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Streeter GL. The status of metamerism in the central nervous system of chick embryos. (1933) J. Comp. Neural. 455-475.

Online Editor Note

This historic 1933 paper by George L. Streeter is an early description of the development of the chicken nervous system.

Modern Notes:
chicken

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Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding. (More? Embryology History \| Historic Embryology Papers)

The Status of Metamerism in the Central Nervous System of Chick Embryos

George Linius Streeter (1873-1948)

By George L. Streeter

Department of Embryology, Carnegie Institution of Washington, Baltimore, Maryland

Six Figures (1933)

Introduction

In the laboratory in which these lines are being written there has been installed recently a memorial exhibit in commemoration of Wilhelm His.‘ It was His who first called attention to the importance of the establishment of an institute for human embryology and who formulated the general plan upon which such an institute should be conducted. It was left for Mall, a devoted apostle, to make the His dream a reality and to guide its organization as it came into existence under the form of the Department of Embryology of the Carnegie Institution of Washington. It is left to us and succeeding generations to pause before this exhibit as a shrine and contemplate the contributions of a great scientist and to rejoice in the fine friendship that existed between him a.nd one of our own great men.

The His historical exhibit was made possible through Prof. W. His, his son, who had in his possession the majority of the items and who has deposited them in the Carnegie Laboratory for the purposes of the memorial. There are included a large number of personal copies of his monographs and separates containing marginal notes and sketches; original manuscript of his important paper “Héiute und Hiihlen des Korpers”; lecture notebook, recorded by W. Braunc, in which are his famous lecture—drawings; microscope and lenses used by Professor His from 1856 to 1904; ombryograph invented by him and with which much of his work was done, the predecessor of the camera lucida; a box of photographic transparencies made by him and on which the illustrations in the “Atlas of human embryos” were based; comparative embryological collection of more than 1000 slides prepared by him, covering a wide range of animal forms and the original material on which many of his researches were made; various portraitphotographs and a copy of the Hans Olde etching.

One of the fields of investigation to which His made important eontributions extend over the same ground with which this paper is concerned, namely, the natural subdivisions of the neural tube. VVhile 11ot regarded as one of his major accomplishments, the subdivisions introduced by him in the “N omina anatomica” have come i11to universal use. From careful studies of the development of the human brain and the brain of embryos of other vertebrate classes, he arrived at subdivisions that are applicable to embryonic stages of the various animals as Well as to the adult brain. He provided the anatomist with a morphological analysis of the brain based on the earliest stages and determined the relations of the separate parts to one another and traced their subsequent increasing complexity of form. In this way he greatly simplified neural topography and advanced our understanding of a most intricate organ.

During the time that His and the other members of his committee, Krause and Waldeyer, were conservatively arriving at brain analysis and terminology based on demonstrable morphological realities, a different direction was taken by the comparative embryologists. Apparently intoxicated by the simplification of the mechanics of development that seemed to be offered by the principle of metamerism, they did not stop their metamerism with the middle germ layer, but were applying it also to the other tissues including the medullary tube. Several investigators in this country joined in this movement. Among them was Dr. Charles Hill, working in the Zoological Laboratory of Northwestern University and under the inﬂuence of Professor Locy? I mention Hill because of the wide use that has been made of his observations. His drawings of the early stages of the brain and his presentation of neural metamerism is in general harmony with the present prevailing views among teachers and writers in this field.

2 Hill, C. 1899. Primary segments of the vertebrate head. Anat. Anz., Bd. 16,

S. 353-369. Also, 1900, Developmental hitory of primary segments of the vertebrate head. Zool. Jahrb., Bd. 13, S. 393-446.

In brief, the Hill-Locy interpretation of metamerism in the embryonic brain holds that the encephalic portion of the neural axis is divided into some eleven ‘similar joints or segments.’ In the fore- and mid-brain these are transient, being soon obliterated and succeeded by secondary modifications or expansions. In the hind-brain, however, the primitive segments persist longer and thus for a time are contemporaneous and might be mistaken as being in series with the secondary expansions of the fore- and mid-brain. It is emphasized that they are not really coordinate with the latter, but are identical with the evanescent segments which have faded out in the more oral levels.

