Paper - Contribution to the structure and development of the vertebrate head
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Locy WA.Contribution to the structure and development of the vertebrate head. (1895) J. Morphol. 11(3): 497-595.
- Locy 1895 Contents: General Introduction | Part I - Metamerism of the Head | Part II - The Sense-Organs | Figures
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Contribution to the Structure and Development of the Vertebrate Head
A Growing interest has been manifested in the prob1ems of craniaI morphology as their importance in comparative anatomy has been more fully rea1ized, and a number of streng investigators have taken up this particular Held of study. Through their researches much has been accomplished ; the views on crania1 anatomy have gradually changed, as advance after advance has been made, unti1 morphologists have come to Iook upon that wonderful comp1ex — the head - not as a structure sui generis, but as the most extensively modified part of the anima1, formed by differentiation and specialization from parts that are structurally homo1ogous with those that follow in the trunk. According to this conception, the distinction between head region and trunk region is one of degree of differentiation and not one of kind.
The modifications which the head has undergone have been brought about gradually, and are so comprehensive in their range that if we could know their complete history, even in one animal, we should have a key to the leading questions of vertebrate phylogeny. But there are so many causes tending to modify the course of deve1opment, that we cannot depend on the steps of phy1ogenetic history being repeated in a complete and order1y way in any one anima1 form. Often a balance of probabi1ities must be struck to determine what is ancestral and whatis secondarily acquired. The chain of evidence is indeed very incomplete, and must always be supplemented by a certain amount of inferencez but what has not been fully enough recognized in the practical study of embryo1ogical development, is that the shortest interva1s of time may be very important in keeping the connection. Coherency of the history must be preserved ; and the difficulty of doing so is great1y increased by the fact that the new is made to proceed out of the old, and, frequent1y, one organ insidiously takes the place of an earlier formed one. Too great stress cannot be laid on the desirabi1ity of having a more complete series for study, and this is especially important in cranial anatomy. The traditional method has been to study one stage and then another «a little older,« and to fill in the intervening gap with inferences. This has proved to be inadequate and mis1eading. It is now required that we shall have stages close enough together to trace the history of the transitory as well as the permanent Organs. The practical diiiiculties in obtaining a sufficiently complete series are very great, and, in many cases, well-nigh impossible. Great effort has been expended in getting the materia1 for the present research; it is a kind of material in which the stages cannot be contro11ed, and my series cannot be regarded in any sense as a complete one. Nevertheless there is represented in it several distinct stages that have not heretofore been described by students of e1asmobranch embryology. Beard, in his study on the Transient Ganglion Cells and their nerves in Raja batis says : "My series of the embryos of this form is what many might judge to be complete, numbering as it does some 300 specimens of all ages and sizes. Nevertheless there are gaps in the collection and these are often of a kind that it may not be easy to fill in. No two embryos are exactly alike in all the pictures which they yield of this apparatus, and in half a dozen specimens that would be taken to be of the same age from their sizes, from a comparison with Balfour’s stages, or from the more certain criteria of number of gi1l—clefts, protovertebræ, etc» etc., it is quite common to lind this transient nervous System, like other Organs, in widely different stages of deve1opment and presenting great variations in detailed characters. In my researches- on Raja I have been compelled to give up completely any attempt to make use of Balfour’s nomenclature. With a limited number of embryos at one’s disposal, it is easy to tit them into one or the other of the well-known stages ; with an increased number this becomes more difiicult or even impossible, so great are the variations met with." I have had a sirnilar experience with my embryos of Squalus acanthias (Acanthias vulgaris). I am conscious there are gaps in the material, and, after making the best use of it that I can, I have, no doubt, missed many things not represented in my collection.
The studies recorded in this paper deal almost exclusively with the early history of the brain and sense—organs. Some of the questions upon which direct evidence is brought are: What was the primitive condition of the nervous system of the Vertebratesk What was the number and nature of the primitive neural segments entering into the braink What has been, in generaL the line of modification along which they have been converted into the braini And what were the early steps in the differentiation of sense-organs?
