View source for Paper - The development and reduction of the tail and of the caudal end of the spinal cord (1920)

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=The Development and Reduction of the Tail and of the Caudal end of the Spinal Cord=

By Kanae Kunitomo


Nagasaki Medical School, Nagasaki, Japan.

(Four plates, two text-figures)

[[Book_-_Contributions_to_Embryology|Contributions to Embryology]]


==Introduction==
A great deal of literature has been published from time to time dealing with the question of whether the human embryo at a certain stage of its development has an actual tail - that is, a structure homologous with the tail of other mammals -  and with the persistence of such tail in after hfe. In biogenetic investigation this is a subject of great interest (Darwin and Haeckel). It was from the assumption of the occurrence of a tail in the human embryo, which was based upon one of Ecker's figures (Icones Physiologicae, 1851-59), that Darwin drew one of his arguments for the descent of man from a race of tailed ancestors. Von KolUker (1884) asserted that the human embryo has a tail-like process at its caudal end which was not, however, recognized by him as a "true" tail. He described it as eine spitze Schwanzartige V erlangerung . Ecker (1851-59) referred to it as Schwanzformige Korperende, and stated that it contained only the notochord and caudal end of the spinal cord, and was converted finally into a coccygeal tubercle (Steisshocker) projecting caudalward. Rosenberg (1876, 1899), who investigated the subject from a morphological standpoint, opposed the theory that the caudal appendage in the human embryo was homologous with the tail of other mammals, as he could not discover any part of the axial skeleton in the caudal projection. 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), on the other hand, held it to be a true tail, even though it contained no vertebral primordia, and from his conclusions on the subject interest and discussion were revived. His (1880a), who coincided in general with Ecker, published his theories under the heading "Besitzt der menschhche Embryo einen 8chwanz?" in his Anatomic menschUcher Embryonen. He recognized a tail-like appendage in his younger specimen (4 mm), but did not regard it as a true tail. In older embryos, in which the primitive vertebrae had developed into cartilaginous tissue, he found 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 cord (caudal filament) and was therefore called non-vertebral tail. In none of his specimens did he find more than the normal number of vertebrae, 34. He states: "''Die Embryonen A and B haben sonach eine achte Schwanze-anlage, die aber ausserordentlich kurz ist und jedenfalls nicht liber zwei Pegmentlangen umfasst.''" The opinions of Ecker and His may be summarized as follows:

# The term tail refers only to that portion of the embryo which projects beyond the cloaca.
# In younger specimens (8 to 15 mm) the tail api)ears as a free, pointed projection from the cloaca, directed caudo-dorsally.
# 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.
# The vertebral portion persists for a while, appearing later as a coccygeal prominence in the caudal region - coccygeal tubercle; then it, too, disappears.


Keibel (1891) published in an important 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 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.


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 two 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. 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 :

# In the reduction of the tail the caudal vertebra* fuse to form the last vertebra.
# The caudal filament represents the renmant of the tail-hud and contains a branch of the middle sacral artery.
# In the reduction of the tail two processes are concerned: (Â«) 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.
# The connective tissue contained in the caudal filament develops into the caudal ligament.


Regarding these changes they state (p. 100) :

:"''Wandelt sich dann durch starkeres Wachstum der Kaudalwirbelsaule der Schwanzhocker in den Steisshocker um, so wird durch den Zug der Haut, deren Wachstumrichtung der der Steisswirbel entgegengesetzt ist, der Schwanzfaden von der Achse der Kaudalwirbel entfernt und mit der Haut aufwarts niitgenommen (cfr. embryo Du. 4.5 cm.). Durch dieses Aufwartsrucken des Schwanzfadens vdrd aber dieser seines Eiickenmarks beraubt, 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 Mesodermrest des friiheren Schwanzfadens hervorgegangen ist. Da auch hier diesen Bindegewebsziigen und dem Schwanzfadeiirest die Endaste der inzwischen durch Bildung des Steisshockers sehr reduzierten Arteria sacraUs media zukommen, so koimen wir sie als einen inmierhin wesenthchen Rest der urspriinghchen Schwanzanlage bezeichnen. Diese Bindegewebsziige sind das Ug. caudale; sie schhessen auch den urspriinglich im Schwanzfaden sich befindhchen kaudalsten Teil des Eiickenmarks ein, in dem sich spater die 'vestiges coccygiens' von Tourneux und Hermann (s. o.) oder 'kaudalen Riickenmarksreste' entwickeln.''"


Our knowledge concerning the development of the caudal end of the spinal cord is very limited, especially as regards its early stages. jMy aim, therefore, has been to study the early development of this part of the spinal structure and to follow the histological changes it undergoes in adaptation to later topographical conditions. Before reporting my investigations, however, I would refer to some of the writers who have preceded me in this field of study. Clarke (1859), in his well-knowTi study of the spinal cord, pictures the ventriculus terminalis as seen in sections of the cord of the ox (plate xxiii, fig. 21). He regarded this structure as a persisting remnant of the lower end of the sinus rhomboidalis, which in other mammals is usually limited to the lumbar enlargement. Krause (1875) discovered 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 means of its ciliated cells, it serves in the maintenance of the circulation of the contained cerebral spinal fluid. Tourneux and Hermann (1887), who studied the caudal end of the spinal cord in the human embryo, discovered the remnant of the neural canal in the caudal region and.caUed it vestiges medullaires coccygiens. Tney describe in detail the process of reduction of the caudal end of the spinal cord. Argutinsky (1898) discussed the morphology of the ventriculus terminalis in older fetuses and newborns, and classified it in three divisions - upper, middle, and lower. Von Kolliker, Ecker, His, and others reported that in younger embryos the spinal cord extends to the extreme end of the tail. Brugsch and Unger briefly summarized their investigations on the ventriculus terminalis in the human embryo as follows (p. 232) :

:"''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.''"


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 blindly in the filum tcrminale.


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. Harrison (1901) describes the histological structure of a large, well-developed tail which was removed from a child six months old. He states:

:"Two weeks after the birth of the child 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 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."


More recently similar observations have been made by Brugsch (1907), Konstantiowitsch (1907), and Schwarz (1912).

==Material and Methods==

The material upon which this study is based consists of 44 specimens in the Carnegie Collection of human embryos, Baltimore. They range from 4 to 125 mm CR length, and a table of them, with their respective measurements, is shown herein. The specimens, for the greater part, had been carefully preserved in 10 per cent formalin and dehydrated in alcohol. The smaller ones were embedded in paraffin, the larger ones in celloidin, and most of them were cut in serial sagittal sections varjdng in thickness from 20 to 200 m in the different specimens. A large proportion of the specimens were stained in toio in alum cochineal and borax carmine before embedding; others were stained on the sUdes with hematoxylin and eosin or similar stains. From 15 to 80 sagittal sections through the median part of each embryo were used in this investigation, and usually one graphic reconstruction was made of each specimen, although all of these are not illustrated. They present a median profile view disclosing some structure - for instance, the winding caudal end of the chorda dorsaUs or the sympathetic ganglia. In the illustrations these are slightly schematicized in order to show cUstinctly their actual relations. The graphic reconstructions were made by copying each section on transparent paper from a projection apparatus. When the drawings under the projection apparatus were completed the sheets were piled so that adjacent sections were accurately fitted one upon another. The desired parts of the sketch on each sheet were then copied on another sheet of paper, due attention being given to the form and relation of the component parts. This procedure was facilitated by the use of a glass table illuminated from below. In the case of cross-sections, a guide-line was established by marking upon each sheet two lines, one perpendicular to the other, thus forming a series of crosses which were exactly superimposed throughout the entire pile. The individual sections were then plotted off on millimeter paper by fitting the crosses to a chosen perpendicular line, the distance between the sections being determined by the thickness of the sections and the enlargement of the drawing. The enlargements with the projection apparatus were as follows: Embryos 4 to 16 mm, X70; embryos 17 to 38 mm, XoO; embryos 39 to 52 mm, X30; embryos 67 to 125 mm, X20.


{{Kunitomo1920 table1}}



==Serial Description of Embryos==

===Embryo No. 836, 4 mm Greatest Length===

This embryo {{CE836}} represents the earliest stage studied in this investigation, and a diagrammatic profile reconstruction of it is shown in figure 1, plate 1. It was sectioned transversely and a series of models of it were reconstructed under the direction of Professor Evans. These were available for comparison and were of particular value in orienting the sections in the lumbo-sacral region, where the caudal extremity curves so that it lies transverse to the axis of the trunk. The embryo contains 28 somites and a long mesodermic remnant which extends about one-third of its length beyond the caudal end of the spinal cord. The chorda dorsalis runs along the ventral surface of the spinal cord in close apposition to it and terminates before reaching the caudal end of the central canal; whereas the caudal gut extends farther down towards the tip of the tail, as indicated in figure 1. The caudal extremity of the embryo consists of a mass of germinating cells into which the ends of the spinal cord, the chorda dorsalis, and mesodermic remnant all merge, their outlines becoming entirely obliterated. The primordium of the caudal arterj- extends to the tip of the tail.

===Embryo No. 786, 4 mm Greatest Length===

This specimen {{CE786}} has 30 somites and a mesodermic remnant ; the latter consists of a long cord joined to 4 globular masses, the last of which is slightly longer and thinner than the others. The counting of the somites in small embryos is always very difficult, as pointed out by Keibel, especially when the material is not well preserved. It was easy, however, to make out the vesicula auditiva in the sagittal sections of this specimen, and caudodorsal to it the glossopharyngeal nerve. The ganglionated vagus nerve in turn is situated caudal to this, being surrounded by the internal jugular vein, which curves around its caudal margin. The first cervical somite lies dorsal-caudal at a distance of 180 to 200 um from the vagus ganglion. The embryo has 3 occipital somites.