In his papers of 1899 and 1900, Hill published a series of drawings of salmon and chick embryos in which neural segmentation as interpreted by him is shown with diagrammatic clarity. Those of the chick are the ones that have been so frequently reproduced in books on embryology, and as typical examples I am herewith showing two of his drawings (fig. 1). They portray the 4-somite stage, at which time he finds the Whole gamut of segments in their maximum distinctness. Since the principle of neural metamerism is so largely based on this and similar evidence, it becomes important to make certain of the morphological accuracy of the underlying observations. If the neural tube is actually segmented as Hill pictures it, one must accept some such interpretation as he and the other segmentalists have elaborated.

It is with this thought that the writer has gone back to the chick embryo and, through utilization of the available improvements in morphological technique, has sought to verify the presence or absence of transitory metameric segmentation of the brain. The results of this study are reported in the following paper. In summarizing these the writer finds himself in full agreement with Froriep,"* who, 40 years ago, expressed the opinion that metameres of the medullary tube, in the sense of primitive members of a segmentally laid-down organ system, are not justified by the available evidence.

‘Froriep, A. 1892. Verhandl. Anat. Gesel1., Wien, Ergi1'.nzungsh., Anat. Anz.

Segmentation of the vertebrate body seems to be originally limited to the middle germ—layer and Where segmental arrangement occurs in derivatives of the ectoderm it is of secondary origin and is an adaptation to the metamerism of the mesoblast.

Photographic records of the brain of the 4—sornite chick do not reveal the segmented condition pictured by Hill (fig. 1), nor do the next older or younger stages exhibit constrictions that can be satisfactorily fitted in with such a system of uniform segmentation. Instead of definable metameres which can be traced into specific brain parts, it is found that certain of the functional subdivisions of the brain reveal their identity Very early and before the closure of the neural folds. As development progresses, new subdivisions and modifying mechanisms detach from these or appear in the intervals between them. To what extent these more tardy subdivisions are predetermined structures and to what extent they are the product of interaction between the pioneer mechanisms is beyond the province of this paper. Morphologically, however, the various nuclear masses as they successively make their appearance and the communicating fiber-laminae can be traced step by step as they individually grow in size and complexity until the final form is attained. All of this takes place in the absence of any true metameric segmentation. Qualification should perhaps be made for the visceral cranial nerves. That portion of the neural axis from which they arise is characterized by marked transverse grooves, and if this group of nerves is to be regarded as segmental then the grooves produced by the proliferating neuroblasts giving origin to the nerves must also be held as segmental. But that is not granting much to neural metamerism.

fig.1 Brain segments in the 4—somito chick according to H.ill (1899). This frequently copied figure illustrates neural segmentation with great daring. When this is compared with photographs of actual specimens (fig. 2), scant justification is found for the segmentation as shown. Only the somite grooves are found in the photographs.

fig.2 Ventral and dorsal views showing tlm neural folds in four chick embryos of about the 4-somite stage for c-,omp:1rison with figure 1. Flnlargmnmut, X 20.

Chick Brain at Closure of Neural Tube

The chick is one of the most thoroughly studied of all embryonic types. It has been elaborately and systematically portrayed in atlas form and, moreover, in a whole series of atlases, beautiful and well known.‘ But even in this abundance of material it is difficult to determine whether I-Iill’s evanescent segments of the brain are present or not. They cannot be seen satisfactorily in total cleared preparations studied by transmitted light, the method heretofore invariably employed. In such specimens one has only incomplete glimpses of surfaces, rendered uncertain by refraction and overlapping of structures and inaccessible to special illumination. Consequently, a large personal equation enters in the selection of significant levels and contours. Nor do serial sections and reconstruction methods suffice. In face of these difficulties, it was concluded that the desired information as

‘E1-dl, M. 1’. 1845. Die Entwiekelung dos Menschen und des Hiihnchens im Eie. Leipzig. Duval, M. .1899. Atlas d’embryologie. Paris.

