Difference between revisions of "Paper - The origin of the motor neuroblasts of the anterior cornu of the neural tube (1922)"

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=The Origin of the Motor Neuroblasts of the Anterior Cornu of the Neural Tube=
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By Raymond A. Dart, M.Sc., M.B., Cu.M. and Joseph L. Shellshear, M.B., Cu.M.
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Demonstrators of Anatomy, University College.
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==Introduction==
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The foundation of His’ conception lies in the ectodermal or neural tube origin of motor neuroblasts. It is essential therefore to analyse carefully the evidence of those who have actually dealt with this phase of the question. With few exceptions the His conception has been given a false appearance of certainty by writers having assumed this neural tube origin from the results of His, Ramon y Cajal, and Schaper.
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As recently as 1921 Neal writes ‘“‘That neuraxones develop as processes of ganglion cells scarcely admits of reasonable doubt in the light of the evidence now in our possession. Few to-day would challenge the truth of Harrison’s assertion that the work on the cultivation of tissues may be said without reserve to have completely proved the correctness of the conception of His and Ramon y Cajal.”
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Coordinated movements occur in the embryo at a time antedating the first appearance of ganglion cells and at a stage in development before those stages on which His, Ramén y Cajal, and Schaper worked. Such being the case the problem must be reinvestigated, for the origin of any structure or mechanism must go back to the ultimate beginnings, and these certainly antedate the stages which these men describe.
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His depicts no stages earlier than Pristiurus of 4-5 mm. in length and Cajal makes his deductions from chicks at stages from 72 hours onwards.
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The work of Harrison on the cultivation of tissues is undoubtedly a demonstration of the protoplasmic activity of nerve cells in general, but upon the subject of the site of origin of the cells we are particularly dealing with, it gives us no information whatever. Further, as Harrison stated in one of his papers, “ There is nothing in the present work which throws any light upon the process by which the first connection between the nerve fibres and its end organ is established.”” We must therefore emphasise at this point that, no matter how instructive researches and experiments may be on the development of axonic processes, if the somites are so functioning as to give rise to movements in the embryo before nerve-fibres, or even ganglionic cells, are demonstrable in the embryo, such researches as these can give us no certain 78 Raymond A. Dart and Joseph L. Shellshear
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information as to the origin of the mechanism concerned in the production of these movements.
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Some consider neuro-fibrillae to be the first evidence of “effector” activity, but the demonstration of these in an embryo after the advent of coordinated movements merely points to a later differentiation going on in a mechanism already established.
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Our aim is to demonstrate the origin of these motor neuroblasts from the myotomes and to establish the principle of both functional and structural continuity between the nervous system and the other systems which it controls.
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TERMINOLOGY
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The motor neuroblast is frequently referred to as the effector neuroblast or neurone when referring to the elements in the reflex arc.
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On the sensory side, as Huxley so clearly pointed out, we have (1) the receptor, a modified epithelial organ, (2) the nerve and ganglion or cell transmittor, and (8) the sensorium; whereas on the effector side we have only the term effector used, in some cases distinctly referring to the neurone itself, in others to the structure effected, or the two elements as a whole unit. In this paper the term “effector” is used for the motor neuroblast (or neurone) and we propose to introduce the term “‘expressor” for the organ acted upon by the effector neurone, be it muscle, electric organ, or other structure; and thus the term “expressor” on the motor side is similar in use to the term “‘ receptor” on the sensory side.
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Thus we could describe a simple reflex arc in the following terms: receptor, transmittor, sensorium, effector, expressor. The old term neuro-muscular, revived by Parker and others, in the case of undifferentiated primitive tissue is open to great objection because such tissue is neither neural nor muscular in that it is undifferentiated. Although the meaning of the term may be clear, in many cases its ambiguity becomes obvious when the undifferentiated tissue of the myotomes, or of the embryonic heart, is directly compared with adult fully differentiated muscle. The substitution here of the term effectorexpressor seems to us less open to objection. Furthermore it clarifies the general conception of the so-called ‘“‘independent-effectors” by dividing them into two obvious classes: independent expressors and independent effector-expressors.
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EVIDENCE OF COORDINATED ACTIVITY
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Recent work on unicellular organisms shows that the cilia, the expressors are coordinated by impulses from receptors in the cirri and we thus find the earliest expression of complicated coordination even in the unicellular organism. Parker shows in Invertebrata a definite effector-expressor (neuro-muscular) undifferentiated tissue. Obviously we have need of a conception sufficiently broad to comprehend these activities as well as those of higher forms. By such a conception the components of the effector-expressor mechanism are traced back to their sources, or, better still, common source in the single indifferent cell.
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We ourselves have observed movements of a coordinated character with abduction and adduction in embryos before the appearance of sensory neuroblasts, and when the apparent relationship between the neural tube and the myotome is one of contact. Such evidence has been to hand since Balfour, who, in his famous monograph on Elasmobranch fishes, states,
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‘Before the appearance of the third visceral cleft in a part of the innermost layer of each protovertebra (which may be called the splanchnic layer from its being continuous with the mesoblast of the splanchnoplane) opposite the bottom of the neural tube, some of the cells commence to become distinguishable from the rest and to form a separate mass. This mass becomes much more distinct a little later, its cells being characterised by being spindle shaped and having an elongated nucleus which becomes deeply stained by reagents. Coincidently with its appearance the young dog-fish commences spontaneously to move rapidly from side to side with a kind of serpentine motion, so that, even if I had not traced the development of this differentiated mass of cells till it becomes a band of muscles close to the notochord, I should have had little doubt of its muscular nature. It is indicated by the letters mp (in figs. 11, 12, and 13). Its early appearance is most probably to be looked upon as an adaptation consequent upon the respiratory requirements of the young dog-fish necessitating movements within the egg.
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Shortly after this date, at a period when three visceral clefts are present, I have detected the first traces of the spinal nerves.” Cf. fig. 1 of this paper.
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Wintrebert (1904), and Paton (1906), have confirmed this demonstration of Balfour’s. Furthermore, Paton points out that the primitive movements of abduction and adduction of the body begin at a time when these phenomena ‘‘may as yet neither be designated as myogenic or neurogenic in origin.”’ So, Paton describes for the vertebrate embryo phenomena comparable with those designated by Parker as newro-muscular in lower invertebrates.
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If the His conception is correct, there is, at this stage, no connection whatever between the neural tube and the myotomes. The effectors are in the neural tube and have not yet grown out to the expressor organ. How then are there coordinated movements of abduction and adduction?
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Of those who stand for discontinuity between the neural tube and myotome, Balfour alone would appear to have appreciated the significance of this question. Keen student of phylogeny as he was, he saw the origin of the effectorexpressor mechanism from a common source (as also did Gaskell) and he failed to account for the apparently insurmountable difficulty in any other way than by a pathologic-like lesion.
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Before describing in detail the actual histological appearance of various embryos at this age, it is expedient to quote Balfour in full on this point:
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‘General considerations. One point of general anatomy upon which my observations throw considerable light is the primitive origin of the nerves. So long as it was admitted that the spinal and cerebral nerves developed in the embryo independently of the central nervous system, their mode of origin 80 Raymond A. Dart and Joseph L. Shellshear
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always presented to my mind considerable difficulties. It never appeared clear how it was possible for a state of things to have arisen in which the central nervous system as well as the peripheral termination of nerves, whether motor or sensory are formed independently of each other; while between them a third structure was developed, which, growing out either towards the centre or towards the periphery, ultimately brought the two into connection. That such a condition could be a primitive one seemed scarcely possible.
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...[t is possible to suppose that in their primitive differentiation contractile and sensory systems may, as in Hydra, have been developed from the protoplasm of even the same cell.
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...When such a condition as that was reached the sensory portion of the cell would be called a ganglion cell, or, terminal sensory organ, the connecting process a nerve, and the contractile portion of the cell a muscle cell. When these organs were in this condition, it might not impossibly happen for the general developmental growth, which tended to separate the ganglion cell and the muscle cell, to be so rapid as to render it impossible for the growth of the connecting nerve to keep pace with it and that thus the process connecting the ganglion cell and the muscle cell might become ruptured. Nevertheless the tendency of the process to grow from the ganglion cell to the muscle cell, would remain, and when the rapid developmental growth had ceased, the two would become united again by the growth of the process which had previously been ruptured.”