===Embryo No. 810, 5.5 mm Crown-Rump Length===

A graphic reconstruction of the caudal end of embryo No. {{CE810}} is shown in figure 2, plate 1, in which the stnictures are diagrammatically straightened out. In reality the caudal end is bent to the right and towards the front, its tip nearly reaching the right side of the face. The first cervical ganglion is about one-third the size of the second and lies close to the Froriep ganglion, which is situated at the dorsal side of the bow-shaped trunk of the accessory nerve and is particularly large on the left side. There are 32 spinal ganglia, the last 2 being small. I was able to count on the right side 37 somites and a mesodermic remnant; on the left side only 30 somites and a remnant. In the cranial portion of the mesodermic remnant there seem to exist potential somites, the outlines of which, however, can not yet be made out. Its caudal part consists of a small, bell-shaped remnant. The entire remnant is about the length and size of 4 somites. The mesoderm in the caudal end of the embryo is primitive in character and can be seen dividing into its parietal and visceral layers. Embedded in these can also be seen the caudal gut and a simple vascular plexus. The spinal cord, with its central canal, extends to the caudal end of the mesodermic remnant. The caudal gut, which extends from the cloaca to the end of the tail, is beginning to disappear in this embryo at a point 200 um caudal to the cloaca and 657 um from the extreme end of the tail between the thirtieth and thirty-first somites, as is indicated in figure 2, plate 1. In figure 31, plate 2, these conditions are shown more in detail. At the site of the beginning degeneration the ends are sharply pointed, but are still connected by a cell-strand. The shorter, cranial portion of the caudal gut opens into the cavity of the cloaca. The longer, caudal portion ends blindly at both extremities, the caudal end merging into the mesodermic cell-mass and uniting with the caudal end of the chorda dorsalis. Each portion of the gut contains a distinct lumen. On the ventral surface of the caudal region of the embryo, where the gut first begins to disappear, is a small groove (indicated by X in fig. 31), which may represent the primordium of the subcaudal epithelial plate of Keibel.

===Embryo No. 371, 6.6 mm Crown-Rump Length===

This embryo {{CE371}} has 37 somites and a mesodermic remnant. The thirty-seventh somite lies close to the remnant, while the others are separated by narrow spaces into which blood capillaries enter. The remnant is constricted at three points, the separations, however, being incomplete. The caudal end of the medullary tube extends to the end of the tail, as can be seen in figures 3 and 32. The caudal gut, which was distinctly recognized in the 5.5 mm specimen, has here undergone a more marked obliteration, and over its greater part is left only a strand of cells which, to judge by their staining reactions, are probably degenerating. The caudal portion of the gut contains a small lumen, however, and shows no change from that noted in the preceding specimen. \Miat constitutes the cranial end of the shorter portion of the caudal gut in the yoimger embryo is here dilated and forms part of the cloacal membrane, ventral and parallel to the gut is a blood-vessel which anastomoses with the middle sacral artery by means of numerous capillaries. The chorda dorsalis runs within the substance of the vertebral column until it reaches the thirtieth somite, when it emerges from the vertebral tissue and continues the remainder of its course in the interval between the primitive vertebrae and spinal cord, until it finalty loses itself in the cell-mass at the end of the tail. The plexiform middle sacral artery follows a course ventral to the vertebral column and can be traced to the tip of the cord.

===Embryo No. 221, 7.5 mm Crown-Rump Length===

Embryo No. {{CE221} has 38 somites and a remnant, the latter showing constrictions at three points. The caudal end of the embryo resembles that of a pig, as described by Keibel in his work on the human embryo. The cloaca, which is situated at a level with the thirty-first somite, is well developed, but as yet there has been no perforation of the membrane. Below the chorda dorsalis is a short remnant of the caudal gut containing a small lumen, as can be seen in figure 4, plate 1. At their caudal extremities the chorda, spinal cord, and caudal gut appear to fuse together. The central canal of the spinal cord is obhterated at its sacral portion, showing a pathological condition, and appears as a solid mass of nervous tissue. The groove between the tail-bud and the cloaca, which in the younger specimens indicated the first jroint of disappearance of the caudal gut, is here situated at the level of the thirty-first somite and is destined to later develop into the subcaudal epithelial plate. In this embryo there are 31 spinal ganglia with nerves.

===Embryo No. 389, 8 mm Crown-Rump Length===

As can be seen in figures 5 and 33, the caudal gut in embryo No. {{CE389}} still persists as a group of cells inclosing a narrow lumen. This mass seems to fuse with the caudal ends of the medullary tube and the chorda. The remnant of the caudal gut is surrounded by a close network of capillaries, which possibly bear some relation to its absorption. At the ventral wall of the spinal cord below the thirtieth somite can be seen two or three folds indicated by X in figure 33. As such folds were not found in the younger specimens, I assume that they begin at about this stage of development, and from now on I shall have occasion to refer to them frequently. The winding chorda emerges from the vertebral column at the thirty-fourth somite and continues its course to the end of the tail in the interval between the vertebral column and the spinal cord. This embryo contains 38 somites and a mesodermic remnant. In the latter I was unable to make out any constrictions, only a mass of germinating cells. The spinal ganglia number 32.


===Embryo No. 721 and No. 422, 9 mm Crown-Rump Length===

As no embryos of this size in sagittal section were available, I have had to make use of two that were cut in coronal sections, although in them the study of the vertebral structures was much more difficult. In embryo No. {{CE721}} the caudal portion is cut transversely, and in the sections through the end of the tail the caudal gut with its round lumen can be distinctly recognized. The extreme end of the gut is surrounded by a network of capillaries communicating with the middle sacral artery and vein. The cells of the caudal gut seem to fuse with those of the spinal cord and the chorda dorsalis in the caudal region. In this specimen there are 37 somites and a long remnant. I was able to count 32 spinal ganglia, the thirty-second being small and without nerves. In embryo No. {{CE422}} there are 38 somites with a remnant, and 32 spinal ganglia. The last one of these also is small and contains no nerve-fibers. The specimen is so poorly preserved that the caudal gut can not be clearly made out.

===Embryo No. 1197, 10 mm Crown-Rump Length===

In embryo No. {{CE1197}} there are 35 primitive vertebra" of Remak, or scleromeres, although the thirty-third and thirty-fourth appear to consist each of two parts fused together. This fusion has occurred, apparently at an earlier stage. At the caudal end of the vertebral column the vertebra" have not completely developed, although the tissue is condensed, showing that development is well under way. The chorda dorsalis is situated dorsal to the primitive vertebrae in the caudal portion and is slightly contorted. Caudally it terminates suddenly with a rounded end ventral to the neural tube at a level with the thirtyfifth vertebra. There are 32 spinal ganglia with nerves, the last 2 nerves being quite delicate. I have carefully examined each section of the caudal region in an effort to locate a remnant of the caudal gut, but could find no trace of it.

===Embryo No. 544, 11 mm Crown-Rump Length===

In embryo No. {{CE544}} the caudal end is bent sharply to the right. In the graphic reconstruction shown in figure 34 it is represented as straightened out in order to show more clearly the relations of the structures in this region. It is pictured in a more diagrammatic way in Kgure 6. Here 38 primitive vertebrae and a remnant are present, the latter differing from that described in the younger embryos. In those instances I have referred to it as the mesodermic remnant, using the terminology of Keibel, whereas in this stage of development (9, 10, and 11 mm.) the mesodermic remnant has been gradually converted into a non-vertebrated tail. The last scleromere or primitive vertebra, as shown in figure 34, is larger than the two or three more cranially situated ones. Two theories as to its development present themselves for consideration: Its growth may be the result of (1) fusion of the last two or three scleromeres which have become separated from the adjacent somites; (2) the addition of cells from the mesodermic remnant. While somewhat in doubt as to which theory to accept, I am inclined to favor the latter, as this embryo contains 38 primitive vertebra*, corresponding to the maximim number of somites found in the younger embryos, and there is no condensed tissue or group of cells in the end of the tail to indicate the primordium of a primitive vertebra.

===Embryo No. 852, 12 mm Crown-Rump Length===

In embryo No. {{CE852}} I found 37 scleromeres and a remnant; also 33 spinal ganglia, the last 2 being small, with slender nerves. The last 3 nerves emerge at the same point to form the caudal nerves, which nm from about the thirty-second to the thirty-sixth scleromere. The central canal of the spinal cord narrows between the thirty-fourth and thirtyfifth scleromeres and on the ventral wall of this narrow part are a few folds. These are indicated in figure 35. The chorda is almost entirely embedded in the tissue of the primitive vertebrae. In the younger specimens it emerges at the thirty-fifth scleromere, while in this one it emerges at about the thirty-seventh and from thence runs ventral to the spinal cord, touching the latter closely. The cloaca is situated at a level between the thirty-third and thirty-fourth scleromeres.

===Embryo No. 485, 13 mm Crown-Rump Length===

This specimen {{CE485}} is cut in transverse section and is therefore well suited for the study of the spinal cord. There are 33 spinal ganglia, but at the thirty-third and thirty-second complete nerve-fibers can not be made out. The thirty-third, in particular, comprises such a small cell-group as to be hardly recognizable. There are 37 primitive vertebrae and a non-vertebrated tail portion 289 ju long. As in the several specimens immediately preceding it, the few caudal vertebrae are fused together, showing no distinct boundaries. The non-vertebrated tail portion consists still of germinating mesenchymal cells, while the more cranially situated scleromeres are gradually becoming converted into precartilaginous tissue.

===Embryo No. 643, 13.5 mm Crown-Rump Length===

This specimen  {{CE643}} is cut in serial sagittal sections, but is, however, rather poorly preserved. There are 37 primitive vertebrae, the last one being incomplete. Caudal to this is a long non-vertebrated tail portion. There are 33 spinal ganglia, the last 2 having slender nerves which, with the thirty-first, form the caudal nerves. These extend caudally along each side of the tail.

===Embryo No. 940, 14 mm Crown-Rump Length===

This specimen {{CE940}} is cut in transverse section, and it is therefore difficult to follow the topography of some of the structures. I was able to count 36 primitive vertebrae, with a remnant, and 33 spinal ganglia. The last two, especially the thirty-third, are small and not provided with nerves. The twenty-ninth, thirtieth, and thirty-first spinal nerves extend down along the sides of the cord to the level of the thirty-sixth vertebra. The central canal of the spinal cord narrows at about the thirty-second vertebra, the portion caudal to this being practically devoid of the mantle layer.