Keibel, F. 12. K. Abraham. 1900. Normentafel zur Entwicklungsgeschichte des I-Iulmes. Jena.

to the true form of the neural tube must be obtained from specimens whose tissues are rendered opaque and can be freely exposed to direct illumination by means of dissection and under circumstances that introduce the minimum amount of artifact. It was found that chick embryos having these requirements could be prepared by fixing blastoderms in Bouin’s solution which are then rinsed and dehydrated up to 80 per cent alcohol. In the latter medium they are dissected in various ways and photographed with carefully adjusted illuminations. At the stages in which we are interested the embryos are very tiny and magnification is necessary, the original negatives being made at an enlargement of 12 diameters. In making prints for study the enlargement is further increased to 25 or 50 diameters. A series of such photographs are shown in figure 2 and they will now be described.

The four stages shown in figure 2 were selected as representing the same period in which Hill found such well-defined brain segments and they should be closely compared with his figure reproduced in figure 1. At this time the neural folds are just beginning to close and there are from 3 to 4 separated somites. A and A’ of figure 2 are dorsal and ventral views of the youngest specimen in the group. The endoderm and heart have been removed, exposing the ventral surface of the primary sheet of mesoderm (A’). Hensen’s node can be seen, and extending forward from it is the ﬂat notochord. Lateral to the notochord on each side are the stripsiof the thickened mesoderm from which the somites are being segmented, three pairs can already be seen. The dorsal View of the same specimen (A) shows the neural folds as still rather ﬂat, except at the front end where there is active proliferation of the constituent cells and a relative excess of tissue, particularly in theregion where the eye evagination is to take place. The proliferation of cells occurs in such a way that their margins are turned up and tend to approximate. In gross appearance it is as though the opposing plates were pressed together with thumb and finger. The point to be noticed is that there is no distinguishable segmentation, except in the region of the mesodermal somites, which appear to be the elements responsible for the phenomenon. There are slight irregularities elsewhere along the margins of the folds, but they are less marked and bear no more evidence of being segmental than those characterizing the margins of the primitive groove in this same specimen.

The next specimen (fig. 2, B and B’) is slightly older. The primitive pit is more distinct and at that point on the ventral surface there is a prominent Hensen’s node, set apart from t.he primitive streak caudal to it. The fourth pair of mesodermal somites ca11 be recognized. The notochord can be seen lyi11g along the mid-line and taking its origin from Hensen’s node. The neural plates lie ﬂat as in the 3—somite stage, except at the front end where they are bent upward and their margins pressed against each other. In the ventral view (B’) the neural axis is correspondingly narrower, although there is already a little‘ fullness where the optic evagination is to take place. As in the previous specimen, there is no evidence of orderly segmentation other than that concerned with the somites. Nor is there so far, if we adhere to the material evidence before us, anything in the way of a brain vesicle.

The next two specimens (fig. 2, C and D) are essentially representatives of the same stage as the last specimen, showing variants in form. In one of them (0') the optic evagination is more advanced than in B’ and in the other |D’) it is less advanced. Otherwise the closing neural tube is smooth and devoid of segmentation, excepting in the region of the somites. That the mesodermal somi.tes inﬂuence the form of the neural ectoderm with which they are in contact can be seen by stripping off the mesoderm, as was done in D’. Here one can speak at least of secondary segmentation. Whether these neural segments would form in the absence of mesodermal somites must be determined before the question can be answered as to their primary or secondary nature. Elsewhere the neural plates during this period are devoid of any such phenomenon. Nor do these photographs show as yet any brain vesicles.