I have been greatly indebted to Professor c. 0. Whitman for courtesies both at Woods Holl Biological Laboratory and at the University of chicago, and I have also to thanlc him for advice and suggestions on the subjecvmatter of this paper.1
- 1. I soon after the appearance of the Zieglers’ paper (Beiträge zur Entwicklungsgeschichte von Torpedo : Art-Eis- jiir Ali-E. ais-at» Bd. 39, Akt. I, Jan. I892) Dr. Whitman suggested to me that I should work over the same gkouncl—— the gastrw lation and formation of the germinal layers—in one of our North American species of Elasmobranchii I collected material (Galeus canis and Squalus acanthias) for that purpose, and. began studies along that 1ine, but, tinding new and undescribed conditions in the head region, my attention was gradually drawn off from the original purpose and directed towards cranial anatomy.
In glancing over the views regarding cranial anatomy that have been he1d since the beginning of this century, we find a suggestive hint in the shift of opinion, as to the decided1y relative nature of all our views on morphological Problems. There has been a distinct rise in the point of vier-v, but scarcely any of the earlier Problems are yet solved ; new ones have been added, but those that have been handed down have grown broader and increased in proportions as a mountain on nearer approach. There has been a gradual change in front, keeping pace with the advance of our knowledge and with the changes in our methods of interpretation. During that period the very sou1 has been breathed into the body of comparative morphology, and as a resu1t, our interpretations are directed towards explaining anatomica1 structures in connection with their past deve1opment. At the beginning of this century, on account of superficial appearances, the conception took rise that the head is divided into definite segments. The obvious division of the slcull by sutures was gradually worked into the theory that the cranium represents a number of modified vertebræ; and then began on the part of anatomists, the efforts to determine the number of these segrnents or vertebræ in the sku1l. Oken and Goethe formu1ated the theory which was taken up by that eminent anatomjst Richard 0wen, who lavished upon it an amount of attention and labor that was worthy of a more substantial theory, but the mistalces of that great man have proved of benefit to other rnorpho1ogists.
This was the beginning of the idea that the head region represents a deiinite number of segments. Originally founded on external features of no segmental importance whatsoever, and not essential to the question which they served to introduce, the problem gradually widened and deepened and reached the essential parts connected with this segmental condition. The Hrst conspicuous change of front came with the delivering of the croonian Lectures, in 1869, by Huxley, in which he completely overthrew the vertebral theory of the sku1l, withdrawing attention from the superHcial sutures in the cranium and directing it to the cranial nerves and branchiæ as bearing evidence to the segmentation of the head. Gegenbaun in I872, studied the cranial nerves especially in relation to the branchial clefts and reached the con(:1usion that there are nine segments represented in the head. Another distinct advance was made by Balfour, who first studied the segmental divisions of the mesoblast in the head of the Elasmobranchs, and identified by this means eight head-somites clearly represented. He also expressed the conviction that there were primitively a larger number of segments but, owing to extreme modifications of the head region, they are no longer clearly represented. This was at bottom the same problem, but it was now shifted upon Organs that are truly segmental.
From the time of its discovery, this segmental division of the mesoblast in the head became a great favorite with morphologists in elucidating the problem of head segmentation. The mesoblastic divisions seemed, so far as the evidence went, to embody the most direct survivals of the original segmentation and, therefore, to be the most promising line along which to work out the problem. Valuable contributions have been made along this line since Balfouns time, by Marshall, Van Wijhe, D0hrn, Killian, 0ppe1, and others, and the myotomes of the head have continued to hold their high Position in the minds of morphologists as the most signiiicant remnants of the original segmentation.
Notwithstanding all these researches the original problem is still unsolved. There is no agreement as to the number or the nature of the primitive segments, and about the only point that may be regarded as settled beyond controversy is that the head and brain were primitively divided into segments.
Recently, there has been added as a factor in the diseussiom observations on segmental divisions of the neura1 tube. Although such divisions have attracted the attention of several observers, their importance in the problem of cranial segmentation has not been appreciated. They have been regarded as of secondary origin, depending on the segmentation of the mesoblast. Nevertheless, I hope to show that these segments are the first to appear in the head region, and that they are entitled to more serious attention on the part of anatomists, as representing the most primitive segmentation of the head of which any traces are preserved.
- Locy 1895 Contents: General Introduction | Part I - Metamerism of the Head | Part II - The Sense-Organs | Figures
Cite this page: Hill, M.A. (2020, July 3) Embryology Paper - Contribution to the structure and development of the vertebrate head. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_Contribution_to_the_structure_and_development_of_the_vertebrate_head
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