===Embryo No. 390, 15.5 mm Crown-Rump Length===

In embryo No. {{CE390}} the primitive vertebrae are differentiated into precartilaginous tissue and between the vertebrae there is a small quantity of embryonic connective-tissue, as indicated in figure 36. At this period the vertebral column consists of 35 precartilaginous vertebrae; no additional segments can be recognized, although in the younger specimens there were 38 somites and one very long mesodermic remnant. There is a long, irregular, mesodermic cell-strand at the caudal end of the thirty-fifth segment, which, with the spinal cord, extends to the end of the tail. Between the two lies the caudal end of the chorda dorsalis, which, however, does not extend to the tip of the tail, as can be seen in figures 8 and 36. About the thirty-fifth segment the chorda becomes embedded in the substance of the vertebral column, the point at which it emerges being characterized by a sharp curve in its course.

We are here confronted with the following questions: What was the fate of the additional somites which could be so distinctly recognized in the earlier stages? And what is the mesodermic cell-strand which extends from the last primitive vertebra? .\s to the first, from the study of this material I am of the opinion that the last few vertebrae, which have earlier developed from the sclerotomes, fu.se together during the process of embrvonic development, thus forming the last vertebra in an embryo of this age. I am, however, aware that the comparison of a series of embryos can not conclusively settle this question. As concerns the cell-strand, it is my belief that it is formed of parts of the tissue which did not go to make up the primitive vertebrae, and constitutes the primordium of the caudal ligament. As is well known, the sclerotomes of the somites develop into not only primitive vertebrae, but also into several other supporting tissues which form the framework of the vertebral column. (Keibel and Mall, Human Embryology, I, page 331.)


The spinal cord narrows suddenly at the thirty-second vertebra, so that the portion caudal to this, together with its canal, presents an appearance distinctly different from the main body of the cord, as can be seen in figure 3b. On account of its regressive appearance, I shall hereafter refer to this part as the atrophic cord. One of its very characteristic features is a slender, narrow canal, and it might be desirable to speak of this as the narrow canal portion of the spinal cord.

===Embryo No. 406, 16 mm Crown-Rump Length===

A graphic reconstruction of embryo No. {{CE406}} is shown in figure 37, and a more diagranunatic sketch in figure 9. These show that the embryo has 36 cartilaginous vertebrae. On the right side the thirty-second and thirty-third segments have fused together. The last vertebra consists of two or three sections, each of which in an earlier stage probably represented a complete somite, these later fusing into one large segment. At the caudal end of this is a group of undifferentiated mesodermal cells - the primordium of the caudal ligament. The caudal end of the chorda dorsalis emerges at the thirty-sixth vertebra and terminates abruptly between the vertebral column and the spinal cord. The spinal cord narrows suddenly at the thirty-fourth vertebra. On the ventral wall of the canal in this narrow or atrophic portion of the cord there are three or four folds (fig. 37). In some of the sections can be seen a larger fold at the level of the twenty-ninth vertebra, which hangs down to the level of the thirty-fourth, both sides adhering to the wall of the spinal cord, thus forming a diverticulum, which is not shown in the illustration. The caudal end of the embryo is bent sharply dorsal, the bent portion being marked off on the surface by a shallow, circular furrow. The spinal cord extends to the end of the tail, conforming to the shape of the bent portion. The extreme end contains a narrow cavity which represents the caudal end of the central canal; the canal is interrupted at the root of the tail, where the cord appears to consist of solid nerve-tissue, as is indicated in figure 37. In this specimen there are 31 spinal ganglia with distinct nerves.

===Embryo No. 43, 16 mm Crown-Rump Length===

This specimen {{CE43}} has 37 cartilaginous vertebrae, the last being divided into three parts. There are 32 spinal ganglia. A graphic reconstruction was made of this embryo, but it is not illustrated in the figures.

===Embryo No. 576, 17 mm Crown-Rump Length===

This embryo {{CE576}} has 35 cartilaginous vertebrae, the last consisting of three small pieces fused together. The demarcation between these pieces can be more clearly recognized in the lateral portions of the column than in the median plane; so in determining the composition of the last segment one nuist study carefully the more lateral line of sections. A profile reconstruction of the embryo is shown in figure 3<S, and a more diagrammatic sketch in figure 10. The tail, with the caudal end of the .spinal cord, is bent shari)ly dor.salward. The spinal cord narrows suddenly at the thirty-second vertebra anil from this point down the central canal, which extends the entire length of the cord, becomes much smaller and rounder, while in the more cranial portion a transverse section of it would form an elongated oval. The ventral wall of the canal in the atrophic jiortion presents several transverse folds, as seems to be usually the case at this stage of reduction. The caudal portion of the chorda dorsalis is convoluted and its end sharply retracted.

===Embryo No. 991, 17 mm Crown-Rump Length===

The caudal end of embryo No. {{CE991}} is somewhat torn, but I feel reasonably sure that it did not exhibit a long caudal process. At a level between the thirty-second and thirty third vertebrae the central canal of the spinal cord narrows suddenly, and on the ventral wall of the atrophic portion of the cord are two folds. There are only 31 spinal ganglia, the first cervical on each side being absent. The others are completely developed, and even the thirty-first has its full complement of nerve-fibers. In embryo No. 576, just described, this nerve was quite slender. The chorda dorsalis runs in a straight line through the cartilaginous vertebral column and emerges from the thirty-fourth vertebra without winding. This is the first specimen of the series that lacks the non-vertebrated tail. At the point where the tail is found in younger embryos this has a small projection resembhng a tail-bud more than an actual tail. Between the last vertebra and this caudal projection is a mesodermic cell-mass into which enter the plexiform branches of the middle sacral artery and vein.

===Embryo No. 432, 18 mm Crown-Rump Length===

Embryo No. {{CE432}} contains 34 cartilaginous vertebrae. The last is larger than the thirty-third and on its lateral side shows three divisions, each consisting of young precartilaginous tissue. The middle part, where the segments are partially fused, is made up of cartilaginous tissue, and here the vertebra is incompletely divided into two segments - cranial and caudal. A little to one side of the median line, therefore, it is possible to count 35, and more laterally, 36 vertebrae. If the scleromeres of the several somites fuse together and develop into the last cartilagiiious vertebra, we may assume that the remaining mesenchymal somites left in the caudal portion of the tail form the non-vertebrated tail, and that in a more advanced stage of embryonic growth this substance develops into the caudal ligament. The chorda dorsalis shows two branches at its caudal end; one is formed at the thirty-second, the other at the thirty-fourth vertebra, and both follow a dorsal course. The more caudal branch is the longer, and its pointed extremity, which represents the caudal end of the chorda, adheres to the ventral wall of the atrophic portion of the spinal cord, as shown in figure 39. The wall of the spinal cord is quite thick in this region and at its upper portion are several folds. This condition is very interesting on account of its possible mechanical relation to the chorda, because as the caudal end of the chorda retracts it would tend to draw up the end of the spinal cord that is in the tail.

===Embryo No. 431, 19 mm Crown-Rump Length===

Embryo No. {{CE431}}  has 33 cartilaginous vertebrae. The last is larger than the thirtysecond and consists of two segments. It would seem most probable, therefore, that it has been developed by the fusion of two or more parts. The chorda dorsalis is considerably distorted in the thirty-third vertebra, thus indicating a fusion of several primitive vertebrae, and its caudal end adheres to the ventral wall of the spinal cord (fig. 13). The spinal cord extends to the tip of the tail, and a short distance from its extremity the ventral wall shows several folds. It appears possible that with the retraction of the chorda dorsalis the neural tube is also drawn up, thus producing folds on its ventral wall. The caudal end of the anterior spinal artery winds through these folds. The primordium of the ventriculus terminalis, between the wide and narrow parts of the central canal, is seen at the level of the thirty-second vertebra. This embryo has 32 spinal ganglia, the thirty-second being incomplete and without nerve-fibers; in all the others, however, the spinal nerves are complete.

===Embryo No. 837, 21 mm Crown-Rump Length===

Embryo No. {{CE837}} has 35 cartilaginous vertebrae, but the last is very small and its transition into cartilage is just beginning. It is surrounded by a voluminous mass of precartilaginous tissue resulting from the fusion of the last few scleromeres. The caudal piul of tlio fliordti dorsalis projects from the oxtrciiic (mkI of the vcrtol)nil foluinii into the region of the non-vertebrated tail and :ipiK>ars as if previously it may have adhered to the wall of the caudal end of the neural tube. In the thirty-fourth vertebra, and between tiie thirty-fourth and thirty-third, the chorda shows a typical loop formation, while iu its main portion it is almost straight and is situated in the midline of the column. The central canal of the spinal cord narrows sharply at the level of the thirty-second vertebra. Its ventral wall shows a few folds in the region of the atrophic portion of the cord. The spinal cord reaches to the tip of the tail and has a continuous canal thoughout. The middle sacral artery and vein extend into the non-vertebrated portion of the tail. 

===Embryo No. 453, 23 mm Crown-Rump Length===

In embryo No. {{CE453}} there are 35 vertebrae, the last consisting of precartilage tissue whicli has not as yet developed into true cartilage. The chorda dorsalis is straight and runs through the vertebral column in the midline. Its caudal end, however, .shows 4 coils, as shown in figure 40, representing a profile reconstruction of the specimen. The neural canal narrows at the thirty-first vertebra. At the level of the thirty-third vertebra a sac-shaped cell-mass lies between the spinal cord and the vertebral column, separated, however, frt)m the wall of the former (fig. 40, diverticulum). This sac seems to have resulted from a diverticulum of the ventral wall of the neural canal, the stalk of which has been obliterated. The ventral wall of the atrophic portion of the spinal cord shows small folds at the level of the thirty-fourth and thirty-fifth vertebr8P. The caudal end of the spinal cord expands slightly and its extreme end enters into the tail, which is now quite reduced. The middle sacral artery communicates with the anterior spinal artery by means of a branch that curves about the tip of the vertebral column. The subcaudal epidermal plate has nearly disappeared, while the post-anal swelling and the coccygeal tubercle have become visible. There is a shallow furrow between the tail-end and the primordium of the coccygeal tubercle, constituting a boundary between them. This embryo has 32 spinal ganglia with nerves.