The outline drawings in figure 3 represent the initial phases in the closure of the neural tube. As we shall see, separation of the tube into subdivisions begins even as early as this and the effort has been made to record the form of the tube as accurately as possible. The contours were obtained by making tracings of enlarged photographs. Thus, the 3- and 4-somite stages of figure 3 are from the same specimens shown in figure 2, A and B, with which they should be compared. The others were made from similar photographs of various other specimens. In all cases the drawings were controlled by the original embryonic material under the binocular microscope. The drawings were all made at the same enlargement so that one can follow any alterations in width or length. For their final rendering in ink I am indebted to the expert hand of Mr. J. F. Didusch.

In comparing these stages the transformation of the caudal part of the germ—disk can be dismissed with few words. This is the zone of primary differentiation where, through the activity of Hensen’s node and the contiguous primitive streak, certain elements, such as mesoderm and notochord, are sepa~ rated out from the ectodermal caudal germ—bed and there is left in the path behind, or rather in front, the median longitudinal chordal scar, 011 each side of which are the paired 11eura.l plates which are to become the central nervous system. This constitutes the primary exodus of non-nervous elements from the ectoderm. As growth proceeds, the zone where this neural purification is taking place is found progressively further and further back, remaining characteristically, to the very last, ﬂat and open just as was the original germ-disk.

Confining our attention to the purified neural plates, it will be seen that their closure into a tube begins in the chick, near the eye region and is effected by the bending up of the plates so that their margins are pressed together and the same process spreads from this region caudalward simultaneously

fig.3 Outline dr:1wi11gs of’ the chick brain during the period of c011versi011 into 3. 11011172.! tube, showing certain precocious primitive parts \vhic,h already 9:411 be recognized {it that time. These structures determine the contours of the neural tube. It will be seen that lnetameric segmentation is present only in the region of the mesodermal somites. The specimens are all enlarged 25 diameters.

with the elongation of the neural axis. At first the margins merely press against each other. This is the condition up to the 6- and 7-somite stages. At the 8-somite stage actual fusion has taken place along the line where the first contact occurred. From this point the phenomenon of closure or fusion of the mid-dorsal seam spreads forward and caudalward and in doing so gives rise to the neural crest which will not be dealt with here. That much will be generally granted. But while the tube is closing there is active proliferation of the cells composing its wall. This proliferation, like the closure itself, occurs earlier at the oral end, spreading thence backward, and, furthermore, it is more active in certain zones resulting in a fullness or expansion of the corresponding areas, characteristic for the stage and the species. It is the significance of the consequent irregularities in contour of the neural tube which I wish to raise in question. I cannot see that these are orderly uniform metameric segments as pietured by Hill, instead they appear to be definite specialized structures, the primordia of the more primitive mechanisms, which can be followed through. into the completed brain. As an example of a definitive part of the brain, whose primordium can be recognized very early in the embryo, the chick offers a very conspicuous one in the visual apparatus. Already in the 3- and 4-somite stages there is a fullness in the region where the eye is to appear. At the 6-somite stage, as can be seen in figure 3, there is on each side a well-demarcated evagination expanding laterally. From then on these bilateral evaginations are unmistakably the eyes. It is to be noted that neither dorsally, anteriorly, nor ventrally do these meet in the middle line. In other words, in the 6- to 7-somite stage we are not dealing with a primary brain vesicle, but with two eye vesicles separated by a part of the brain Wall that is to form other important structures less far advanced. Furthermore, the eye vesicles are there before the dorsal margins actually come together and before one could postulate a primary brain vesicle. Following closely after the formation of the eye vesicles are the optic thalami and the optic lobes which are also essentially bilateral and, as we shall presently see, are limited to the dorso-lateral part of the neural wall. They can be distinctly seen in the last two stages (8- and 11-somite) of figure 3, but can be followed better in figures 4 and 5.