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From our present knowledge of the origin of sensory neuroblasts, of the constitution of the neurones, and of primitive neuro-epithelial and neuromuscular cells, it is evident that to establish His’ conception, the demonstration of some pathologic-like lesion is logically necessary, but even then the presence of coordinated movements of abduction and adduction would be entirely unexplained.
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THE HISTOLOGICAL APPEARANCE OF THE EMBRYO AT THE TIME OF THE FIRST APPEARANCE OF , COORDINATED MOVEMENTS
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Fig. 1 is a section of a drawing of Squalus acanthias (No. 1498, Sect. 110, 8) from the embryological collection at Harvard inaugurated by Dr C. Sedgwick Minot.
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This embryo is described as being 3-8 mm. in length and shows the actual appearance of the embryo when the first movements occur. It corresponds almost exactly with Balfour’s figures. The myotome shows the differentiation and thickening of the inner wall (fig 1 6). In the interval between the myotome and notochord is to be seen the commencement of the so-called breaking-down of the inner wall of the myotome. The relationship between the myotome and neural tube is certainly one of contact, but it is not possible in this specimen to determine continuity.
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Fig. 2, Squalus acanthias, 4-0 mm. (H.C.) (No. 7050 a, Sect. 106, 10,2) shows a further stage in the differentiation of the myotome.
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Protoplasmic continuity between the myotome and neural tube is now very definitely established.
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The differentiation of the inner wall reveals the type of tissue shown in fig. 1, for we see the beginnings of the division of the effector-eapressor mechanism of the myotome into its two components effector and expressor. As to which of the cells lying outside the neural tube are motor neuroblasts, there is nothing in the specimen to tell. The cells are indifferent.
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Fig. 8. Squalus acanthias, 5-2 mm. (H. C.) (No. 1855 a, Sect. 251) shows a later differentiation. For further evidence as to the nature of these cells readers are referred to Held’s monograph in which similar stages are clearly depicted.
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Fig. 5.
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His publishes in his paper on the development of neuroblasts numerous pictures of Pristiurus in the same stages. He regarded the myotome of so little account that in all but a few pictures he leaves it out altogether. Comparing his pictures, however, of embryos of 44 mm. with fig. 2, we note a striking similarity between his figs. 838 and 39. He depicts a breakingdown of the myotome in the same area as fig. 2. We do not desire to fall into the error of assigning to the so-called sclerotome a purely neuroblastic function. We are alive to the outcome of this having been done in the case of the “neural crest.” Placodes give rise to neuroblasts and connective tissue. We consider that the inner wall of the myotome belongs to this placodal type of structure. But whereas we do not at this stage commit ourselves as to which cells are neuroblasts in the figures of this paper, His did commit himself and depicts neuroblasts outside the neural tube at this early period. The study of his figures leaves the impression that these cells have come from the myotome. His’ figs. 38 and 39 are reproduced as figs. 4 and 5. His proof that these motor neuroblasts are derived from the germinal layer of the neural tube will be found on pages 323 and 324 of his paper, “‘ Die Neuroblasten und deren Entstehung im Embryonalen Mark.”
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Briefly stated, his proof rests on two facts:
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(1) That the site of origin of the motor neuroblasts is the germinal layer, for when he sees the motor neuroblasts (in the outer part of the neural tube) present, there are holes in the ependymal layer from which they have sprung.
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(2) That the cubic capacity of a protoplasm of a germinal cell allows of sufficient volume of material to stretch from the neural tube to the myotome:
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“das Volum der Gesammtzelle ses .» 697 cub. ” des Kernes ee wee . 65 ,, ” des Zellenleibes ohne den Kern 632. ,,
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Die Breite eines Axencylinders betragt (mit einem Nadelzirkel am Projectionsbilde des Zeichnungsprismas gemessen) ca. 0-94. Aus obigen 632 cub. p Protoplasma wiirde demnach eine Faser von rund 250» Lange gebildet werden k6énnen.”
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Our criticism is not simply captious, for it is remarkable that His was able to determine so much with material showing such obvious lacunae after preparation. But surely the time has come for a reconsideration of this problem, and with evidence other than that used seriously by His.
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Figs. 6 and 7 are, respectively, Squalus acanthias, 6mm. (No. 298, Sect. 208, 10) and 7-5mm. (No. 149, Sect. 404, 8), both (H.C.). They show the further changes in the myotome and region between the myotome and neural tube. These figures illustrate the banking up of the nuclei lying in the protoplasm between the neural tube and the myotome. The inner wall of the myotome is reduced in thickness and in the number of rows of nuclei, whether due to growth of the embryo or to change in position of nuclei it is difficult to express an opinion. For comparison with figs. 3, 6 and 7, we represent figs 8, 9 and 10, which are a continuation of His’ pictures. If histological evidence is our criterion, these pictures clearly point to a centripetal rather than a centrifugal movement, for the neuroblasts are outside the neural tube in the early stages and inside in the later stages although no explanation of the relative change in position of the motor neuroblasts is offered by His.
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DISCUSSION OF EVIDENCE
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On the evidence so far as we have represented it, certain facts stand out:
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(1) A mechanism capable of coordinated movements is in existence in very early stages in the embryos of those vertebrates which take on a free existence at an early stage in ontogeny.
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Fig. 10.
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(2) This mechanism is in the myotome itself, and the subsequent differentiation of the myotome reveals the stage of development chosen by most investigators for the solution of the problem.
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(3) If there is no protoplasmic continuity between the myotome and the neural tube at these earliest stages, and if the motor neuroblasts are in the neural tube, His’ hypothesis can give us no explanation of the behaviour of the embryo unless we suppose that the adult mechanism has no relationship with the primitive embryonic mechanism.
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(4) In Squalus acanthias the motor ganglion outside the neural tube is seen in fig. 7 to be in the same stage of development as the so-called neural crest (in the sense of recent text-books). ©
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Thus the nuclei belonging to the motor neurones would seem to be differentiated from the myotome at a time which synchronises with the formation of sensory neuroblasts from neuroepithelium. This is no chance circumstance, but it reveals to us a definite phylogenetic story. We have developed an effector-ecpressor mechanism to respond to a receptor-transmittor mechanism. In brief, we have a vivid demonstration of the fact that the myotomic mechanism was developed in response to the exteroceptive side of the reflex are.
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By the development of the segmentally repeated reflex arc, characterised by its “invariability of response,” the animal kingdom becomes to a greater extent the master of its environment; the development of such a mechanism is the common characteristic of all animals with a segmented mesoderm, i.e. of the Annulata and higher Invertebrata and of the Vertebrata. The neural tube (or sensorium) itself is the later and typically vertebrate achievement in phylogeny developed for more accurate coordination and association.
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So far the problem has been approached from the standpoint of the lack of protoplasmic connection between the neural tube and the myotome, and also a type has been used where the collection of anterior cornual nuclei embedded in protoplasm seems to arise from the myotome en masse. Both Balfour and Beard refer to Léwe’s statement that the cells of this mass are, in part, of myotomic origin, and remark that this statement is a ‘“‘ gratuitous assumption.” Surely it is likewise a gratuitous assumption to derive them from the neural tube. If they have wandered out from the neural tube, what are they? And why do they wander out?
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With regard to the question of protoplasmic continuity! We were very interested in the question of fixation and felt that, whatever certain fixed specimens showed, there is a living connection between the myotome and the neural tube. Observing very many embryos in the course of fixation, we found that with a very great variety of fixatives the invariable response was violent contractions of the embryo extending over quite an appreciable interval of time, and this sometimes after preliminary chloretone treatment.
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Despite such violent contractions during fixation, we have found obvious continuity between the neural tube and myotome at very early stages. Our observations confirm those of Graham Kerr, Hensen and others on this point.
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Urodele material in our possession for which we are indebted to Dr Landacre of Ohio State University show unbroken lines of yolk and pigment passing between the neural tube and myotome, and by this fact leave no doubt of absolute protoplasmic connection. To our knowledge continuity, by virtue of a study of the disposition of the mitachondrial elements, has not been urged previously. :
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What is the nature of this connection? It is surely a protoplasmic pathway between the neural tube and the myotome. Graham Kerr (fig. 12, a, b, c) says “The nerve trunk is lengthened out and externally is continued into the muscle cell of the myotome.” There is no doubt he means motor nerve trunk (m.n.r.). There is no proof that there is a differentiated motor nerve trunk at so early a stage, meaning by nerve trunk the effector axon. Again, muscle cells have not been seen at this stage. The cells to which he refers are at that time primitive types in the process of differentiation, and many changes are going to take place on the inner wall of the myotome before there are seen muscle cells differentiated out of the cells of which he speaks.