===Embryo No. 382, 23 mm Crown-Rump Length===

Embryo No. {{CE382}} has 34 cartilaginous vertebrae. The last three do not lie exactly in a row in the median line, as can be seen in figures 15 and 41. The ventral portions of the thirty-second and thirty-fourth vertebrae, and the dorsal portion of the thirty-third, have become converted into cartilage, whereas the remainder of these two vertebrae still consists of precartilage tissue, as in younger specimens. .\t the thirty-third vertebra the chorda dorsalis gives ofT a short branch in a dorsal direction. Caudal to this the chorda winds and finally disappears in the caudo-dorsal portion of the thirty-fourth vertebra. Opposite the end of the chorda the wall of the spinal cord is so thickened as to gi\e the iini)ression that the two might have l>een attached. It is to be regretted that in most of the specimens the caudal end of the chorda dorsalis is torn. At the caudal end of the last vertebra the middle sacral artery anastomoses with the anterior spinal artery through a branch, similar to that mentioned in the last specimen. At the caudal end of the embryo there is a bud-like structure of the skin. This proves to be a stunted tail, for at it.s root can be recognized the sharp caudal end of the spinal cord and the terminal branches of the middle sacral artery and vein. The central canal of the spinal cord narrows at the thirty-second vertebra, but the ventral wall of its atrophic portion does not exhibit the folding that usually occurs in this region.

===Embryo No. 332, 24 mm Crown-Rump Length===

Embryo No. ({{CE332}} is quite similar to No. {{CE382}}, just described, except that it has no tailbud. There are 35 vertebra' and 31 .sj)inal ganglia. The caudal end of the spinal cord is represented by a strand of loosely arranged cells which extends to the epidermis at a point corresponding approximately to the root of the tail in a younger embryo, as can be seen in figure 16.


===Embryo No. 584a, 2.5 mm Crown-Rump Length===

A profile reconstruction of the caudal end of embryo No.  {{CE584a}} is shown in figure 42 and a simplified sketch is shown in figure 17. There are 34 vertebrae, the last being larger than the thirty-third. Around the cartilaginous mid-portion of the last vertebra there is considerable pre-cartilaginous tissue, which has been formed by the fusion of several scleromeres. At the thirty-first and thirty-second vertebrae the column curves ventrally. The chorda dorsalis makes a loop in the thirty-fourth vertebra and gives off short branches. The central canal narrows sharply at the level of the thirty-second vertebra, but expands again at the dorsal portion of the thirty-third and thirty-fourth. This atrophic portion, however, is not the prunordium of the ventriculus terminalis. Unger and Brugsch compare it with the sinus terminalis found in the amphibian Embryo. It is my behef that it represents the primordium of the coccygeal medullary vestige. The extremity of the atrophic portion extends into the tip of the tail and is provided with a lumen throughout. The caudal end of the chorda appears to have adhered to the ventral wall of the spinal cord. There are 32 spinal ganglia, the thirty-second having slender nerves. The middle sacral artery and vein enter into the tail, anastomosing through branches with the anterior spinal artery.

===Embryo No. 405, 26 mm Crown-Rump Length===

Embryo No {{CE405}} has 34 vertebrae, as indicated in figure 18. The last one inchnes dorsally from the axis of the vertebral column. Between the thirtieth and thirty-first vertebrae the axis of the ventral column shows a decided angle and below this point bends ventrally, presenting the coccygeal curve which is characteristic of the adult. The chorda dorsalis presents a spindle-shaped swelling at each intervertebral space and its caudal end shows a loop-formation in the thirty-third vertebra. The spinal cord narrows at a level between the thirtieth and thirty-first vertebrae; the atrophic portion, with its narrow canal, is spiral at its caudal end and enters mto the blunt tail-bud. On the dorsal surface of this spiral part of the cord the epidermis is lacking; whether this is due to mechanical injury or is a natural phenomenon could not be determined. From the ventral side two branches of the anterior spinal artery enter. This embryo has 31 spinal ganglia completely supplied with ner\Ts. The middle sacral artery anastomoses through a branch with the anterior spinal artery.

===Embryo No. 1008, 26 mm Crown-Rump Length===

Embryo No. {{CE1008}} has 34 vertebrae, the last being larger than any of the others and divided incompletely into two segments, as diagrammatically shown in figure 19. The end of the chorda dorsalis presents a number of intricate coils and its caudal extremity lies against the thick wall of the spinal cord. The spinal cord narrows at the thirty-second vertebra, thus marking a boundary between the atrophic portion and the upper part of the cord. The walls of the atrophic portion show two folds - one on the ventral, the other on the dorsal wall. The former lies in the region of the thirty-fourth vertebra and is similar to those already seen. The one on the dorsal wall, at a level between the thirtysecond and thirty-third vertebrae, is a diverticulum which projects caudo-dorsally and contains a long, slender cavity continuous with the central canal. Such folds or diverticula are seldom seen on the dorsal wall of the caudal end of the spinal cord. In the other specimens studied folds were frequently encountered at this end of the cord, but always on the ventral wall. On the ventral wall of the upper, wider part of the canal, at a level between the twenty-ninth and thirtieth vertebrae, is another and much larger fold, extending down about the length of two vertebrae. Both of its margins fuse with the ventral wall, thereby forming a channel in the midline of the fold which unites with the central canal. In this specimen the spinal ganglia are 31 in number.


===Embryo No. 875, 27 mm Crown-Rump Length===

In embryo No. {{CE875}} there are 34 vertebra?. The last is small and contains the winding part of the chorda dorsalis. The spinal cord narrows between the thirty-first and thirty-second vertebrae, as shown in figure 20. Its caudal end expands slightly and the extreme tip enters into the tail-bud. On the ventral wall of the central canal there are a few small folds. Near the caudal end of the vertebral column is a long, solid strand of cells, similar in structure to the cells of the spinal cord, which may have become separated from the latter at an earlier stage. Dorsal to the thirty-third and thirty-fourth vertebrae is a small papilliform tail, which is non-vertebrated and contains the caudal end of the vessels and a group of cells representing a remnant of the caudal end of the spinal cord. There are 31 spinal ganglia with nerve-fibers. The coccygeal tubercle and post-anal swelling are distinctly evident.

===Embryo No. 75, 30 mm Crown-Rump Length===

At the caudal end of embryo No. {{CE75}} there is a small papilliform tail containing a group of cells which merge into the wall of the spinal canal, as shown in figure 43. The spinal cord narrows suddenly at the mid-level of the thirty-second vertebra, and its atrophic portion is further constricted at a level between the thirty-third and thirty-fourth vertebrae, as indicated in figure 43 (constrict). The part below this constriction is the primordium of the coccygeal medullary vestige and the upper part is destined in a later stage to undergo retrogression, leaving a small cell-sac as a second coccygeal medullary vestige. There are two large folds on the ventral wall of the spinal cord at a level with the thirty-first vertebra. In the median plane they are triangular in shape and consist of ependymal and mesenchymal cells that have been inverted, together with the wall. A large diverticulum lies between these two folds. The space below the folds probably represents the primordium of the ventriculus terminalis. Only the branches of the anterior spinal artery enter into these folds. There are 34 cartilaginous vertebrae, and at thirty-first and thirty-second vertebrae the column presents a typical curve. The chorda dorsalis shows a spindle-shaped swelling between the vertebrae, and is much convoluted at the caudal end, as seen in figure 43. There are 31 spinal ganglia; the nerves of the last pair are quite slender.


===Embryo No. 145, 33 mm Crown-Rump Length===

Embryo No. {{CE145}} has 35 vertebrae, as diagrammatically shown in figure 22. The last one is situated on the dorsal side of the axis of the column, while the thirty-third and thirty-fourth lean towards the ventral side. There are 31 spinal ganglia, the thirty-second pair of nerves having no ganglia. In the caudal region there is a peculiarly shaped remnant of the neural tube, possibly an anomaly of development, which is connected with the main cord by a cell-strand. This cell-strand is directly continuous with the ependymal layer of the primordium of the ventriculus terminalis, and may possibly be regarded as the fihim terminale. It emerges from the membranous sheath of the spinal cord, the more cranial portion branching irregularly, while the caudal portion bends dorsally to enter the minute tail-bud. The ends of the middle sacral artery and vein enter into the root of the tail. In this embryo no coccygeal tubercle can be seen.

===Embryo No. 211, 33 mm Crown-Rump Length===

Embryo No. {{CE211}} has 34 vertebra and 31 spinal ganglia. The vertebral column curves ventrally at the thirty-first and thirty-second vertebra. The caudal end of the chorda dorsalis is undergoing regression and appears to be branching. The caudal end of the spinal cord may be divided into three portions: (1) the primordiinn of the conus medullaris, which includes the primordium of the ventriculus terminalis; (2) the filum terminale; (3) the coccygeal medullary vestige. The first extends about the length of the thirtieth and thirty-first vertebrae, tapering gradually towards its caudal end. The ventriculus terminalis, which is included in the conus medullaris, expands in the medial part dorsoventrally and transversely. The upper part of this cavity, which marks the entrance of the central canal, narrows slightly; the caudal end narrows sharply and forms a canal which terminates blindly at the end of the conus medullaris. The wall of the ventriculus terminalis consists of gray and white substance and the cavity is lined with a layer of ependymal cells. The dorsal wall is thicker than the ventral wall. The filum terminale extends from the caudal end of the conus medullaris, wathout definite boundaries, to a level between the thirty-second and thirty-third vertebrae. Its caudal end is represented by a slender bundle of nerve-fibers, and in its cranial portion there is a strand of ependymal cells. The large coccygeal medullary vestige is situated dorsal to the thirty-third and thirty-fourth vertebrae and its wall is thrown into a number of folds. At the caudal end it has two processes, one extending ventrally, the other dorsally. The latter enters into a rounded eminence at the caudal end of the embryo which represents a tail-bud, termed by Unger and Brugsch caudal tuberck. The post-anal swelling is well developed, while the coccygeal tubercle is scarcely to be made out.

===Embryo No. 199, 35 mm Crown-Rump Length===

In number and development of its vertebrae embryo No. {{CE199}} is about the same as Xo. 972, description of which follows. The coccygeal vestige, however, shows greater expansion.

===Embryo No. 449, 36 mm Crown-Rump Length===

Embryo No. {{CE449}} contains only 32 vertebrae, the last one being the smallest, as indicated in figure 24. The vertebral column shows a slight ventral curve at the point between the thirtieth and thirty-first vertebrae. The chorda dorsalis exhibits no convolutions at its caudal end. The spinal cord narrows at a level between the twenty-ninth and thirtieth vertebrae and the ventral wall of the atrophic portion presents a few folds. The remnant of the medullary tube was not found in the caudal region of this specunen. There are 31 spinal ganglia with nerves.