Primary Subdivisions of Brain

To draw conclusions solely from dorsal views of the developing brain has its dangers. From a casual glance at the 8—somite stage in figure 3, one might say that here are three brain vesicles. If, however, one splits the brain along the middle line and studies the median sagittal View (fig. 4, 8-somite), he finds that from that aspect an actual division into brain vesicles does not exist. In.stead of an anterior brain vesicle, one sees one of the optic vesicles projecting laterally, separated from its mate by the incompletely fused thick dorsal margins of the neural plates. It is not a single uniform common cavity but one already bilaterally specialized into parts. Lying next and caudal to this is a surface where the thalamus is to appear. In the dorsal view this in some specimens is partially obscured by the bulging eye vesicles. Back of this in turn comes the enlarging wall that 011 each side is to go into the optic lobe. finally there can be recognized slight bulging areas that are forming the nuclei of origin of the trigeminal and the acoustico-facial nerves. Caudal to this point the neural plates are still open.

Surfaces and contours which are subtle in the 8-somite stage become distinct in the 11-somite stage. With suitable illumination and manipulation of shadows one can emphasize the depressed areas where the neural wall bulges outward, and this means the areas where proliferation is more active. At 17-somites there can be no further question as to the contours of these foci of development and they can be readily traced into the older stages with constant addition of new areas or subdivisions of the earlier ones. figures 4 and 5 present a series of stages in which these are labeled and they can be followed step by step as the brain grows in size and in complexity. It is 11ot necessary to describe all these in detail here, but it will be noted that there is an absence of serial repetition or uniformity among them, such as is demanded by metamerism. It is true that they are arranged along a longitudinal axis which could scarcely be otherwise in an elongated organism, but otherwise their individuality and appropriateness in position and size for particular purposes are their main characteristics. In this connection it may be pointed out that the central nervous system is nicely adapted to the structures it serves, from the very early stages. It is adapted to the body just as much as the body is adapted to it. Furthermore, in its early stages it is adapted to the simple embryonic requirements which are chieﬂy the muscle primordia, and as the requirements of the organism become more complex the nervous system keeps abreast. If this is kept in mind one can better interpret the early morphology of the neural axis. As an illustration of this point, there is shown in figure 6 a sketch illustrating how in the region of the segmented mesoderm the neural axis conforms to the consequent requirements, whereas, in the region of the facial processes and branchial bars there is supplied an appropriate set of cranial nerves. Further forward the primary require» ments are the visual and olfactory mechanisms, which obligations the brain meets early and adequately. It can be understood how subsequently in rapid succession new centers and super-centers made their appearance for control, elaboration, and coordination of the brain itself. These with their various communications find their places efiiciently and with striking economy of space. It is apparently in such terms that the transformation of the simpler embryonic form into the final brain must be interpreted. This is quite opposite to the conception that a metameric tube after the fashion of Hill must first be subdivided into three homogeneous brain—vesicles which then become further subdivided into the definite parts.

For the latter view the writer finds no confirmation in the brain of the chick.

fig.4 Dissections of the brain of the chick, enlarged 25 diameters, showing in a series of stages the :11'e:3.s of more rapid development as judged by the expansion of the neural wall. It will be seen that the bulging areas correspond to primitive functional mechanisms which can be traced in figure 5 to definite

brain parts.

fig.5 A continuation of figure 4. The en1a.rgement for each specimen is specified. 470 G. L. STREETER

The Visual apparatus which is such a dominating feature of the chick brain is a favorable structure to follow through the stages shown in figures 4 and 5. It is a good representative of the principle of early determination of the various regions of the neural tube. As early as the 11-somite

fig. 6 Outline drawing of centrzil nervous system of a 25~s0mite human embryo (Carnegie Collection, No. 6097). The somites and the condensed tissues of the branchial region are the only body parts that seem to be calling for help from the nervous system and the form of the neural tube is nicely fitted to their service. The character of segmentation shown by them is correspondingly reﬂected in the neural tube. Otherwise, the environment at this time offers the brain no inducements to segmentation. The endothelial apparatus is at first quite indi— pendent of the nervous system.