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Assuming for the moment that these connections could be motor nerves, then it would mean that the neuroblast itself has differentiated in ontogeny at a time antedating any known differentiation of neuroblasts, and, furthermore, that the effector-expressor mechanism has differentiated long before the ganglion-receptor differentiation in association with which it undoubtedly arose in phylogeny. We will return to this question in the next section of our paper dealing with migration.
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THE MIGRATION OF NERVE CELLS
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The phenomenon of cell migration is accepted by all as being of fundamental importance in questions of development. The movements of blood cells, of phagocytic cells of various kinds, of mesenchyme cells are real, that these cells do move is amply shown by the examination of living embryos. Dr Sabin’s work on the chick and the Clarks’ on the tadpole are illustrative of the phenomenon.
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It is not surprising therefore, in the face of a familiar parallel instance, that practically all writers on the origin and development of the peripheral nervous system have given expression to some conception of migration in the case of the nerve cell, perhaps the most extreme example being that of Neumayer, who speaks of cells passing out of the neural tube and then being “fetched back” again. No one has actually seen the migration of a nerve cell in its entirety in the living embryo and without some selective method of intra-vitam staining such demonstration would be almost impossible.
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What do we mean by migration of the nerve cell?
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If a movement of the complete cell occurs from a place x to a place y, then it is legitimate to speak of the migration of that cell in its entirety. Evidence of such migration requires to a certain extent that those structures 88 Raymond A. Dart and Joseph L. Shellshear
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in the vicinity of the cell, by which we determine the relative alteration in position of the cell, should be fixed in position and in time. Again to show migration of a cell, in the sense in which we speak, requires that we demonstrate a movement of the whole cell and, since we ourselves in an extensive study of embryos, have never been able to define cell boundaries in their entirety, we hold that despite the suggestive appearances offered, the question is not proved either way from purely histological methods.
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To illustrate our meaning, let us refer to figs. 8 and 7. It could be granted, for the sake of argument, that the nuclei are moving either towards or away from the neural tube, but, if the protoplasm under the influence of any particular nucleus is, on the one hand, fixed in relation to the neural tube and, on the other hand, to the myotome, the cell as a whole does not move with reference to these structures if those relations are maintained. The nucleus may move however, and we believe it does do so. It is conceivable further, that, without any alteration in the relative position of the cell, the nucleus may be in earlier periods in relation to the myotome and in later periods become absorbed into the neural tube. Such an interpretation is justified not only from the figures of Selachian embryos we here produce, but also from His’ own figures despite the fact that they bear only the faintest resemblance to the embryos they are designated to depict.
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Assuming, then, that the nuclei may migrate, is there any other way in’ which the nuclei of motor neuroblasts may find themselves in the ventral horn of the neural tube and yet be primarily extraneural in origin?
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There are three possible lines of approach to this problem.
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(1) The examination of a close series of embryos.
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(2) The experimental method.
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(3) By the observation of living embryos in such a manner as Drs Sabin and Clark have done for cells of other types.
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THE SERIAL METHOD
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This method is recognised by all to have its serious limitations, and for this reason ‘has been supplanted at various times by newer fashions, such as the culture method and the experimental method. These in their turn have limitations just as serious if not more so.
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The serial method seems to us to give an almost complete answer to this problem and yet with many observers has not only failed, but has led to erroneous interpretations for the reason that an elementary law of topographical survey has been disregarded.
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The fundamental principle in surveying is that the “base line” or “datum line,” from which subsequent measurements are taken in order to determine the relative position of objects, must be fixed in space. No surveyor takes the shore line of a beach which is continually silting up for this purpose. For, should he do so, his map would soon have little value. The “datum line” which has been used by embryologists consciously or otherwise, has been the external limiting membrane and so any cells within the membrane have been stated to come from the neural tube itself.
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If.we take any embryo.in early stages we find, as His depicts in his fig. 39 (fig. 5 of this paper), blood forming cells (Bd.) just outside the external limiting membrane—in brief, the rudiment of a branch of the anterior spinal artery. We find this artery takes a constant position. We refer readers to current text-books of embryology, to His’ own work, to Hensen’s pictures and others. We reproduce different embryonic stages of this vessel, figs. 12, 13 and 14, showing its constancy with relation to. adjacent structures.
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The order of structures from within outwards is:
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# The original neural tube which is in most cases clearly distinguishable.
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# Commissural fibres running from the posterior horn to the anterior horn of the opposite side.
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# The blood vessel in question.
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# The motor nuclei.and also that which formed at the same time the included portion of the neural crest designated by Balfour as the commissure of the {{neural crest}}.
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The longitudinal connection between successive sensory segments has become included at the same time and so is laid the foundation for the columns of Goll and Burdach. The transference of the posterior root from a dorsal to a lateral position in the cord, so puzzling to Balfour, is explained by the increasing prominence of these longitudinal columns.
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This fact of inclusion into the neural tube is borne out by the studies of Kolliker upon reptilian and avian forms, where the process of inclusion is incomplete and motor cells lie extra-neurally even in the adult. That these cells are somatic motor cells is shown by the later work of Sterzi and is indicated in fig. 14, of a toad fish embryo, here provided. It seems reasonable to assume, concerning these pictures, that the differentiation of motor neuroblasts is relatively later in these particular animals.
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We are now in a position to discuss the observations of Ramon y Cajal. The earliest stage in which he could obtain a result with his method of staining was a 72 hour chick. His figure is reproduced as fig. 15.
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Upon our hypothesis the neuroblast designated as motor is already included. Cajal’s picture gives explanation of the true status of the so-called “motor nerve fibres” of Graham Kerr, previously referred to. These may well be the protoplasmic bridges in which the intercalated axones (sensorium of Huxley) are formed, and at this stage these axones, as depicted by Cajal, are becoming completely included into the neural tube.
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As motor neuroblasts are differentiated before the 72 hour stage is reached, and before a silver reaction can be secured, it is obvious that Cajal’s work can give us no certain information concerning their origin.
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So far we have demonstrated the fact that motor neuroblasts and other cells may become incorporated into the neural tube without having to call in the aid of complete cell migration as a factor. We are nevertheless in accord with Ariens Kappers that neurobiotaxis is a real thing. Ariens Kappers has shown that the position of the motor nuclei in the medulla oblongata is in great part determined by the particular sensory tracts that predominate. If such is the case and motor nuclei are attracted thereby, centripetal rather than centrifugal movements on the part of motor neuroblasts should be expected. Consequently the inclusion of the motor cells of the anterior cornu in the neural tube forms the most striking verification of the neurobiotactic conception of Kappers.
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Fig. 12.
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We have so far dealt with the problem of the origin of the motor neuroblast from fixed serially sectioned material and from the physiological side.
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==Evidence from the Experimental Method of Approach==
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A considerable volume of experimental work has been done on the problem of the behaviour of nerve cells and the questions of development generally. But that the experimental method gives results capable of antagonistic explanations is shown by the anomalous findings in the works of Shorey and Harrison. It is instructive at the same time to compare the experimental results of Kuntz and Eric Muller on the origin of the sympathetic system.
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At Wood’s Hole, during the summer, we subjected the early developing tadpole of the Bull Frog to treatment with -7—-8 per cent. NaCl as suggested to us by Dr Streeter.
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Stockard from the experimental side, and Murk Jansen from the clinical side have shown that, if embryos are exposed to certain influences at definite times in development, certain definite defects or arrests in development follow. Anencephalia and amyelia are examples of such developmental arrests.