===Embryo No. 972, 37 mm Crown-Rump Length===

Embryo No. {{CE972}} has 34 vertebrae, the last two having fused together at the center. The vertebral column curves ventrally at the thirtieth and thirty-first vertebrae, the curve being so sharp that the thirty-second, thirty-third, and thirty-fourth vertebrae are situated in a row nearly horizontal to the trunk, as can be seen in figure 44. Cranial to thirty-first the chorda dorsalis expands between the vertebrae. The caudal end is winding and broken.

At the caudal end of the spinal cord one can recognize the primordia of the conus medullaris, filum terminale, and coccygeal medullary vestige, as shown in figure 44. The primordium of the ventriculus terminalis, which is included in the conus medullaris, appears as a continuation of the central canal of the spinal cord without any line of demarcation, and is situated at a level with the twenty-ninth vertebra. Its ventral wall is thinner than the dorsal wall and shows a few small folds (fig. 44, x). The primordium of the filum terminale extends from the caudal end of the conus medullaris, viz, at the level of the under part of the thirtieth vertebra, to the middle of the thirty-second vertebra. It contains an incomplete canal which is lined by a remnant strand of ependymal cells which are directly continuous with the ependyma of the ventriculus terminalis. At its caudal end is an ependymal strand which is directly continuous with the coccygeal medullary vestige. In addition to this ependymal substance, there is a small bundle of nerve-fibers along the ventral border of the filum terminale which extends into the white substance of the cord above. The primordium of the coccygeal medullary vestige is situated dorsal to the last two vertebra and contains a slender cavity. There are 30 spinal ganglia supplied with complete nerves. The thirty-first ganglion has almost completely disappeared on each side, leaving the nerves exposed.


===Embryo No. 362, .3!) mm Crown-Rump Length===

Embryo No. {{CE362}} has 34 vertebrae, the first one being situated on the dorsal side of the vertebral axis, as is diagrammatically shown in figure 26. At the thirtieth and thirtyfirst vertebrie the column is bent ventrally. The eaudal end of the chorda dorsalis winds and branches in the last three vertebrae. There are 30 spinal ganglia with nerves, but the thirty-first pair of nerves has no ganglia, their degeneration probably having occurred before that of the nerves. The caudal portion of the spinal cord is divided into the conus inedullaris, filum terniinale, and coccygeal medullary vestige. The ventriculus terminalis, which is included in the conus medullaris owing to the folding of its walls, is subdivided into two parts - an upper part, triangular in shape, and a lower, which is oblong and communicates with the upper by a narrow channel. The filum terminale reaches from the caudal end of the conus medullaris to the ventral side of the coccygeal vestige, being enveloped by the membrane of the spinal cord, the dura mater. This embryo presents a small tail-bud at its caudal end, containing a group of cells which connects with the coccygeal medullary vestige.

===Embryo No. 95, 50 mm Crown-Rump Length===

Although embryo No. {{CE95}} is recorded in the catalogue of the Carnegie Collection as 40 mm. crown-rump length, its state of development more nearly corresponds with a 50 mm. embryo, and on this account I have used the latter measurement in the heading. This specimen has 35 vertebrae. The last one is very small and partly fused with the one above it. The column presents a ventral bend at the thirty-first vertebra, giving the typical coccygeal curve. The chorda dorsalis is disappearing in certain areas in the vertebral bodies as far down as the thirtieth vertebra, but in each intervertebral space a fragment remains. Caudal to the thirtieth vertebra the condition of the chorda remains the same as in the younger specimens, and in the thirty-second it gives off a short dorsal branch. The caudal end is more simple in form than in the younger stages, but I am inclined to believe that at an earlier stage it too was winding, as one can see in the thirty-fifth vertebra a few detached globules which probably at an earlier stage were continuous with the chorda and with it formed a terminal loop.

At the caudal end of the spinal cord are two groups of cells connected by a cell-strand. The more caudal one is situated dorsal to the thirty-fourth and thirty-fifth vertebra*; it is somewhat larger than the other, is oblong in form and incloses an oval cavity - a fragment of the central canal of the spinal cord. The other group of cells is situated dorsal to the thirty-second and thirty-third vertebra* and incloses a long, narrow cavity. The ventriculus terminalis extends the length of two vertebrae - the twenty-ninth and thirtieth. At this stage it has acquired its adult form. In none of the earlier specimens have I noted it so perfectly developed, although embryos No. 449, 30 mm., and No. 199, 35 mm., show a cavity at the caudal end of the central canal as the ])rimordium of the ventriculus. In this specimen the structure is cylindrical in shape, has six walls, and measures 0.87 mm long, 0.23 mm. deep, and 0.52 mm. wide. The ventral wall is concave, the dorsal convex, the sides slightly concave. The upper wall or ceiling is irregular and at the front presents a long, narrow diverticulum directed cranio-ventral. Behind this diverticulum is a narrow channel which connects the ventriculus terminalis and the central canal of the spinal cord. The ventriculus terminalis is embedded in the nerve-fibers of the cord. The filum terminale extends from the caudal end of the conus meduUaris, at the level of the thirty-first vertebra, to a point between the thirly-third and thirty-fourth vertebrae close to the column. It is covered by a membrane of the spinal cord and passes through the ventral side of the cell groups at the caudal end of the medullary tube. The pia mater covers closely the whole surface of the spinal cord; it contains blood capillaries, and is visible at the conus meduUaris. The dura mater, which envelops loosely the pia mater, adheres to the wall of the vertebral canal as far as the midlevel of the thirty-first vertebra, at which point it leaves the wall and unites with the caudal end of the conus medullaris. This portion constitutes the primordium of the bursa durge matris. After the dura mater reaches the conus medullaris it envelops the pia mater quite closely, both following a caudal course and forming a sheath for the filum terminale. The point at which these membranes terminate can not be definitely decided. It is probable that the pia mater extends nearly to the end of the filum terminale between the thirty-third and thirty-fourth vertebrae. The fibers of the dura mater appear to enter into the caudal and dorsal portions of the last vertebra.

===Embryo No. 184, 50 mm Crown-Rump Length===

Embryo No. {{CE184}} has 34 vertebrae, the last one being the smallest, as is indicated in figure 28. At the thirty-first vertebra the column presents a ventral curve, bringing the thirty-second, thirty-third, and thirty-fourth vertebrae in about a horizontal row and at right angles with the main column. The chorda dorsalis is disappearing in the 29 upper vertebral bodies, but at the thirtieth and below there is no change from the earlier stages, except that the chorda is relatively more slender. Its caudal end is bent caudo-dorsally before terminating; from this point the caudal ligament takes its origin. The middle sadral artery at this stage is a relatively delicate vessel, running from the ventral to the dorsal side of the vertebral column, and curving about the apex of the thirty-fourth vertebra. Its branches are plexiform, and in their meshes are groups of cells resembling neuroblast cells. The caudal end of the spinal cord contains a large cavity representing the ventriculus terminalis at a more advanced stage of development. The upper end of this cavity connects with the central canal of the spinal cord; its lower end terminates in two horns, the dorsal one of which is a blind pouch; the ventral horn is united with the caudal remnant of the spinal cord by a strand of ependymal cells and many transverse folds. The caudal remnant of the spinal cord consists of three separated portions. The first, which is attached to the caudal end of the ventriculus terminalis by an ependymal cell-strand, lies between the thirtieth and thirty-first vertebrae. This portion is embedded in nervefibers. As in younger specimens, it incloses a narrow cavity interrupted about midway. The second portion of the remnant is situated between the thirty-first and thirty-second vertebrae and leans to the dorsal side of the filum terminale. It also contains a small lumen. The third and largest portion is situated at the level of the thirty-third vertebra; its cavity is larger than the others and its caudal end enters into the caudal ligament.


The pia mater envelops the spina.l cord and contains blood capillaries. It traverses the course of the filum terminale, completely inclosing it, and appears to reach the dorsal portion of the thirty-third vertebra, at which point the filum terminale ends. The dura mater also covers the spinal cord over the pia mater. At the caudal end of the conus medullaris, about the thirtieth vertebra, the dura mater adheres closely to the pia mater. At the dorsal side of the thirty-third vertebra the fibers of the dura mater merge with the fibers of the caudal ligament.


This embryo has 31 spinal ganglia on the right side and 30 on the left. The last ganglion on either side is very small, being in process of retrogression. The right thirtieth and thirty-first ganglia and the left thirtieth are not located between the vertebrae, but at the dorsal side of the upper vertebral bodies.

===Embryo No. 448, 52 mm Crown-Rump Length===

A profile reconstruction of the caudal end of embryo No. {{CE448}} is shown in figure 45 and a more diagrammatic sketch is shown in figure 29. The embryo has 34 vertebrae, the last of which is only three-fourths the size of the thirty-third. The last three have begun to fuse, so that a section cut through the axis of the vertebral column shows one large vertebraI body representing the three vertebrae, as shown in figure 45. The vertebral column is bent ventrally between the thirtieth and thirty-first vertebrae, forming an obtuse angle and creating the tjTiical coccygeal curve. Within the bodies of the vertebrae, from the first to the twenty-ninth, the chorda dorsalis is disappearing, but a remnant still remains in each intervertebral space. From the thirtieth to the thirty-fourth vertebrae it continues without convolutions, but the caudal end is branched and winding, partially disappearing at the dorso-caudal portion of the last vertebra close to the remnant of the spinal cord. The spinal cord tapers to a point as the conus medullaris and proceeds as the filum terminale from a level between the thirtieth and thirty-first vertebrae. Four portions of the neural tube can be distinguished at the caudal end of the spinal cord: (1) the sacral region of the spinal cord; (2) the conus medullaris and its contained ventriculus terminalis; (3) the filum terminale; (4) a remnant. The first consists of the ependymal zone, the mantle zone which contains the germinating nerve-cells, and the marginal zone, as is typical for the cord as a whole. The conus medullaris extends from the twenty-eighth to the thirtieth vertebrae, tapering gradually. In this region there is a large cavity, which in a median sagittal section shows four walls. Through the front of the upper wall the ventricle joins with the central canal of the spinal cord. The lower wall is narrow and from it extend two ependymal cell-strands. The longer of these goes straight downward to the first cell-group of the renmant of the medullary tube, through the axis of the conus medullaris. The shorter strand can be seen at the corner between the lower and ventral wall in figure 45. At the ventral wall is a diverticulum, the entrance to which appears as a narrow stalk consisting of a solid cord of ependymal cells and connecting with the ependymal cells of the cavity. This diverticidum is divided into two parts which are united by a cell-strand - a small upper sac and a larger lower sac. The ventral walls of both sacs are situated close to the surface of the conus medullaris, but do not open into it. The lower part of the conus medullaris consists chiefly of nerve fibers of the spinal cord, and here the central canal is entirely obliterated, leaving a long strand of ependymal cells. The conus medullaris extends to a point between the thirtieth and thirty-first vertebrae, and from there continues as the filum terminale, which extends to the last vertebra, skirting close along the dorsal side of the vertebral column. At the dorsal side of the filum terminale there are two remnants of the primitive neural tube. One of these is situated just dorsal to the apex of the conus medullaris. It contains a slender lumen, the remains of the central canal of the spinal cord. The other remnant (fig. 45, res. m. co.) is situated dorso-caudal to the thirty-third and thirty-fourth vertebrae. It is oblong in shape and likewise contains a cavity, somewhat larger, which represents a remnant of the central canal. Its caudal end is sharp and fuses with the caudal ligament. The latter is not so distinct in this specimen as in the younger ones.


