marked alteration of form.
The transverse process is at ﬁrst a short projection which lies at some distance from its corresponding rib (Fig. 26). The cartilagenous rib rapidly increases in size and at the same time the transverse process grows outward and forward to meet it (Figs. 29, 32 and 34). At ﬁrst the developing cartilage of the rib and that of the transverse process are embedded in a continuous blastema, but before chondriﬁcation has proceeded far, branches from successive intervertebral arteries become anastomosed in the area between the neck of the rib and the transverse process and separation is eﬁected (Figs. 36, 38 B and 39).
Between the extremity of the transverse process and the rib a joint is developed (Figs. 39, 40, 41 and 42), and the surrounding blastema converted into costo-transverse ligaments.
The articul-ar processes develop slowly from the cartilage. Extension takes place anteriorly, A. A. Pr., and posteriorly, P. A. Pr., in the interdorsal membrane. In an embryo of 14 mm. (Figs. 25, 26 and 27) these articular plates are separated by a distinct interval. In one of 17 mm. they have approached each other very closely (Fig. 37) ; and in one of 20 mm. not only do the articular processes show distinctly more form (Figs. 28, 29 and 30), but in addition the superior articular process slightly overlaps the inferior (Fig. 38). This overlap of the superior articular processes is distinctly more advanced in an embryo of 28 mm. (Fig. 39), and still more so in one of 33 mm. (Figs. 31-33). In an embryo of 50 mm. (Figs. 34, 35 and 40) conditions essentially like the adult have been reached.
The laminar processes scarcely exist in Embryo CXLIV (Fig. 26). In Embryo XXII (Fig. 29) they have begun to project posteriorly to the region of the articular processes (Fig. 29). The dense embryonic connective tissue covering the laminar processes at this stage gives attachment to a membrane covering, the dorsal musculature, F. D. M ., and to a membrane surrounding the spinal cord, M. R. D. This accounts for the two projections seen dorsally on the side of the model representing the membranous tissue. In Embryo CXLV, length 33 mm., the laminar processes extend well toward the dorsal line (Figs. 32 and 33) ; in Embryo LXXXIV, length 50 mm. (Figs. 34, 35 and 40), they completely encircle the spinal canal and from the region of fusion of each pair a spinous process extends distally, though not so far as in the adult.
Alterations in the cartilage of the neural processes preliminary to Charles R. Bardeen 171
ossiﬁcation begin at about the time they take place in the vertebral bodies. They are ﬁrst seen in an area which corresponds to that in which the neural cartilage begins. The earliest calciﬁcation appears in Embryo CLXXXIV, length 50 mm., in the arches of the ﬁrst cervical to the sixth thoracic vertebrae.
The development of the ribs I shall not attempt in this place to describe in detail. Figs. 25-34 and 37-42 show suﬂiciently well the relations of the proximal ends of the ribs to the vertebrae. They are developed opposite the intervertebral disks. The blastemal tissue which surrounds the developing heads of the ribs becomes converted into eosto-vertebral ligaments. Diiferentiation in the cartilage preliminary to ossiﬁcation takes place in the shafts of the ribs even earlier than in the vertebral bodies and in the neural processes. Ossiﬁcation is well under way in the shafts of the ribs of Embryo LXXIX, length 33 mm. ; XCVI, length 44 mm.; XCV, length, 46 mm. ; and LXXXIV, length 50 mm.
Each cartilagenous vertebra is developed from four centers of chondriﬁcation. In addition, a separate center appears for each rib. In comparing these centers with the blastemal formative centers, we ﬁnd that each primative center of blastemal condensation enters into union with tissue derived from the anterior half of the body-segment next posterior and then gives rise to three centers of chondriﬁcation, one for the neural arch, one for the rib and one for half a vertebra. When ossiﬁcation ﬁrst takes place the centers for the ossiﬁcation of the neural arches and the ribs correspond to the original chondriﬁcation centers in the blastema, but the centers for ossiﬁcation of the bodies show little. trace of the bilateral condition which marks the cartilagenous fundaments.
The processes of chondrogenous form differentiation are shown in the drawings of the models. The period of ossiﬁcation of the vertebrae has been so often and so well described that no attempt will be made to enter upon a further acount of it in this paper. I have, however, not found two primary ossiﬁcation centers, such as Renault and Rambaud have described, for each neural arch.
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Explanation of Figures
Abbreviations Used to Designate Structures Illustrated in the Figures
A. A. Pr., anterior articular process. N. P1'., neural process.
C. V., cardinal vein. Pch. 8., perichordal sheath.
C. Pr., costal process. P. A. Pr., posterior articular process. 0021., C(B10lI.l. Pd., pedicle.
Ch. (1., chora dorsalis. Rib, rib.