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In these particular experiments, careful histological examination showed that we were not successful in producing complete absence of the neural tube, i.e., complete anencephalia and amyelia. However, many cases histologically approached very closely this condition. In these embryos with defective neural tubes the myotomes developed and became fused with their fellows of the opposite side. Collections of ganglionic cells also developed ventral to, and, in many places, separated from, the neural tube in the substance of the fused myotomes. These embryos were capable of “vortex”? movements. We reproduce fig. 16, a type of such specimens, which are certainly confirmatory of our histological investigation in normal embryos; for if neuroblasts develop from the substance of the myotome, such an extra-neural situation of motor cells is to be anticipated where the neural tube is incomplete.
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Sherrington states that the anencephalic foetus suckles at the breast. Accurate information concerning the activities of such monsters is difficult to secure, but in Cincinnati we received first-hand information concerning the behaviour of an anencephalic monster. The mother gave birth to twins, of which the second was an anencephalic monster. The doctor and nurse placed it on one side and attended to the other child. Shortly afterwards the nurse called the doctor’s attention to the fact that the monster was moving its limbs about and gasping for breath. A cast of the head of this foetus, preserved at the Cincinnati hospital, leaves no doubt as to its anencephalic character.
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The explanation of these ‘“‘reflex”” phenomena is not yet apparent, but as in the tadpoles discussed, may well be bound up with the phenomenon of the inclusion of extra-neural elements into the neural tube. They are certainly inexplicable upon the His hypothesis. It is hoped that when the attention of obstetricians is called to these facts, still more precise evidence will be forthcoming concerning the neuro-physiology of these curious monsters.
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The remarks of Andral concerning teratomata are of great interest: “‘The progress, which has recently been made in the cultivation of embryology and comparative anatomy, has taught us that- the greater number of the organs are much more independent of each other in their respective formation, than was for a long time supposed; and that consequently any arrest in the development of one organ, but seldom necessarily produces a similar arrest in the development of others. For instance, we now know that the nerves may be perfectly developed independently of the existence of the brain or spinal cord; as has been abundantly proved in several cases of anencephalia and amyelia. It appears that the nerves are primarily formed in those organs, which it is their office to connect with the centres of the nervous system; and that they do not unite with these centres for a considerable time after their first rudiments are perceptible. Where these organs are deficient, the nerves are likewise deficient; so that the existence of the nerves depends much more on the development of the organ which they are destined to supply, than on that of the nerve centres. |
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M. Serres has recorded a remarkable illustration of this fact in the case of a monster with two brains and a single body in which case there were only 94 Raymond A. Dart and Joseph L. Shellshear
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two pneumogastrics found, arising one from the external side of each brain. In this case there were only two pneumogastric nerves, because there was only a single pulmonary and digestive apparatus for them to supply. In other cases on the contrary, which M. Serres has cited, when these organs were double, and the brain single, there were two sets of nerves destined for the two sets of organs” (1832).
 +
 +
It is consequently clear that the revival of so ancient a conception can searcely be regarded as revolutionary. The most modern views concerning developmental teratology and neurology found support in very salient evidence almost a century ago.
 +
 +
In conclusion we desire to express our + deep sense of indebtedness to Professor Grafton Elliot Smith for his generous advice and assistance in the preparation of this paper and to Professor Frederic T. Lewis of Harvard for placing the Minot Embryological Collection at our disposal during our stay in Boston.
 +
 +
 +
==Description of Figures==
 +
 +
A. Neural tube. D. Ventral ganglion. B. Inner wall of myotome. £. Dorsal ganglion. Cc. Notochord. V. Vessel.
 +
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Figs. 1, 2, 3, 6, 7, see text.
 +
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Figs. 4 and 5. (His’ figs. 38 and 39.) Pristiwrus-embryo von 44 mm. Lange. Neuroblasten aus der Markflache hervorbrechend. M. Myotomozellen. Bd. Bindegewebszellen.
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Fig. 8. (His’ fig. 40.) Pristiurus von 6mm. Linge. Gruppe von Neuroblasten, ihre Fortsitze an eine vordere Wurzel abgebend. Gekreuzer Verlauf der Fasern, die zum Ramus dorsalis und die zum R. ventralis gehen. C. Gruppe von Bindegewebszellen an der Stelle, wo spater ein Blutgefass liegt.
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Fig. 9. (His’ fig. 43.) Pristiurus-embryo von 8 mm. Lange. Randschnitt eines Wurzelstammchens. Das von einer kleinen Neuroblastengruppe ausgehende Staémmchen endet in kurzer Entfernung vom Mark schrig abgeschnitten. Bd. langs und quer gelagerte Bindegewebszellen in dessen Umgebung. Bei C. ein capillares Langsgefiss.
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Fig. 10. (His’ fig. 44.) Pristiurus-embryo von 14mm. Lange. Der Randschleier ist deutlicher ausgebildet, die Intermediarschicht vorhanden. Die vordere Wurzel zeigt einen cylindrischen axialen Strang, von einzelnen Bindegewebszellen umlagert. Innerhalb des Marks sind einige Fasern durch den Randschleier hindurch bis zur Intermediirschicht verfolgbar, andere treten schon vorher aus der Schnitt-flache heraus. Bei dieser und bei der vorigen Figur sind die reifen Neuroblasten an ihren hellen Kernen erkennbar. Bei C. das Langegefass.
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Fig. 11. Chick No. 2. 49 hours, sect. 7. 2. 4, illustrating the position of the anterior spinal artery in an earlier stage than fig. 12.
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Fig. 12. Toad fish embryo, No. 638, sect. 13. 2. 7, to illustrate the branch of the anterior spinal artery and the outlying group of motor cells.
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Fig. 13. Pig embryo 8 mm.
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Fig. 14. Toad fish embryo, to illustrate laterally placed motor ganglion. Note position of blood vessel entering the neural tube.
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Fig. 15. (Ramon y Cajal’s fig. 7 a.) Coupe de la moélle d’embryon du poulet au 3¢ jour de l’incubation.
 +
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Fig. 16. Bull frog embryo after treatment with -8 per cent. NaCl.
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 +
 +
==Literature Cited==
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 +
ANDRAL, G. (1832). A Treatise on Pathological Anatomy, translated from the French by Richard Townsend and Wm. West. New York, Samuel Wood and Sons, 261, Pearl St.
 +
 +
Barour, F. M. (1876). “The development of elasmobranch fishes.” Journ. of Anat. and Physiol. vols. x and XI.
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 +
Brarp, J. (1888). “Morphological Studies. II. The development of the peripheral nervous system of vertebrates. Part 1. Elasmobranchii and Aves.” Quart. Journ. Micr. Sci. vol. 29.
 +
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Ciark, E. R. (1912). “Further observations on living growing lymphatics: their relation to the mesenchyme cells.”” Amer. Journ. of Anat. vol. xin, No. 3.
 +
 +
Harrison, R. G. (1907). “Observations.on the living developing nerve fibre.” Anat. Rec. vol. 1, No. 5.
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 +
Hep, H. (1909). Die Entwickelung des Nervengewebes bei den Wirbelthieren. Barth, Leipzig. Hensen, V. (1903). Die Entwickelung hanik der Nervenbahnen im Embryo der Séugetiere. Kiel and Leipzig.
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His, W. (1889). “Die Neuroblasten und deren Entstehung im Embryonalen Mark.” Abh. sachs. Gesell. Wiss. Leipzig. Math.-phys. Cl., Bd. 15, No. 4.
 +
 +
Kerr, Grauam J. (1902-3). “The development of Lepidosiren paradoxa.” Quart. Journ. Micr. Sct. vol. XLVI.
 +
 +
KotxiKer, A. (1902). ‘‘ Ueber die oberflachlichen Nervenkerne im Marke der Vagel und Reptilien.” Zeitschrift f. wissen. Zool. LXxu. 1.
 +
 +
Kuntz, ALBERT and Bateson, O. V. (1920). “Experimental observations on the histogenesis of the sympathetic trunks in the chick.” J. Comp. Neur. vol, XXXII.
 +
 +
Motuer, Eric and Inevar, SVEN, (1921). “Ueber den Ursprung des Sympathicus bei den Amphibien.” Upsala Lakdref. forh. Ny foljd. Bd. xxv1, H. 5-6.
 +
 +
Nezat, H. V. (1903). “The development of the ventral nerves in Selachii. 1. Spinal ventral nerves.” Mark Anniversary Volume, Article XV.
 +
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—— (1921). “Nerve and plasmodesma.” J. of Comp. Neur. vol. xxx, No. 1.
 +
 +
Neumayer, L. “Entwickelungsgeschichte der Wirbelthiere.”’ Hertwig, vol. 11, Teil 3.
 +
 +
Parker, G. H. (1918). “The elementary nervous system.” Monographs on Experimental Biology. J. B. Lippincott & Co., Philadelphia.
 +
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Parton, S. (1908). ‘The reactions of the vertebrate embryo to stimulation and the association changes in the nervous system.” Mitt. aus der Zool. Stat. zw Neapel, Bd. 18.
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Ramon y Casa (1890). “A quelle époque apparaissent les expansions des cellules nerveuses de la moélle épiniére du poulet?” Anat. Anz. vol. v, p. 638.
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Sastn, F. R.(1917). “Origin and development of the primitive vessels of the chick and the pig.” Contributions to Embryology, vol. vi. Carnegie Instit. Wash. Pub. 226.
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Scnaper, A. (1897). “Die friihesten Differenzirungsvorginge im Centralnervensystem.” Archiv f. Entwick. mech. der Organismen, Bd. v, Heft 1.
 +
 +
SHERRINGTON, C.S. Schafer’s Text-book of Physiology.
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Suorzy, M. 8. (1909). “The effect of the destruction of peripheral areas on the differentiation of the neuroblasts.” Journ. of Exper. Zoology, vol. vi, No. 1.
 +
 +
Srerz, Anprea I. (1904). “I gruppi cellulari periferici della midolla spinale dei rettili.” Inst. Anat. della Regia Univ. di Pisa.
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Wintresert, M. P. (1904). “Sur lexistence d’une irritabilité excito-matrice primitive indépendante des voies nerveuses chez les embryons ciliés des Batraciens.” Comptes Rendus de la Soc. de Biol. vol. tv.
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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!