The caudal end of the sympathetic nerve-trunk lies between the middle sacral artery and vein, the three passing along the ventral side of the thirty-third and thirty-fourth vertebrae, where they curve around the apex of the last vertebra. The caudal ligament forms at the caudal end of the thirty-fourth vertebra and extends dorso-cranial to the coccygeal vestige. The caudal portions of the sympathetic trunks unite ventral to the thirtieth vertebra and become as one. After the union of these cords two additional ganglia can be seen - one at the thirty-first, the other at the thirty-third vertebra. From the latter the sympathetic nerve-trunk follows the midline of the vertebral column and curves around the last vertebra to the dorsal side, as shown in figure 45. The condition of the dorsocaudal portion of the nerve-trunk can not be clearly recognized.


The pia mater covers entirely the surface of the spinal cord and is rich in blood capillaries. It also envelops that portion of the filum terminale containing the cell-groujis which connect with the ependymal cells of the ventriculus terminalis. The dura mater traverses the wall of the vertebral canal enveloping the spinal cord and its covering of pia mater. In the caudal region of the spinal cord there does not appear to be a distinct space between the pia mater and dura mater and hence the arachnoid membrane is not visible at this point. A short distance from this, however, where the membranes envelop the conus medullaris, there is a marked space between the two membranes and here the arachnoid can be fairly well made out, forming a fibrous network of embryonic connectivetissue. At the level of the caudal third of the thirtieth vertebra where the filum terminate begins, the dura mater fuses with the pia mater and the two become separated from the wall of the vertebral canal and extend caudalward. The second group of cells, which lies caudo-dorsal to the thirty-third vertebra, does not seem to be covered by the pia mater or dura mater, these membranes having disappeared a short distance above.

===Embryo No. 1656, 67 mm Crown-Rump Length===

There are 34 vertebrae in embryo No. {{CE1656}}, the last being the smallest. At the thirty-first and thirty-second the vertebral column shows a ventral curve, the angle being sharper than in the younger specimens. The vertebrae are separated by embryonic tissue which is to develop at a later stage into intervertebral fibro-cartilage. This separation becomes progressively more marked above the thirtieth vertebra. Between the vertebrae which still lie close together is a small space where the chorda dorsalis coils as it emerges from the vertebral bodies in the median line. Several of these coils can be seen in figure 46, which is a profile reconstruction through the caudal end of the embryo. The blood-vessels enter the vertebral bodies from the ventral and dorsal side.

In the conus medullaris there are two medullary ventricles. The more cranially situated one is somewhat smaller tnan the other, measuring 0.55 by 0.25 by 0.33 mm. Its form, as seen in the sagittal plane, can be recognized in figure 46 {vent. t. cran.). The lower cavity is oblong in shape, measures 1.1 by 0.3 by 0.36 mm., and presents a canallike appendage 1.7 mm. in length, as seen in figure 46 {Append.). This appendage tapers to a point and continues as a cell-strand. Toward the caudal end of the strand, in the path of the filum terminale, are two small groups of cells which represent the remnants of the ependymal cells of the medullary tube (fig. 46, Ee. epend.).

The phenomenon of dedifferentiation at the caudal end of the spinal cord is well shown in this specimen. The appendage of the lower cavity was a complete ventriculus terminalis at the first stage; the main body of the cavity was a complete one at the second stage, and the upper cavity is the ventricidus terminalis at the present stage, thus showing a progressive upward trend. The gray substance which primarily existed around the ventriculus terminalis has now disappeared as the result of degeneration, and the caudal end of the central canal has gradually enlarged. The caudal end of the lower cavity, however, is becoming gradually narrow because the caudal portion of the conus medullaris, which contains the ventriculus terminalis, has also gradually become atrophied and lost its cellUke substances. The septum between the two cavities is a remnant of the gray substance of the spinal cord, in which the degeneration is not yet complete.

The filum terminale follows a downward course from the end of the conus medullaris and nerve-fibers can be recognized as far down as the caudal portion of the thirty-second vertebra. In the caudal region are found two cell-groups representing remnants of the neural tube; one, which lies between the thirty-second and thirty-third vertebrae, contains no lumen, and the epithelial cells are undergoing degeneration. The other is situated dorsally between the thirty-third and thirty-fourth vertebrae and incloses a small lumen.

The membranes of the spinal cord are more easily made out in this specimen than in the younger ones. The dura mater is separated from the periosteum of the vertebral bodies, especially at the ventral wall of the vertebral canal, by a dense plexus of blood vessels, connective -tissue, and small spaces. This separation occurs at a level between the twenty-seventh and twenty-eighth vertebrae, and the dura mater becomes adherent to the conus medullaris between the twenty-eighth and twenty-ninth vertebrae, following an oblique course from the periphery to the center of the vertebral canal. There is thus laid out the early form of the dura! sac. Outside of this .sac the fibers are .separated into tufts which run parallel and caudalward. In the space between the dural sac and the conus medullaris the arachnoid membrane can be seen developing. The pia mater envelops closely the spinal cord and supports the blood-vessels; between the twenty-fifth and twenty-eighth vertebra it is separated from the dura mater and the arachnoid by a still wider space.

===Embryo No. 662, 80 mm; No. 928, 100 mm; No. 142, 125 mm Crown-Rump Length===

As the investigation with embryos Nos. {{CE662}}, {{CE928}}, and {{CE142}} was not very satisfactory, I shall not attempt to give its results in detail at this time. I have, however, made a special study of the coccygeal medullary vestige because of its importance in comparison with the same structure in younger specimens. In the 80 and 100 mm. embryos the coccygeal vestige is very small and its contained cavity narrower than in the younger specimens. In the 125 mm embryo (negro) the structure is well developed and shows one long offshoot stretching under the epidermis at the sacral region. It is quite different in form and condition from the case reported by Tourneux (Precis d'embryologie humain), and therefore does not present the loop formed by a more deeply situated limb (segment coccygien direct) and a more superficial limb [seginent coccygien refleche). In this embryo the coccygeal vestige contains a slender cavity.


==Development and Reduction of the Tail==

In considering the process of reduction of the tail I should like, in the first place, to refer to the important study of this condition in mammals made by Braun (1882), w'hose conclusions in general are as follows:

# The tail of the mammalian embryo consists of two portions - a vertebral ed part and a non-vertebrated part, the latter situated caudal to the former.
# The non-vertebrated part appears usually in the form of a thread at the end of the vertebrated tail, and consequently may be designated the caudal filament (Schwanzfaden). Being usually thinner than the tail itself, it is consequently sharply marked off from the latter.
# The vertebrated part of the tail can again be subdivided into two parts according to whether it projects from the body or not. The projecting part is designated as tail, although it has long been well known that this is directly continuous with the sacral vertebra*. The relative size of the internal and external tail varies, and hence we meet with long-tailed, short-tailed, and tailless mammals.
# The caudal filament is a transitory structure, although for a time it contains the end of the spinal cord, the chorda dorsalis, and the caudal gut. These structures muiergo resorption and the last tissue to persist is the epidermis, the caudal thread for a time persisting of only epidermis cells.
# The caudal gut originally extends into the tail; before being resorbed it separates into fragments which disappear, the last to persist being the part constricted off at the tip of the tail.
# The chorda dorsalis always projects beyond the caudal vertebra?, where it separates into forked processes or curls in irregular loops. This part disappears completely.
# The spinal cf)rd originally extends to the tip of the tail. The latter, however, soon exceeds it in length, when it terminates at the base of the caudal filament. It was possible to show in specific embryos that the ascensus medulK-r is due not alone to the overgrowth of the vertebra, but that also there is degeneration and aljsorption of the caudal end of the spinal cord, to which in part the formation of the filum terminate owes its origin.


It would appear, therefore, that in his subdivision of the mantunahan embryonic tail Braun included in the caudal filament that portion which lay between it and the vertebrae. My own position on the subject is briefly this: Is the caudal filament, through all the stages of mammalian embryonic life, one and the same thing as the non-vertebrated tail? That an intermediate portion exists between the two was apparently not recognized by Braun, but in man it constitutes a most important factor in the reduction of the tail vertebra;. After detailed investigation wdth the material at hand, numbering about 40 embryos ranging from 4 to 50 mm. in length, I was able to divide the caudal structure as follows: (1) the vertebrated portion; (2) the mesodermic end portion. In somewhat older embryos the first is subdivided into a proximal portion with persisting vertebrae, and a portion from which the primitive vertebrae have disappeared (lost-vertebrae portion). This point can better be understood by referring to the embryos themselves.


