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| ==The development and reduction of the tail and of the caudal end of the spinal cord==
| | #REDIRECT [[Paper - The development and reduction of the tail and of the caudal end of the spinal cord (1920)]] |
| By Kanae Kunitomo.
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| (Four plates, two text-figures)
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| :'''Links:''' [[Book_-_Contributions_to_Embryology|Carnegie Institution of Washington - Contributions to Embryology]]
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| ==Introduction==
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| A great deal of literature has been published from time to time dealing with
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| the question of whether the human embryo at a certain stage of its development
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| has an actual tail that is, a structure homologous with the tail of other mammals
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| and with the persistence of such tail in after life. In biogenetic investigation this
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| is a subject of great interest (Darwin and Haeckel). It was from the assumption
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| of the occurrence of a tail in the human embryo, which was based upon one of
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| Ecker's figures (Icones Physiologicae, 1851-59), that Darwin drew one of his
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| arguments for the descent of man from a race of tailed ancestors. Von KolUker
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| (1884) asserted that the human embryo has a tail-like process at its caudal
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| end which was not, however, recognized by him as a "true" tail. He described
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| it as eine spitze Schwanzartige V erlangerung . Ecker (1851-59) referred to it as
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| Schwanzformige Korperende, and stated that it contained only the notochord and
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| caudal end of the spinal cord, and was converted finally into a coccygeal tubercle
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| (Steisshocker) projecting caudalward. Rosenberg (1876, 1899), who investigated
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| the subject from a morphological standpoint, opposed the theory that the caudal
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| appendage in the human embryo was homologous with the tail of other mammals,
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| as he could not discover any part of the axial skeleton in the caudal projection.
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| He believed, therefore, that the latter must be concerned more with the development of the spinal cord which he found embedded in its dorsal side. Ecker (1880),
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| on the other hand, held it to be a true tail, even though it contained no vertebral
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| primordia, and from his conclusions on the subject interest and discussion were
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| revived. His (1880a), who coincided in general with Ecker, published his theories
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| under the heading "Besitzt der menschhche Embryo einen 8chwanz?" in his
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| Anatomic menschUcher Embryonen. He recognized a tail-like appendage in his
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| younger specimen (4 mm.), but did not regard it as a true tail. In older embryos,
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| in which the primitive vertebrae had developed into cartilaginous tissue, he found
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| that one or two vertebrae entered into the root of the tail. This portion he designated as the vertebral tail. The remainder contained only notochord and medullary
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| cord (caudal filament) and was therefore called non-vertebral tail. In none of his
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| specimens did he find more than the normal number of vertebrae, 34.
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| He states:
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| :"Die Embryonen A and B haben sonach eine achte Schwanze-anlage, die aber ausserordentlich kurz ist und jedenfalls nicht liber zwei Pegmentlangen umfasst."
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| The opinions of Ecker and His may be summarized as follows:
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| # The term tail refers only to that portion of the embryo which projects beyond the cloaca.
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| # In younger specimens (8 to 15 nun.) the tail api)ears as a free, pointed projection from the cloaca, directed caudo-dorsally.
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| # The tail consists of two portions —that containing; vertebra* and that without vertebrae. The latter contains only chorda dorsalis and medullary tube. In time this portion disappears, the medullarA- tube atrophying and the chorila becoming converted into a knot.
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| # The vertebral portion persists for a while, appearing later as a coccygeal prominence in the caudal region coccygeal tubercle; then it, too, disappears.
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| Keibel (1891) published in an imi)()rtant paper his findings in regard to the development of the caudal gut in 4, 8, and 11.5 mm. embryos. The existence of this structure, which forms a small canal or cell-strand at the caudal end of the body axis, he regards as irrefutable evidence of a tail primordium. He found the gut to be longer in the 8 mm. embryo than in the others. He asserts (page 378) that in this stage the caudal gut extends through the whole length of the tail, and apparently at this time attains its maximum length. This author defines the line of demarcation between the tail and the body in two ways: (1) he designates as the tail the caudal portion beyond the attachment of the pelvic joint; (2) in the
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| younger embryos, in which the primordia of the legs have not yet appeared, he defines the first 8 trunk segments as the cervical segments, the next 12 as the dorsal, the next 5 as the lumbar, the next 5 as the sacral, and the remaining segments as caudal vertebra". These he found were usually 6 in number. The last one he called the mesodermic remnant and regarded it as one segment, although it was two or three times as long as those cranial to it.
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| Braun (1882) published his observations on the development and reduction of the embryonic tail among mammalia, having at his disposal a great number of specimens. As a rule he found a caudal filament at the extreme end of the tail in the mammals that he studied, and therefore believed this structure to be of general occurrence, and probably true also of the human tail. On the other hand, Ecker and His, who studied the same condition in human embryos, did not consider the two exactly homologous. Braun classified the tw^o portions of the tail as internal and external, and subdivided the latter into vertebral and non-vertebral tail, the caudal filament being part of the latter. Waldeyer (1896) takes exception to this division into internal and external tail, as he does not believe the former is a tail.
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| Rodenacker (1898) uses the terms cauda aperta and cauda occulta instead of internal and external tail, linger and Brugsch (1903) give the following results of their investigations :
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| # In the reduction of the tail the caudal vertebra* fuse to form the last vertebra.
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| # The caudal filament represents the renmant of the tail-hud and contains a branch of the middle sacral artery.
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| # In the reduction of the tail two processes are concerned: («) the formation of the caudal tubercle; (h) the formation of the coccygeal tubercle. The first is the reduced tail; the second is formed by the bulging of the caiudal end of the vertebral column. This is due to the fact that the growth of the vertebra' is more rapid than tliut of the skin and spinal cord.