Dart RA. and Shellshear JL. The origin of the motor neuroblasts of the anterior cornu of the neural tube. (1922) J Anat. 56: 77-95. PMID 17103945

Online Editor  
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This historic 1922 paper by Dart and Shellshear is an early description of spinal cord motor development.



Dart RA & Shellshear JL. (1922). The Origin of the Motor Neuroblasts of the Anterior Cornu of the Neural Tube. J. Anat. , 56, 77-95. PMID: 17103945



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The Origin of the Motor Neuroblasts of the Anterior Cornu of the Neural Tube

By Raymond A. Dart, M.Sc., M.B., Cu.M. and Joseph L. Shellshear, M.B., Cu.M.

Demonstrators of Anatomy, University College.

Introduction

The foundation of His’ conception lies in the ectodermal or neural tube origin of motor neuroblasts. It is essential therefore to analyse carefully the evidence of those who have actually dealt with this phase of the question. With few exceptions the His conception has been given a false appearance of certainty by writers having assumed this neural tube origin from the results of His, Ramon y Cajal, and Schaper.


As recently as 1921 Neal writes ‘“‘That neuraxones develop as processes of ganglion cells scarcely admits of reasonable doubt in the light of the evidence now in our possession. Few to-day would challenge the truth of Harrison’s assertion that the work on the cultivation of tissues may be said without reserve to have completely proved the correctness of the conception of His and Ramon y Cajal.”

Coordinated movements occur in the embryo at a time antedating the first appearance of ganglion cells and at a stage in development before those stages on which His, Ramén y Cajal, and Schaper worked. Such being the case the problem must be reinvestigated, for the origin of any structure or mechanism must go back to the ultimate beginnings, and these certainly antedate the stages which these men describe.

His depicts no stages earlier than Pristiurus of 4-5 mm. in length and Cajal makes his deductions from chicks at stages from 72 hours onwards.

The work of Harrison on the cultivation of tissues is undoubtedly a demonstration of the protoplasmic activity of nerve cells in general, but upon the subject of the site of origin of the cells we are particularly dealing with, it gives us no information whatever. Further, as Harrison stated in one of his papers, “ There is nothing in the present work which throws any light upon the process by which the first connection between the nerve fibres and its end organ is established.”” We must therefore emphasise at this point that, no matter how instructive researches and experiments may be on the development of axonic processes, if the somites are so functioning as to give rise to movements in the embryo before nerve-fibres, or even ganglionic cells, are demonstrable in the embryo, such researches as these can give us no certain 78 Raymond A. Dart and Joseph L. Shellshear

information as to the origin of the mechanism concerned in the production of these movements.

Some consider neuro-fibrillae to be the first evidence of “effector” activity, but the demonstration of these in an embryo after the advent of coordinated movements merely points to a later differentiation going on in a mechanism already established.

Our aim is to demonstrate the origin of these motor neuroblasts from the myotomes and to establish the principle of both functional and structural continuity between the nervous system and the other systems which it controls.

TERMINOLOGY

The motor neuroblast is frequently referred to as the effector neuroblast or neurone when referring to the elements in the reflex arc.

On the sensory side, as Huxley so clearly pointed out, we have (1) the receptor, a modified epithelial organ, (2) the nerve and ganglion or cell transmittor, and (8) the sensorium; whereas on the effector side we have only the term effector used, in some cases distinctly referring to the neurone itself, in others to the structure effected, or the two elements as a whole unit. In this paper the term “effector” is used for the motor neuroblast (or neurone) and we propose to introduce the term “‘expressor” for the organ acted upon by the effector neurone, be it muscle, electric organ, or other structure; and thus the term “expressor” on the motor side is similar in use to the term “‘ receptor” on the sensory side.

Thus we could describe a simple reflex arc in the following terms: receptor, transmittor, sensorium, effector, expressor. The old term neuro-muscular, revived by Parker and others, in the case of undifferentiated primitive tissue is open to great objection because such tissue is neither neural nor muscular in that it is undifferentiated. Although the meaning of the term may be clear, in many cases its ambiguity becomes obvious when the undifferentiated tissue of the myotomes, or of the embryonic heart, is directly compared with adult fully differentiated muscle. The substitution here of the term effectorexpressor seems to us less open to objection. Furthermore it clarifies the general conception of the so-called ‘“‘independent-effectors” by dividing them into two obvious classes: independent expressors and independent effector-expressors.

EVIDENCE OF COORDINATED ACTIVITY

Recent work on unicellular organisms shows that the cilia, the expressors are coordinated by impulses from receptors in the cirri and we thus find the earliest expression of complicated coordination even in the unicellular organism. Parker shows in Invertebrata a definite effector-expressor (neuro-muscular) undifferentiated tissue. Obviously we have need of a conception sufficiently broad to comprehend these activities as well as those of higher forms. By such a conception the components of the effector-expressor mechanism are traced back to their sources, or, better still, common source in the single indifferent cell.

We ourselves have observed movements of a coordinated character with abduction and adduction in embryos before the appearance of sensory neuroblasts, and when the apparent relationship between the neural tube and the myotome is one of contact. Such evidence has been to hand since Balfour, who, in his famous monograph on Elasmobranch fishes, states,

‘Before the appearance of the third visceral cleft in a part of the innermost layer of each protovertebra (which may be called the splanchnic layer from its being continuous with the mesoblast of the splanchnoplane) opposite the bottom of the neural tube, some of the cells commence to become distinguishable from the rest and to form a separate mass. This mass becomes much more distinct a little later, its cells being characterised by being spindle shaped and having an elongated nucleus which becomes deeply stained by reagents. Coincidently with its appearance the young dog-fish commences spontaneously to move rapidly from side to side with a kind of serpentine motion, so that, even if I had not traced the development of this differentiated mass of cells till it becomes a band of muscles close to the notochord, I should have had little doubt of its muscular nature. It is indicated by the letters mp (in figs. 11, 12, and 13). Its early appearance is most probably to be looked upon as an adaptation consequent upon the respiratory requirements of the young dog-fish necessitating movements within the egg.

Shortly after this date, at a period when three visceral clefts are present, I have detected the first traces of the spinal nerves.” Cf. fig. 1 of this paper.