In embryo No. {{CE221}}, 7.5 mm long, the tail contains 38 somites and a long mesodermic remnant. The somites, which later develop into preeartilaginous vertebrae, are well defined by the presence of small blood capillaries between them. On the dorsal surface of the tail the boundaries of the somites can be recognized distinctly as transverse shallow grooves. In the last somite, which is in contact with the mesodermic remnant, the boundary is not nearly so clear as in the others and would probably have disappeared altogether in the retrogressive process had the embryo Uved. In this specimen the tail is entirely a vertebrated tail, as each somite is capable of development into a vertebra. The long mesodermic remnant at the caudal end, although sejiarated by segmentation from the mesodermal sheet, evidently would not have developed into preeartilaginous tissue. This part I have differentiated from the vertebrated tail as a mesodermic remnant, using the term employed by Keibel. I was able to distinguish in this embryo, therefore, two divisions of the tail - a long somitic portion, the vertebrated tail, and a short mesodermic portion, the caudal end of which may be compared with the caudal filament but not with the lost-vertebrae tail. In this stage the non-vertebrated portion has not as yet developed - that Ts, the portion in which the somites or pre-cartilaginous vertebrae have disappeared. The last somite, however, shows signs of disappearing, and after a time, therefore, the lost-vertebrae portion will appear in place of the last somite. In other words, the reduction phenomenon has begun in the last somite; this progresses in the tail from one somite to the other, each losing its distinct boundaries, the blood capillaries fusing and disappearing.


In somewhat older specimens (8 mm., fig. 33), the last somite is larger than the preceding one and evidently represents the fusion of two pieces - the thirty-eighth somite and the mesodermic remnant. At the 11 mm. stage the lost-vertebrae portion of the tail becomes well developed (fig. 34). In the 12 and 15 nam. embryos the boundaries of the thirty-sixth, thirty-seventh, and thirty-eighth somites have become indistinct. In the 15.5 mm. specimen these three somites are converted into a cord ?.'hich extends to the end of the tail (fig. 36, str. cell). This cord consists of embryonic cells which at an earlier stage of development existed in the somites as sclerotomes. In the median portion of the cell-strand are three or four segments, and the last vertebra also shows two or three divisions. At this stage three types of vertebra can be recognized at the caudal end of the vertebral column: (1) the vertebra; which have developed from the sclerotomes into precartilaginous or primitive vertebrae; (2) the incomplete primitive vertebrae, or the parts of the thirty-sixth and thirty-seventh sclerotomes which form the last vertebrae; (3) the cell-strand formed by the fusion of the last two somites and perhaps the thirty-sixth as well. I have not been able to determine whether or not the mesodermic remnant has merged into this strand. This mesodermic cell-strand - the primordium of the caudal ligament - is diagrammatically shown in figures 36 and 37 (str. cell). In the 16, 17, and 18 mm embryos the last vertebra is larger than the more proximally situated ones and consists of two or three pieces united in the median plane; 35 vertebrae, developed into precartilage or cartilage tissue, were found in the 15.5, 17, and 18 mm. embryos, and 36 in the 16 mm. embryos. In those 17 mm. and larger the last two or three primitive vertebrae were usually found to be fusing at the center of the column, while in the lateral parts they show the divisions quite distinctly.


His did not find any extra vertebrae in the tail, but many other authors have recognized from 2 to 4. Perhaps the material upon which His based his studies did not include specimens in the same stages of development as my 15.5, 17, and 18 mm. embryos, in which the last vertebra consisted of two or three pieces. The 21 mm. embryo also represents the typical condition at this stage, the tail showing extra vertebrae. In this embryo can be clearly demonstrated a short tail consisting of two portions, such as has been described by His, the extreme non-vertebrated or, more correctly speaking, the lost- vertebrae portion, and the vertebrated portion. I am sure that embryos of this age never present a caudal filament homologous with that of other mammals, and I can not therefore agree with His, Braun, Keibel, linger, and others, who describe the non-vertebrated portion of the tail in the human embryo as a caudal filament, since this portion at an earUer period contained somites capable of development into primitive vertebrae. Ecker, who studied human and mammalian embryos, asserted that the human embryo never has the caudal filament such as is the rule for mammalia. I could not recognize clearly a caudal filament in any of my specimens. In one of the three embryos (6.5, 7.5, and 8 mm.) which showed a portion of the caudal gut in the end of the tail, the caudal end was demarcated by a bend, and this might have been mistaken for a caudal filament. I am of the opinion that in the 8 and 10 mm. embryos the portion of the tail beyond the pointed end of the vertebral column, as shown in figure 34, can be compared with the caudal filament in mammals, but is not the true caudal filament described by Braun. It consists of the caudal end of the mesodermic remnant and contains the end of the neural canal and, in the 8 mm embryo (fig. 33), a part of the caudal gut. I believe, also, that the non-vertebrated portion of the mammalian tail, which is not included in the caudal filament, is homologous with the non-vertebrated tail of the human embryo.


Having compared the non-vertebrated portion of the manunalian tail with that of human embryos, I have concluded that in the former the reduction process occurs at an early stage, just as it does in the human embryo. This theory is based upon two facts: First, the last caudal vertebra is larger than the next proximally situated one. as in the cow embryo described by Braun, its increased size being due to the fusion of the last three pieces. This phenomenon represents the reduction proce.ss in the lower segments at a certain stage. Second, the number of vertebrae in the tail of sheep embryos, as asserted by Braun, is variable, and this variation must be due to a stronger or weaker effect of reduction.


In the 19 and 23 mm. specimens there is a very short tail with a caudal tubercle. In the 19 mm. embryo the vertebrae of the tail have fused together into one large vertebra - the thirty-third - in which can be recognized two or three pieces. The caudal end of the chorda dorsalis within this vertebra shows several coils, indicating a fusion of the last few vertebrae. The lost-vertebrae portion of the tail is represented in this specimen by the caudal extremity which contains the ends of the neural canal, the middle sacral artery and vein, and the chorda dorsalis. The latter adheres to the ventral wall of the neural canal and it appears as if the neural tube is retracted cranialward. In the 23 mm embryo the development of the caudal end is farther advanced. The furrow between the tail root and the primitive anus becomes gradually more shallow, and the vertebral portion of the tail is embedded in the embryonic tissue which will later develop into the coccygeal tubercle. This shortening of the tail is evidently brought about by three factors: (1) fusion of the last few caudal vertebrae ; (2) rapid growth of the aUmentary canal and its surrounding structures ; (3) the flexion of the caudal portion of the vertebral column.


(1) The disappearance of the last few caudal vertebrae by fusion, leaving only the winding end of the chorda dorsalis, is a well-estabUshed proof of the compression and final disappearance of the caudal vertebrae and the chorda dorsalis which was within them. Unger and Brugsch took the \aew that in spite of the presence of an external tail one could still speak of the formation of a coccygeal tubercle, inasmuch as the segments of the caudal region, which in their most caudal portion are already reduced, have begun to show a moderate, ventrally directed curve in their axis, which is eventually to be the coccygeal eminence. They point out that two factors are of importance in the formation of the coccygeal eminence: (a) the fusion (reduction) of the most caudal segments; (6) the bending in the axis of the caudal vertebrae. In the 25 and 27 nam. embryos the lost-vertebrae portion of the tail becomes rounded off and is shown as the caudal tubercle. Its extremity appears as a bud-shaped appendage and contains the caudal ends of the spinal cord, with its central canal, and of the middle sacral artery and vein. This budhke appendage is called by many authors the caudal filament, but this is incorrect for the reason, as stated above, that it represents only a part of the lost-vertebrae portion of the tail which was primarily the vertebral portion, and therefore could never be considered as the caudal filament described by Braun.


(2) The area between the vertebral column and the rectum, especially the root of the tail, increases rapidly in a caudo-ventral direction. The caudal region of the rectum also extends down, its growth being in proportion to that of the vertebral column (figs. 39, 40, and 43). It is the behef of many authors - Rosenberg. Eckcr, Keibd, and others - that the tail and the coccygeal tubercle in human embryos become shorter and finally disajipear by an increase in volume of the caudal soft tissues, muscular tissue, svibcutaneous connective-tissue, etc., which surround the caudal part of the vertebral column.


At an earlier stage the swelling between the primitive anus and the root of the tail is called the post-anal swelling. Keibel asserts that in embryos 11 mm. and larger the root of the tail is separated from the ventral trunk by double plates of epithelial cells which lie between it and the anus. Therefore, by means of these plates, which consist of two sheets of epidermal cells connected ventrally to the l)Ost-anal epidermis and dorsally to the ventral side of the tail, the tail-root is distinctly marked off. Following Keibel's idea, Tourneux speaks of it as depression sous caudale de Vintegument externe. linger and Brugsch describe the stages of disappearance of this post-anal swelling in embryos 25 and 45 mm. long. In my specimens it is quite clear. In the 13 and 14 mm. embryos these plates are visible, but in the 45 mm. specimen they have disappeared. After observing the specimens in the various stages, my conclusions on this point are as follows: At a certain stage (13 mm. and older) the digestive tube grows more rapidly than the vertebral canal, so that the depression gradually straightens out.* At this time the caudal region of the digestive tract - viz, the cloacal region - develops faster than the caudal end of the vertebral column, which constitutes the caudal end of the internal tail. Moreover, the mesodermic tissue between the primitive anus and the root of the tail develops rapidly and gradually bulges downward. By the swelling of the caudo-ventral region of the tail-root the fold of epidermis, or so-called epithelial plate, is stretched by degrees and at last disappears. The growing of the coccygeal tubercle would also aid in this process. In his paper Keibel asserts that the mesodermic tissue between the primitive anus and the tail-root grows luxurianth' at certain stages and bulges downward. He terms this swelling die postanalen wulst (post-anal swelling). In this way the epithelial plates disappear. This epidermal plate between the anus and tail-root moves gradually caudalward. In the 12 mm. embryo it is situated at the level of the thirty-third vertebra, and in the 46 mm. specimen has moved down to the level of the thirty-fourth vertebra. The caudal end of the rectum - viz, the caudal end of the digestive tract, and perhaps that of the genito-urinary organs as well - has likewise moved caudalward.