Den, dermis. Sc1., sclerotome.
Dz‘sIc., intervertebral disk. Sptm., perichordal septum.
D. L., dorsal ligament. Sp. 0., spinal cord.
D. M ., dorsal musculature. Sp. G., spinal ganglion.
F. 17. E., ﬁssure of v. Ebner. Sp. N., spinal nerve.
F. D. M., fascia of dorsal musculature..S'p. Pr., spinous process.
Ids. M., interdiscal membrane. T'rap., trapezius.
Idr. M ., interdorsal membrane. Tr. P12, transverse process.
Is. A., intersegmental artery. V. L., ventral ligament.
L., lamina. V. B., vertebral body.
Myo., myotome. 5, 6‘, 7, 5th, 6th and 7th thoracic vertebrae.
M. R. D., membrane. reuniens dorsalis.
FIGS. 1, 2, 3 and 4. Frontal sections through the thoracic region of several embryos during the blastemal period of vertebral development. 47.5 diam. (1) Embryo CLXXXVI 186, length 3.5 mm. (2) Embryo LXXX 80, length 5 mm (3) and (4) Embryo CCXLI 241, length 6 mm. (3) Through the region of the chorda dorsalis, (4) through a. more dorsal plane. Figures 1, 3 and 4 represent sections cut somewhat obliquely so that the right side of the sections is ventral to the left. In Figs. 2 and 4 on the right side the bodies of several embryonic vertebrae are represented in outline. In Figs. 2 and 3 owing to artefacts the myotomes are pulled away from the sclerotomes.
FIGS. 5, 6 and 7. Cross-sections through midthoracic segments during the blastemal period of vertebral development. 55 diam. (5) Embryo LXXVI, length 4.5 mm. The right side of the section passes through the middle, the left side through the posterior third of the 5th segment. (6) Embryo II, length 7 mm. 5th thoracic segment. The right side of the drawing represents a. section anterior to that shown at the left. (7) Embryo CLXXV, length 13 mm. The left half of the 6th vertebral body, neural process and rib are drawn in detail, the body-wall, spinal cord and spinal ganglion are shown in outline.
FIGS. 8, 9, 10, 11, 12 and 13. Views of models representing the blastemal stage of vertebral development. (8-10) Embryo II, length 7 mm., 33% diam. (11-13) Embryo CLXIII, length 9 mm., 25 diam. (14-16) Embryo CIX, length 11 mm., 25 diam. 8, 11 and 14 views from in front; 9, 12, 15, views from the side; 10, 13, 16, views from behind 174 Development of Thoracic Vertebrae in Man
Frcs. 17-24. Sagittal sections in the mid-line through the 6th, 7th and 8th thoracic segments of a series of embryos from 12 to 50 mm. long. (17) Embryo CCXXI, length 12 mm. This section includes several segments anterior and posterior to the three above mentioned, 6th, 7th and 8th. (18) Embryo CXLIV, length 14 mm. (19) Embryo CVIII, length 22 mm. (20) Embryo LXXXVI, length 30 mm. (21) Embryo CXLV, length 33 mm. (22) Embryo LXXIX, length 33 mm. (23) Embryo XCVI, length 44 mm. (24) Embryo CLXXXIV, length 50 mm.
Fxos. 25-35. Dorsal, lateral and ventral views of models made by the Born method to illustrate vertebral form-differentiation in the 6th, 7th and 8th thoracic vertebrae during the chondrogenous period. On the left side the cartilagenous, on the right the enveloping ﬁbrous tissue is shown. The latter is also shown on the eighth vertebra in Figures 29 and 35. (25-27) Embryo CXLIV, length 14 mm., 20 diam. (28-30) Embryo XXII, length 20 mm., 13 diam. (31-33) Embryo CXLV, length 33 mm., 10 diam. (34, 35) Embryo LXXXIV, length 50 mm., 10 diam. (34) Dorsal view, left half; (35) median view.
Figs. 36-42. Transverse sections through mid-thoracic vertebrae of a series of embryos. 5 diam. (36) Embryo CVI, length 17 mm. (37) Embryo CCXVI, length 17 mm. (38) Embryo XXII, length 20 mm. (39) Embryo XLV, length 20 mm. (40) Embryo LXXXIV, length 50 mm. (41) Embryo XLIV, length 70 mm. (42) Embryo XXIII, length 70 mm.
The models from which the illustrations in this article were drawn have been reproduced by Dr. B. E. Dahlgren at the American Museum of Natural History, New Y orh, N. Y ., and arrangements may be made for securing copies by applying to the Director of the Museum.
Cite this page: Hill, M.A. (2021, January 18) Embryology Paper - Development of the thoracic vertebrae in man. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_Development_of_the_thoracic_vertebrae_in_man
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