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| # The connective tissue contained in the caudal filament develops into the caudal ligament.
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| Regarding these changes they state (p. 100) :
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| :"Wandelt sich dann durch starkeres Wachstum der Kaudalwirbelsaule der Schwanzhocker in den Steisshocker um, so wird durch den Zug der Haut, deren Wachstumrichtung
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| der der Steisswirbel entgegengesetzt ist, der Schwanzfaden von der Achse der Kaudalwirbel
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| entfernt und mit der Haut aufwarts niitgenommen (cfr. Embryo Du. 4}^ cm.). Durch
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| dieses Aufwartsrucken des Schwanzfadens vdrd aber dieser seines Eiickenmarks beraubt,
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| d. h. er ist reduziert. Seinen Inhalt steUt nun ein Gewebe vor, das in Form von Bindegewebsziigen mit der kaudalen Flache des letzen Kaudalwirbelsverbunden ist, und das aus dem
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| Mesodermrest des friiheren Schwanzfadens hervorgegangen ist. Da auch hier diesen
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| Bindegewebsziigen und dem Schwanzfadeiirest die Endaste der inzwischen durch
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| Bildung des Steisshockers sehr reduzierten Arteria sacraUs media zukommen, so koimen
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| wir sie als einen inmierhin wesenthchen Rest der urspriinghchen Schwanzanlage bezeichnen.
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| Diese Bindegewebsziige sind das Ug. caudale; sie schhessen auch den urspriinglich im
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| Schwanzfaden sich befindhchen kaudalsten Teil des Eiickenmarks ein, in dem sich spater
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| die 'vestiges coccygiens' von Tourneux und Hermann (s. o.) oder 'kaudalen Riickenmarksreste' entwickeln."
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| Our knowledge concerning the development of the caudal end of the spinal
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| cord is very limited, especially as regards its early stages. jMj' aim, therefore, | |
| has been to study the early development of this part of the spinal structure and to
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| follow' the histological changes it undergoes in adaptation to later topographical
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| conditions. Before reporting mj' investigations, however, I woiUd refer to some
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| of the writers who have preceded me in this field of study. Clarke (1859), in his
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| well-knowTi study of the spinal cord, pictures the ventriculus terminaUs as seen
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| in sections of the cord of the ox (plate xxiii, fig. 21). He regarded this structure
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| as a persisting remnant of the lower end of the sinus rhomboidalis, -which in other
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| mammals is usually limited to the lumbar enlargement. Krause (1875) discovered
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| the ventriculus in the spinal cord of the human embryo and describes it as persisting in adults as a rudimentary organ. He suggests that in the embryo, by
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| means of its ciliated cells, it serves in the maintenance of the circulation of the
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| contained cerebral spinal fluid. Tourneux and Hermann (1887), who studied the
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| caudal end of the spinal cord in the human embryo, discovered the remnant of the
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| neural canal in the caudal region and.caUed it vestiges medullaires coccygiens. Tney
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| describe in detail the process of reduction of the caudal end of the spinal cord.
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| Argutinsky (1898) discussed the morphology of the ventriculus terminahs in older
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| fetuses and newborns, and classified it in three di\dsions — upper, middle, and
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| low^er. ^'on KoUiker, Ecker, His, and others reported that in younger embryos the
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| spinal cord extends to the extreme end of the tail.
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| Brugsch and Unger briefly summarized their investigations on the ventriculus terminahs in the human embryo as follows (p. 232) :
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| :"Kurz gesagt stellt der V. t. also eine konische Erweiterung des CentraLkanals im unteren Ende des Conus medullaris und im .\nfange des fihmi terminale vor, dessen oberer weiter Abschnitt meistens Ausbuchtungen besitzt. Der untere Abschnitt endigt blind im filum terminale."
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| In describing this structure they make the following divisions: (1) an upper, wider part, which is continuous viith the central canal of the more cephahc part of tho spinal cord, and which forms an irre{j;uhir, evaginated space in the conus inedullaris; (2) an under part, which gradually narrows toward its caudal end and terminates bUndly in the filum tcrminale.
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| The various investigations of the occurrence of tails among adults, children, and newborn infants have given rise to a great deal of discussion. Bartels (1884) published an exhaustive study of the occurrence of tails among the human race. Other publications on the subject have appeared from time to time, notably by Virchow (1884), Oskar Schaeffer (1892), Pyatnitski (1892), Dickinson (1894), Berry (1894), Kohlbriigge (1898), Watson (1900), and others.
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| Harrison (1901) describes the histological structure of a large, well-developed tail wliich was removed from a child six months old. He states:
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| "Two weeks after the birth of the cliikl the tail was 4.4 cm. long: at the age of two months it had grown to 5 cm., and at six months, when it was removed, it had attained
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| a length of 7 cm., showing altogether a fairly rapid rate of growth. The most remarkable characteristic of the tail was hsmovability. Beneath the skin the main bulk of the tail was made up of areolar tissue containing much fat. Blood vessels, nerves, and striated muscle fibers are embedded in this mass. There is no trace of anything like the medullary cord or of notochordal tissue."
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| More recently similar observations have been made by Brugsch (1907), Konstantiowitsch (1907), and Schwarz (1912).
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| -----Content to be added----
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| {{Template:Historic Disclaimer}}
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| {{Glossary}}
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| {{Footer}}
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| [[Category:Historic Embryology]] [[Category:Human]] [[Category:Neural]]
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