stage the eye vesicle begins to acquire a stalk—like constriction and at 17 somites one may speak of an optic foramen. This actually diminishes in size up to the 21—somite stage and then, due to growth of the structures in the adjoining wall, it is converted into the optic recess and becomes finally a mere slit. Of the other parts of the Visual apparatus the thalamus and optic lobe appear early. The older anatomists spoke of the ‘optic thalamus.’ In the embryo the relation between the thalamus and the optic evagination is very close, being contiguous at the 8—somite stage. This continuity is later expressed in the optic tract. The optic lobe as a terminal visual center undergoes an extraordinary growth. As is true for the cerebral hemisphere in man, it first establishes a great surface expansion. This mechanically favors the subsequent stratification and correlating fiber sheets which characterize the later thickening of the wall. In the 3- and 4-day chick the right and left optic lobes open widely into each other and give the appearance of being one optic lobe, which we all delight to call the mid-brain vesicle. In reality the optic lobes start as bilateral expansion of the neural wall and maintain their bilaterality throughout. This is conspicuously true in the chick where in adaptation to the available space the two lobes become separated like saddle-bags on the sides of the brain-stem, connection being maintained by a broad lamina of commissural fibers.

In studying the optic lobes one can see that they are distinct and can be demarcated ventrally from the basal plate. This basal plate is destined to become an important part of the brain. It becomes the main connecting trunk of the brainstem. It is the cerebral ‘main street’ or an elongated ‘market’ along Whose margins are the great nerve centers from which it receives and to which it distributes fibers. While its primary purpose appears to be concerned with traffic, various effecting, regulating, and correlating centers are interspersed along its course. From the point of View of this paper it is to be noted that it has an identity and that it is a longitudinal structure which is not interrupted by transverse partitions such as would be essential in a system of three brain vesicles. To draw a line across it as we have been accustomed to do in the past, saying what is in front of the line is one thing and what is behind it another, is an act of rank pedagogic violence. Anteriorly, the basal plate disappears below the thalamus and corpus striatum blending with the great fiber systems which come from them and course on their lateral surfaces. With its extension caudalward the basal plate undergoes enormous growth throughout the peduncular and pontine regions where it will be recognized as the formatio reticularis which in turn is continuous with the large ground bundles of the spinal cord. The term ‘peduncles of the brain’ has been granted by custom to specific bundles of fibers which have not reached the basal plate in the earlier stages. Otherwise one might wish to use the term ‘pedunculus cerebri’ instead of basal plate for this structure from the beginning. This, however, is not the place to discuss the terminology of this part of the adult brain for which there is so great an opportunity-»a region where we yet speak of a ‘tcctum’ of the ‘tegmentum.’

Between the optic lobe and the thalamus a new subdivision or focal area of active proliferation appears at the 21-somite stage, and this has been designated as the metathalamus abiding the time when a more specific designation can be arrived at. It appears to correspond to a group of pretectal nuclei.

A conspicuous feature of the early neural tube is a segmental-like effect which is associated with the innervation of the facial processes and the branchial bars. Already. at the 8- and 11—somite stages one can see expansions of the neural wall at foci of active proliferation producing transverse grooves and intervening ridges. With some variation between Vertebrate classes and orders, there is a general similarity in the existence of these grooves among all verte— bratcs. The grooves in the chick are not quite the same as those in pig and man, but as in these two orders they show a constant relationship to particular visceral cranial nerves. The points at which the respective nerves emerge from the neural wall and their relation to the grooves were plotted as carefully as possible and are shown for the successive stages in figures 4 and 5. It is to be noted that the different grooves possess certain individualities, in shape, width, length, and point of nerve exit. Furthermore, there are more grooves than nerves. The first two must be allotted to the trigeminal nerve and there is no nerve for the groove between the facial and glossopharyngeal nerves unless it be the acoustic nerve Which is not a visceral nerve. None of the grooves extend across the median line, even at their maximum develop ment at the end of the second day, being interrupted by the median raphe. The basal plate gradually spreads out ventral to them and as it increases in mass, being unsegmental itself, the grooves gradually disappear. These grooves are much more marked and appear to have no continuity with the type of segmentation that characterizes the neural tubes opposite the somites, which is quite contrary to the conclusion arrived at by Hill. Peripherally, the visceral cranial nerves correspond precisely to the condensed cell masses they innervate, as can be seen in figure 6, and it seems probable that their growth is a neural response to their particular environment. They would therefore be expected to exhibit the same type of segmentation prevailing there, which is a very different sort from that of the somites.