Wintrebert (1904), and Paton (1906), have confirmed this demonstration of Balfour’s. Furthermore, Paton points out that the primitive movements of abduction and adduction of the body begin at a time when these phenomena ‘‘may as yet neither be designated as myogenic or neurogenic in origin.”’ So, Paton describes for the vertebrate embryo phenomena comparable with those designated by Parker as newro-muscular in lower invertebrates.

If the His conception is correct, there is, at this stage, no connection whatever between the neural tube and the myotomes. The effectors are in the neural tube and have not yet grown out to the expressor organ. How then are there coordinated movements of abduction and adduction?

Of those who stand for discontinuity between the neural tube and myotome, Balfour alone would appear to have appreciated the significance of this question. Keen student of phylogeny as he was, he saw the origin of the effectorexpressor mechanism from a common source (as also did Gaskell) and he failed to account for the apparently insurmountable difficulty in any other way than by a pathologic-like lesion.

Before describing in detail the actual histological appearance of various embryos at this age, it is expedient to quote Balfour in full on this point:

‘General considerations. One point of general anatomy upon which my observations throw considerable light is the primitive origin of the nerves. So long as it was admitted that the spinal and cerebral nerves developed in the embryo independently of the central nervous system, their mode of origin 80 Raymond A. Dart and Joseph L. Shellshear

always presented to my mind considerable difficulties. It never appeared clear how it was possible for a state of things to have arisen in which the central nervous system as well as the peripheral termination of nerves, whether motor or sensory are formed independently of each other; while between them a third structure was developed, which, growing out either towards the centre or towards the periphery, ultimately brought the two into connection. That such a condition could be a primitive one seemed scarcely possible.

...[t is possible to suppose that in their primitive differentiation contractile and sensory systems may, as in Hydra, have been developed from the protoplasm of even the same cell.

...When such a condition as that was reached the sensory portion of the cell would be called a ganglion cell, or, terminal sensory organ, the connecting process a nerve, and the contractile portion of the cell a muscle cell. When these organs were in this condition, it might not impossibly happen for the general developmental growth, which tended to separate the ganglion cell and the muscle cell, to be so rapid as to render it impossible for the growth of the connecting nerve to keep pace with it and that thus the process connecting the ganglion cell and the muscle cell might become ruptured. Nevertheless the tendency of the process to grow from the ganglion cell to the muscle cell, would remain, and when the rapid developmental growth had ceased, the two would become united again by the growth of the process which had previously been ruptured.”

From our present knowledge of the origin of sensory neuroblasts, of the constitution of the neurones, and of primitive neuro-epithelial and neuromuscular cells, it is evident that to establish His’ conception, the demonstration of some pathologic-like lesion is logically necessary, but even then the presence of coordinated movements of abduction and adduction would be entirely unexplained.

THE HISTOLOGICAL APPEARANCE OF THE EMBRYO AT THE TIME OF THE FIRST APPEARANCE OF , COORDINATED MOVEMENTS

Fig. 1 is a section of a drawing of Squalus acanthias (No. 1498, Sect. 110, 8) from the embryological collection at Harvard inaugurated by Dr C. Sedgwick Minot.

This embryo is described as being 3-8 mm. in length and shows the actual appearance of the embryo when the first movements occur. It corresponds almost exactly with Balfour’s figures. The myotome shows the differentiation and thickening of the inner wall (fig 1 6). In the interval between the myotome and notochord is to be seen the commencement of the so-called breaking-down of the inner wall of the myotome. The relationship between the myotome and neural tube is certainly one of contact, but it is not possible in this specimen to determine continuity.

Fig. 2, Squalus acanthias, 4-0 mm. (H.C.) (No. 7050 a, Sect. 106, 10,2) shows a further stage in the differentiation of the myotome.

Protoplasmic continuity between the myotome and neural tube is now very definitely established.


The differentiation of the inner wall reveals the type of tissue shown in fig. 1, for we see the beginnings of the division of the effector-eapressor mechanism of the myotome into its two components effector and expressor. As to which of the cells lying outside the neural tube are motor neuroblasts, there is nothing in the specimen to tell. The cells are indifferent.

Fig. 8. Squalus acanthias, 5-2 mm. (H. C.) (No. 1855 a, Sect. 251) shows a later differentiation. For further evidence as to the nature of these cells readers are referred to Held’s monograph in which similar stages are clearly depicted.


Fig. 5.

His publishes in his paper on the development of neuroblasts numerous pictures of Pristiurus in the same stages. He regarded the myotome of so little account that in all but a few pictures he leaves it out altogether. Comparing his pictures, however, of embryos of 44 mm. with fig. 2, we note a striking similarity between his figs. 838 and 39. He depicts a breakingdown of the myotome in the same area as fig. 2. We do not desire to fall into the error of assigning to the so-called sclerotome a purely neuroblastic function. We are alive to the outcome of this having been done in the case of the “neural crest.” Placodes give rise to neuroblasts and connective tissue. We consider that the inner wall of the myotome belongs to this placodal type of structure. But whereas we do not at this stage commit ourselves as to which cells are neuroblasts in the figures of this paper, His did commit himself and depicts neuroblasts outside the neural tube at this early period. The study of his figures leaves the impression that these cells have come from the myotome. His’ figs. 38 and 39 are reproduced as figs. 4 and 5. His proof that these motor neuroblasts are derived from the germinal layer of the neural tube will be found on pages 323 and 324 of his paper, “‘ Die Neuroblasten und deren Entstehung im Embryonalen Mark.”

Briefly stated, his proof rests on two facts:

(1) That the site of origin of the motor neuroblasts is the germinal layer, for when he sees the motor neuroblasts (in the outer part of the neural tube) present, there are holes in the ependymal layer from which they have sprung.

(2) That the cubic capacity of a protoplasm of a germinal cell allows of sufficient volume of material to stretch from the neural tube to the myotome:

“das Volum der Gesammtzelle ses .» 697 cub. ” des Kernes ee wee . 65 ,, ” des Zellenleibes ohne den Kern 632. ,,

Die Breite eines Axencylinders betragt (mit einem Nadelzirkel am Projectionsbilde des Zeichnungsprismas gemessen) ca. 0-94. Aus obigen 632 cub. p Protoplasma wiirde demnach eine Faser von rund 250» Lange gebildet werden k6énnen.”

Our criticism is not simply captious, for it is remarkable that His was able to determine so much with material showing such obvious lacunae after preparation. But surely the time has come for a reconsideration of this problem, and with evidence other than that used seriously by His.

Figs. 6 and 7 are, respectively, Squalus acanthias, 6mm. (No. 298, Sect. 208, 10) and 7-5mm. (No. 149, Sect. 404, 8), both (H.C.). They show the further changes in the myotome and region between the myotome and neural tube. These figures illustrate the banking up of the nuclei lying in the protoplasm between the neural tube and the myotome. The inner wall of the myotome is reduced in thickness and in the number of rows of nuclei, whether due to growth of the embryo or to change in position of nuclei it is difficult to express an opinion. For comparison with figs. 3, 6 and 7, we represent figs 8, 9 and 10, which are a continuation of His’ pictures. If histological evidence is our criterion, these pictures clearly point to a centripetal rather than a centrifugal movement, for the neuroblasts are outside the neural tube in the early stages and inside in the later stages although no explanation of the relative change in position of the motor neuroblasts is offered by His.

DISCUSSION OF EVIDENCE

On the evidence so far as we have represented it, certain facts stand out:

(1) A mechanism capable of coordinated movements is in existence in very early stages in the embryos of those vertebrates which take on a free existence at an early stage in ontogeny.


Fig. 10.

(2) This mechanism is in the myotome itself, and the subsequent differentiation of the myotome reveals the stage of development chosen by most investigators for the solution of the problem.

(3) If there is no protoplasmic continuity between the myotome and the neural tube at these earliest stages, and if the motor neuroblasts are in the neural tube, His’ hypothesis can give us no explanation of the behaviour of the embryo unless we suppose that the adult mechanism has no relationship with the primitive embryonic mechanism.