Summary

From morphological evidence one must conclude that the nearest approach to metamerism in the embryonic central nervous system is that occurring in the spinal region, and even this appears to be an adaptation to the Somites that lie alongside. The segmentation is evidently primary in the Somites, which during the early stages make an impress on the cord. ‘

A different type of segmentation is found in the branchial region——a type that is not continuous with and is different in histological character from the somite—type of the cord. The condensed cellular masses that are to form the tissues and musculature of the maxillary, mandibular, and throat regions receive appropriate and early innervation from a series of nerves, the visceral cranial nerves, which are clearly associated with a series of grooves in the neural Wall. These bilaterally placed transverse grooves are the expression of active cell proliferation and represent foci of origin with the associated nuclei, of these visceral cranial nerves. Although resembling each other in general, these grooves possess individualities which distinguish them from one another, and in the different vertebrate orders there are certain variations in their finer morphological details. The trigeminal and facial are the first to establish themselves.

A third category of ‘segmentation’ is brought about by expansions of the neural wall with intervening constrictions associated with the development of definite brain parts. These subdivisions give evidence of early determination of the more fundamental structures which can be followed through to their final form in the mature brain. From the outset they are characterized in each case by individuality of form and are not serial repetitions of a uniform pattern. Reexamination of similar material does not substantiate the well-known figure of Hill showing a neural tube, metameric from stem to stern. Instead of a rigid metameric system, the neural tube shows itself responsive at all levels to its environment. VVhere the environment is truly segmented the tube takes on that character in some degree. At levels where the environment is branchiomerie there we find the neural tube responding with suitable cranial nerves. At levels still further forward, aside for certain special-sense organs, there appear to be no environmental demands, at least in the early stages, and there the neural tube devotes itself to its own requirements in the way of centers of correlation and control, the subdivision of which bears no resemblance to true segmentation.

The subdivision of the embryonic brain into three primary brain vesicles is an arbitrary expedient rather than a natural phenomenon. The scheme is not to be blamed on His. 'When he and his associates on the Committee for revision of Anatomical Nomenclature utilized these divisions of the brain and their subdivisions, it was done only as an aid in regional localization. No such far-reaching interpretation was inferred as that of the metameric enthusiasts. There is a certain convenience in the terms fore-brain, mid-brain, and hindbrain, and if it is remembered that there never are three primary vesicles serially equivalent and precisely demarcated, corresponding to the three brain divisions, then no harm is done. If, however, the student is taught that the neural tube becomes dilated anteriorly into three equivalent homogeneous vesicles which are to become the brain, and that it is subsequent to the acquirement of their vesicular state that these become differentiated and subdivided into actual brain structures, the said student will have been taught something that he can never verify, and it is likely that his conception of organogenesis will lean more toward the principles of the tailor—shop than toward those typical of living embryonic tissue.

If intellectual stimulation is the object in view, how much more thrilling than the three—brain—vesicle lore is the knowledge which is now available concerning the potentialities of embryonic tissues and concerning the inﬂuence of the tissues upon each other. These interactions in their general principles are demonstrable facts and may be safely intrusted to the embryological armamentarium of the student. It is inconceivable that any rigid geometrical schema could be as marvelous as the reality of the blending adaptation maintained between the embryonic nervous system and the body structures which it is to serve, as they exist at the moment, whereby the two are continuously and perfectly coordinated. Thought of in these terms, the form of the neural tube and its modifications from stage to stage take on important significance.

Paper - The status of metamerism in the central nervous system of chick embryos: Difference between revisions