(4) In Squalus acanthias the motor ganglion outside the neural tube is seen in fig. 7 to be in the same stage of development as the so-called neural crest (in the sense of recent text-books). ©

Thus the nuclei belonging to the motor neurones would seem to be differentiated from the myotome at a time which synchronises with the formation of sensory neuroblasts from neuroepithelium. This is no chance circumstance, but it reveals to us a definite phylogenetic story. We have developed an effector-ecpressor mechanism to respond to a receptor-transmittor mechanism. In brief, we have a vivid demonstration of the fact that the myotomic mechanism was developed in response to the exteroceptive side of the reflex are.

By the development of the segmentally repeated reflex arc, characterised by its “invariability of response,” the animal kingdom becomes to a greater extent the master of its environment; the development of such a mechanism is the common characteristic of all animals with a segmented mesoderm, i.e. of the Annulata and higher Invertebrata and of the Vertebrata. The neural tube (or sensorium) itself is the later and typically vertebrate achievement in phylogeny developed for more accurate coordination and association.

So far the problem has been approached from the standpoint of the lack of protoplasmic connection between the neural tube and the myotome, and also a type has been used where the collection of anterior cornual nuclei embedded in protoplasm seems to arise from the myotome en masse. Both Balfour and Beard refer to Léwe’s statement that the cells of this mass are, in part, of myotomic origin, and remark that this statement is a ‘“‘ gratuitous assumption.” Surely it is likewise a gratuitous assumption to derive them from the neural tube. If they have wandered out from the neural tube, what are they? And why do they wander out?

With regard to the question of protoplasmic continuity! We were very interested in the question of fixation and felt that, whatever certain fixed specimens showed, there is a living connection between the myotome and the neural tube. Observing very many embryos in the course of fixation, we found that with a very great variety of fixatives the invariable response was violent contractions of the embryo extending over quite an appreciable interval of time, and this sometimes after preliminary chloretone treatment.

Despite such violent contractions during fixation, we have found obvious continuity between the neural tube and myotome at very early stages. Our observations confirm those of Graham Kerr, Hensen and others on this point.


Urodele material in our possession for which we are indebted to Dr Landacre of Ohio State University show unbroken lines of yolk and pigment passing between the neural tube and myotome, and by this fact leave no doubt of absolute protoplasmic connection. To our knowledge continuity, by virtue of a study of the disposition of the mitachondrial elements, has not been urged previously. :

What is the nature of this connection? It is surely a protoplasmic pathway between the neural tube and the myotome. Graham Kerr (fig. 12, a, b, c) says “The nerve trunk is lengthened out and externally is continued into the muscle cell of the myotome.” There is no doubt he means motor nerve trunk (m.n.r.). There is no proof that there is a differentiated motor nerve trunk at so early a stage, meaning by nerve trunk the effector axon. Again, muscle cells have not been seen at this stage. The cells to which he refers are at that time primitive types in the process of differentiation, and many changes are going to take place on the inner wall of the myotome before there are seen muscle cells differentiated out of the cells of which he speaks.

Assuming for the moment that these connections could be motor nerves, then it would mean that the neuroblast itself has differentiated in ontogeny at a time antedating any known differentiation of neuroblasts, and, furthermore, that the effector-expressor mechanism has differentiated long before the ganglion-receptor differentiation in association with which it undoubtedly arose in phylogeny. We will return to this question in the next section of our paper dealing with migration.

THE MIGRATION OF NERVE CELLS

The phenomenon of cell migration is accepted by all as being of fundamental importance in questions of development. The movements of blood cells, of phagocytic cells of various kinds, of mesenchyme cells are real, that these cells do move is amply shown by the examination of living embryos. Dr Sabin’s work on the chick and the Clarks’ on the tadpole are illustrative of the phenomenon.

It is not surprising therefore, in the face of a familiar parallel instance, that practically all writers on the origin and development of the peripheral nervous system have given expression to some conception of migration in the case of the nerve cell, perhaps the most extreme example being that of Neumayer, who speaks of cells passing out of the neural tube and then being “fetched back” again. No one has actually seen the migration of a nerve cell in its entirety in the living embryo and without some selective method of intra-vitam staining such demonstration would be almost impossible.

What do we mean by migration of the nerve cell?

If a movement of the complete cell occurs from a place x to a place y, then it is legitimate to speak of the migration of that cell in its entirety. Evidence of such migration requires to a certain extent that those structures 88 Raymond A. Dart and Joseph L. Shellshear

in the vicinity of the cell, by which we determine the relative alteration in position of the cell, should be fixed in position and in time. Again to show migration of a cell, in the sense in which we speak, requires that we demonstrate a movement of the whole cell and, since we ourselves in an extensive study of embryos, have never been able to define cell boundaries in their entirety, we hold that despite the suggestive appearances offered, the question is not proved either way from purely histological methods.

To illustrate our meaning, let us refer to figs. 8 and 7. It could be granted, for the sake of argument, that the nuclei are moving either towards or away from the neural tube, but, if the protoplasm under the influence of any particular nucleus is, on the one hand, fixed in relation to the neural tube and, on the other hand, to the myotome, the cell as a whole does not move with reference to these structures if those relations are maintained. The nucleus may move however, and we believe it does do so. It is conceivable further, that, without any alteration in the relative position of the cell, the nucleus may be in earlier periods in relation to the myotome and in later periods become absorbed into the neural tube. Such an interpretation is justified not only from the figures of Selachian embryos we here produce, but also from His’ own figures despite the fact that they bear only the faintest resemblance to the embryos they are designated to depict.

Assuming, then, that the nuclei may migrate, is there any other way in’ which the nuclei of motor neuroblasts may find themselves in the ventral horn of the neural tube and yet be primarily extraneural in origin?

There are three possible lines of approach to this problem.

(1) The examination of a close series of embryos.

(2) The experimental method.

(3) By the observation of living embryos in such a manner as Drs Sabin and Clark have done for cells of other types.

THE SERIAL METHOD

This method is recognised by all to have its serious limitations, and for this reason ‘has been supplanted at various times by newer fashions, such as the culture method and the experimental method. These in their turn have limitations just as serious if not more so.

The serial method seems to us to give an almost complete answer to this problem and yet with many observers has not only failed, but has led to erroneous interpretations for the reason that an elementary law of topographical survey has been disregarded.

The fundamental principle in surveying is that the “base line” or “datum line,” from which subsequent measurements are taken in order to determine the relative position of objects, must be fixed in space. No surveyor takes the shore line of a beach which is continually silting up for this purpose. For, should he do so, his map would soon have little value. The “datum line” which has been used by embryologists consciously or otherwise, has been the external limiting membrane and so any cells within the membrane have been stated to come from the neural tube itself.

If.we take any embryo.in early stages we find, as His depicts in his fig. 39 (fig. 5 of this paper), blood forming cells (Bd.) just outside the external limiting membrane—in brief, the rudiment of a branch of the anterior spinal artery. We find this artery takes a constant position. We refer readers to current text-books of embryology, to His’ own work, to Hensen’s pictures and others. We reproduce different embryonic stages of this vessel, figs. 12, 13 and 14, showing its constancy with relation to. adjacent structures.

The order of structures from within outwards is:

  1. The original neural tube which is in most cases clearly distinguishable.
  2. Commissural fibres running from the posterior horn to the anterior horn of the opposite side.
  3. The blood vessel in question.
  4. The motor nuclei.and also that which formed at the same time the included portion of the neural crest designated by Balfour as the commissure of the neural crest.


The longitudinal connection between successive sensory segments has become included at the same time and so is laid the foundation for the columns of Goll and Burdach. The transference of the posterior root from a dorsal to a lateral position in the cord, so puzzling to Balfour, is explained by the increasing prominence of these longitudinal columns.

This fact of inclusion into the neural tube is borne out by the studies of Kolliker upon reptilian and avian forms, where the process of inclusion is incomplete and motor cells lie extra-neurally even in the adult. That these cells are somatic motor cells is shown by the later work of Sterzi and is indicated in fig. 14, of a toad fish embryo, here provided. It seems reasonable to assume, concerning these pictures, that the differentiation of motor neuroblasts is relatively later in these particular animals.

We are now in a position to discuss the observations of Ramon y Cajal. The earliest stage in which he could obtain a result with his method of staining was a 72 hour chick. His figure is reproduced as fig. 15.

Upon our hypothesis the neuroblast designated as motor is already included. Cajal’s picture gives explanation of the true status of the so-called “motor nerve fibres” of Graham Kerr, previously referred to. These may well be the protoplasmic bridges in which the intercalated axones (sensorium of Huxley) are formed, and at this stage these axones, as depicted by Cajal, are becoming completely included into the neural tube.

As motor neuroblasts are differentiated before the 72 hour stage is reached, and before a silver reaction can be secured, it is obvious that Cajal’s work can give us no certain information concerning their origin.

So far we have demonstrated the fact that motor neuroblasts and other cells may become incorporated into the neural tube without having to call in the aid of complete cell migration as a factor. We are nevertheless in accord with Ariens Kappers that neurobiotaxis is a real thing. Ariens Kappers has shown that the position of the motor nuclei in the medulla oblongata is in great part determined by the particular sensory tracts that predominate. If such is the case and motor nuclei are attracted thereby, centripetal rather than centrifugal movements on the part of motor neuroblasts should be expected. Consequently the inclusion of the motor cells of the anterior cornu in the neural tube forms the most striking verification of the neurobiotactic conception of Kappers.



Fig. 12.


We have so far dealt with the problem of the origin of the motor neuroblast from fixed serially sectioned material and from the physiological side.


Evidence from the Experimental Method of Approach

A considerable volume of experimental work has been done on the problem of the behaviour of nerve cells and the questions of development generally. But that the experimental method gives results capable of antagonistic explanations is shown by the anomalous findings in the works of Shorey and Harrison. It is instructive at the same time to compare the experimental results of Kuntz and Eric Muller on the origin of the sympathetic system.

At Wood’s Hole, during the summer, we subjected the early developing tadpole of the Bull Frog to treatment with -7—-8 per cent. NaCl as suggested to us by Dr Streeter.


Stockard from the experimental side, and Murk Jansen from the clinical side have shown that, if embryos are exposed to certain influences at definite times in development, certain definite defects or arrests in development follow. Anencephalia and amyelia are examples of such developmental arrests.

In these particular experiments, careful histological examination showed that we were not successful in producing complete absence of the neural tube, i.e., complete anencephalia and amyelia. However, many cases histologically approached very closely this condition. In these embryos with defective neural tubes the myotomes developed and became fused with their fellows of the opposite side. Collections of ganglionic cells also developed ventral to, and, in many places, separated from, the neural tube in the substance of the fused myotomes. These embryos were capable of “vortex”? movements. We reproduce fig. 16, a type of such specimens, which are certainly confirmatory of our histological investigation in normal embryos; for if neuroblasts develop from the substance of the myotome, such an extra-neural situation of motor cells is to be anticipated where the neural tube is incomplete.

Sherrington states that the anencephalic foetus suckles at the breast. Accurate information concerning the activities of such monsters is difficult to secure, but in Cincinnati we received first-hand information concerning the behaviour of an anencephalic monster. The mother gave birth to twins, of which the second was an anencephalic monster. The doctor and nurse placed it on one side and attended to the other child. Shortly afterwards the nurse called the doctor’s attention to the fact that the monster was moving its limbs about and gasping for breath. A cast of the head of this foetus, preserved at the Cincinnati hospital, leaves no doubt as to its anencephalic character.

The explanation of these ‘“‘reflex”” phenomena is not yet apparent, but as in the tadpoles discussed, may well be bound up with the phenomenon of the inclusion of extra-neural elements into the neural tube. They are certainly inexplicable upon the His hypothesis. It is hoped that when the attention of obstetricians is called to these facts, still more precise evidence will be forthcoming concerning the neuro-physiology of these curious monsters.

The remarks of Andral concerning teratomata are of great interest: “‘The progress, which has recently been made in the cultivation of embryology and comparative anatomy, has taught us that- the greater number of the organs are much more independent of each other in their respective formation, than was for a long time supposed; and that consequently any arrest in the development of one organ, but seldom necessarily produces a similar arrest in the development of others. For instance, we now know that the nerves may be perfectly developed independently of the existence of the brain or spinal cord; as has been abundantly proved in several cases of anencephalia and amyelia. It appears that the nerves are primarily formed in those organs, which it is their office to connect with the centres of the nervous system; and that they do not unite with these centres for a considerable time after their first rudiments are perceptible. Where these organs are deficient, the nerves are likewise deficient; so that the existence of the nerves depends much more on the development of the organ which they are destined to supply, than on that of the nerve centres. |

M. Serres has recorded a remarkable illustration of this fact in the case of a monster with two brains and a single body in which case there were only 94 Raymond A. Dart and Joseph L. Shellshear

two pneumogastrics found, arising one from the external side of each brain. In this case there were only two pneumogastric nerves, because there was only a single pulmonary and digestive apparatus for them to supply. In other cases on the contrary, which M. Serres has cited, when these organs were double, and the brain single, there were two sets of nerves destined for the two sets of organs” (1832).

It is consequently clear that the revival of so ancient a conception can searcely be regarded as revolutionary. The most modern views concerning developmental teratology and neurology found support in very salient evidence almost a century ago.

In conclusion we desire to express our + deep sense of indebtedness to Professor Grafton Elliot Smith for his generous advice and assistance in the preparation of this paper and to Professor Frederic T. Lewis of Harvard for placing the Minot Embryological Collection at our disposal during our stay in Boston.


Description of Figures

A. Neural tube. D. Ventral ganglion. B. Inner wall of myotome. £. Dorsal ganglion. Cc. Notochord. V. Vessel.

Figs. 1, 2, 3, 6, 7, see text.

Figs. 4 and 5. (His’ figs. 38 and 39.) Pristiwrus-embryo von 44 mm. Lange. Neuroblasten aus der Markflache hervorbrechend. M. Myotomozellen. Bd. Bindegewebszellen.

Fig. 8. (His’ fig. 40.) Pristiurus von 6mm. Linge. Gruppe von Neuroblasten, ihre Fortsitze an eine vordere Wurzel abgebend. Gekreuzer Verlauf der Fasern, die zum Ramus dorsalis und die zum R. ventralis gehen. C. Gruppe von Bindegewebszellen an der Stelle, wo spater ein Blutgefass liegt.

Fig. 9. (His’ fig. 43.) Pristiurus-embryo von 8 mm. Lange. Randschnitt eines Wurzelstammchens. Das von einer kleinen Neuroblastengruppe ausgehende Staémmchen endet in kurzer Entfernung vom Mark schrig abgeschnitten. Bd. langs und quer gelagerte Bindegewebszellen in dessen Umgebung. Bei C. ein capillares Langsgefiss.

Fig. 10. (His’ fig. 44.) Pristiurus-embryo von 14mm. Lange. Der Randschleier ist deutlicher ausgebildet, die Intermediarschicht vorhanden. Die vordere Wurzel zeigt einen cylindrischen axialen Strang, von einzelnen Bindegewebszellen umlagert. Innerhalb des Marks sind einige Fasern durch den Randschleier hindurch bis zur Intermediirschicht verfolgbar, andere treten schon vorher aus der Schnitt-flache heraus. Bei dieser und bei der vorigen Figur sind die reifen Neuroblasten an ihren hellen Kernen erkennbar. Bei C. das Langegefass.

Fig. 11. Chick No. 2. 49 hours, sect. 7. 2. 4, illustrating the position of the anterior spinal artery in an earlier stage than fig. 12.

Fig. 12. Toad fish embryo, No. 638, sect. 13. 2. 7, to illustrate the branch of the anterior spinal artery and the outlying group of motor cells.

Fig. 13. Pig embryo 8 mm.

Fig. 14. Toad fish embryo, to illustrate laterally placed motor ganglion. Note position of blood vessel entering the neural tube.

Fig. 15. (Ramon y Cajal’s fig. 7 a.) Coupe de la moélle d’embryon du poulet au 3¢ jour de l’incubation.

Fig. 16. Bull frog embryo after treatment with -8 per cent. NaCl.


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Cite this page: Hill, M.A. (2020, September 22) Embryology Paper - The origin of the motor neuroblasts of the anterior cornu of the neural tube (1922). Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_The_origin_of_the_motor_neuroblasts_of_the_anterior_cornu_of_the_neural_tube_(1922)

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