(3) Originally the caudal portion of the vertebral column is nearly a straight line, but in embryos about 20 mm. long the axis of the column shows a distinct ventral flexion at about the level of the thirtieth or thirty-first vertebra. There is a second flexion which is dorsal at the caudal end of the vertebral column, between the vertebrated portion and the lost-vertebra? portion of the tail, which is seen in younger embryos. The caudal remainder of the lost-vertebrie tail has, therefore, moved to the dorsal side of the vertebral column, being joined to the last vertebra by bands of embryonic connective-tissue. These bands arc the so called caudal ligament. In these embryos the ridge or epidermal plate between the coccygeal tubercle and the rectum has become shallow almost to the point of disappearance, as shown in figures 40 and 42. In the 30, 33, and 39 mm. embryos the lost- vertebrae portion of the tail has almost entirely disappeared from the surface of the skin. In the first embryo the tail remnant is surrounded by a minute furrow, while in the 33 mm. specimen it appears as a rounded eminence; and finally, at the 39 mm. stage, the remnant of the tail is represented by a small papilla. These remnants contain groups of cells from the primitive neural canal. The apex of the caudal conical eminence, the caudal tubercle, according to Unger and Brugsch, in which the cell-strand of the neural canal enters, is a part of the lost vertebrae tail, or so-called non-vertebrated tail. The various stages in the reduction of the tail as shown on the skin surface do not present the same appearance in every embryo; but on section evidences of its reduction and disappearance are invariably found dorsal to the caudal end of the vertebral column - that is, dorsal to the coccygeal tubercle and in the median line of the embryo. I have seen no case in which the remnant of the tail is situated just at the caudal end nearly in line -with the extended axis of the vertebral column - namely, at the top of the coccygeal tubercle. At this stage the caudal ligament is well developed and consists of bands of connective-tissue. The curve of the vertebral column is quite prominent at the thirtieth, thirty-first, and thirty-second vertebrae. This flexion of the caudal portion of the column begins at about the 25 mm. stage, although sometimes it does not appear until the 33 mm. stage. These embryos show a small tail at the caudo-dorsal end. In the 30 mm. embryo (fig. 43) and the 39 mm. embryo, where the tail is disappearing from the surface of the skin, this curving of the vertebral column becomes more marked than in the younger specimens.


Concerning the disappearance of the tail in the human embryo, I am of the opinion that, while the lost-vertebrae portion of the tail disappears from the skin surface, a few vertebrae of the tail fuse with the one above, usually the thirty-fourth, a part of which disappears by dedifferentiation ; and that the caudal portion of the column, which consists of the thirty-first, thirty-second, thirty-third, and thirty-fourth vertebrae, is bent to the ventral side, sinking into the embryonic tissue between the rectum and the coccygeal tubercle. After the tail entirely disappears there appears outside of the ventral region of the tail root a blunt conical eminence known as the coccygeal tubercle, or eminentia coccygealis. This tubercle is a temporary swelling formed by bulging of the caudal end of the vertebral column and the addition of embryonic tissue contained in the lost-vertebrae tail at an earlier stage. The tubercle disappears at some time between the 33 and 52 mm. stage, while the caudal end of the vertebral column, the so-called internal tail (after Braun), sinks deeper into the soft tissues which surround and envelop it (figs. 44 and 45).


In describing the embryonic tail in mammals, Braun divides it into internal tail {die innere Schwanz) and external tail (aussere Schwanz). This theoretical arrangement may be the better one. In the human embryo, at least in my specimens, it can be clearly demonstrated. In an embryo of 21 mm. one can recognize the external tail, which may be divided into two portions - vertebrated and nonvertebrated. In the 27, 30, and 39 mm. specimens the thirty-first, thirty-second, thirty-third, and thirty-fourth vertebrae belong to those of the internal tail. I agree with other authors that the human embryo has a true tail at a certain stage of its development and that the second coccygeal vertebrae and those caudal to it in the adult are the true tail vertebrae in the philogenetic sense.


==Chorda Dorsalis==

In the 4 mm. embryo the chorda dorsalis lies close to the ventral side of the neural tube, but cranial to the twenty-first segment it is separated from the tube by the tissue of the primitive vertebrae. At this stage it forms a long, narrow tube, its caudal end consisting of only a small cell-strand which terminates in a cell-mass above the caudal extremities of the neural tube and caudal gut. In the 5 mm. embryo, which is shown in figure 31, the chorda cranial to the thirty- third somite is separated from the ventral side of the neural tube, while caudal to the thirtythird the two are contiguous. The chorda terminates m the mesodermic renmant, being covered by the ventral wall of the neural tube, and at its caudal end is united with that of the caudal gut by a cell-strand. In specimens 7.5 to 11 mm. the greater part of the chorda dorsalis cranial to the thirty-fourth or thirty-fifth somite is embedded in the primitive vertebral column and shows considerable winding. Caudal to the thirty-first somite the chorda is placed between the neural canal and the primitive vertebral column. In passing down through the column it shows a series of segmental undulating curves - that is, it alternately bends ventrally and dorsally. The dorsal bends occur at the foci of vertebral formation which eventually become the intervertebral spaces. In older embryos - 12 to 14 irmi. - this segmental undulation of the chorda gradually disappears. In the 12 mm. embryo (as shown in figure 35) the chorda is more completely embedded in the column, although here its terminal portion emerges to he in the space between the spinal cord and the tissue of the column. As the embryo advances in age this bending of the chorda gradually decreases, until at about the 18 mm. stage it becomes straight in its main portion, while the caudal part, which was hitherto straight, now becomes curved, the first indication of the formation of undulations (compare figs. 35, 36, 37, and 39), which, however, are probably not segmental like those above described, but are a phenomenon of the process of reduction in the caudal primitive vertebrae.


When we compare the 7.5, 8, and 11 mm. embryos with those from 15 to 19 mm. it is easy to see that at first the few caudally situated primitive vertebrae - the scleromeres - fuse together, and the chorda which is within them becomes convoluted and recedes cranialward. The winding portion of the chorda is, therefore, situated in the last vertebra which has developed by the fusion of several vertebrae. This condition remains the same up to the 18 mm. or even more advanced stage, and finally, in the 23 mm. stage the chorda presents a spiral appearance, as shown in figure 40. Braun found this same process in sheet and other mammalian embryos, the end of the chorda projecting caudalward from the last vertebra. Ecker also noted this projection of the chorda in his mammalian material. These authors recognized the winding or branched end of the chorda in the caudal filament or in the extreme end of the tail and concluded that this was its primitive state. In human embryos, however, as mentioned above, at the earliest stage when the chorda reaches the extreme end of the tail, its caudal end is straight and shows no winding until the reduction of the tail begins. In embryos from 15 to 23 mm the caudal end projects almost caudalward from the last vertebra which has been formed by the fusion of .several vertcbne. His did not find such a condition in human embryos, although it is usual. According to Braun, the occurrence of a free, naked end of the chorda is due to the disappearance of the last primitive vertebrae by which it had previously been surrounded. In the human embryo the caudal end of the chorda was never surrounded by primitive vertebrae, but was situated between the neural canal and the primitive vertebral column. In the 25 mm. (fig. 42) and the 30 mm. embryo (fig. 43) the coil-like appearance of the chorda is typical, and these stages, therefore, are the clearest of any thoughout embryonic life. Later on, for example, in the 37 mm. embryo (fig. 44), the caudal end of the chorda is disappearing, leaving a few remnants which have become separated from the main chordal strand. This degenerative fragmentation and partial absorption of the terminal portion of the chorda results in a great variety of forms. Very seldom is the caudal end branched in the earlier stages. From the 39 mm. stage the chorda becomes gradually reduced and is finally converted into a more simple form, as shown in figures 45 and 46. While the short caudal portion shows the above-mentioned variations, the main strand changes but sUghtly. After it becomes straightened some portions of it which he in the intervertebral fibro-cartilage show spindle-shaped sweUings, as shown in figiires 39 and 42 (18 to 25 mm.). At last, in the 50 and 52 mm. embryos, the parts embedded in the vertebral bodies disappear, leaving small remnants in the intervertebral spaces. Frequently these remnants show visible coils, as can be seen in figure 46. These remain often until a later stage. The disappearance of the chorda below the thirtieth vertebra occurs later than that of the main strand, and we can therefore still recognize it in the 67 mm. embryo as a continuous cord through the caudal vertebral bodies.

==Development of the Caudal End of the Spinal Cord==

In the 4 mm. embryo the caudal end of the neural tube fuses with the soUd mass of mesodermal cells which extends to the ventral side of the tail. The caudal ends of the chorda dorsalis and caudal gut also merge with this cell-mass. In the 5.5 to 7.5 mm. embryos the caudal end of the neural tube, with its central canal, extends to the apex of the tail and merges into the mesodermal cell-mass, entirely losing its boundaries. In the 8 nun. specimen a difference can be plainly recognized between the caudal portion of the spinal cord and the portion that Ues cranial to the thirty-second somite. Caudal to this level the central canal is distinctly narrower. Thus it may be divided into two portions, an upper, wider canal and a caudal narrow or atrophic canal. The former constitutes the main part of the central canal of the spinal cord. On cross-section it is oval in shape and its walls show no folds. The caudal part is narrower in its dorso-ventral diameter than is the main canal, and therefore on section presents a more rounded form. Sometimes a large fold is found between the two parts and in the 11 and 12 nun. specimens can be seen the primordia of other folds on the walls of the atrophic canal, especially on the -entral side, as shown in figiures 34 and 35. The distinction between the atrophic portion of the spinal cord and the main part is quite marked in the 15.5, 16, 17, and 19 mm. embryos. The caudal end of the wider canal expands transversely, and where it narrows into the atrophic canal constitutes the initial form of the ventriculus terminalis. The portion of the spinal and uliicli incloses the ventriculus is the primitive conus inedullaris.


The ventriculus terininalis was found by Argutinsky in a 45 inm. embryo; by Brugsch and Unger in a 25 mm. embryo, and what may be considered as its primordium is already apparent in my specimens of 11 and 12 mm. respectively, as shown in figures 34, 35, and 36. It can be seen from stage to stage retreating cranialward, while the atropine canal gradually lengthens. In the 18, 23, and 25 mm. embryos the latter expands noticeably in the median or caudal region, as shown in figures 39, 40, and 42. This was also observed by Unger and Brugsch, who, however, did not regard it as the primordium of the ventriculus terminalis, but rather as a homologue of the sinus terminalis of the amphibians, which develops at the caudal end of the central canal of the spinal cord. In the majority of my specimens from 18 to 30 mm. the caudal end of the atrophic canal shows diverticula such as those described by Unger and Brugsch. In such embryos the spinal cord becomes


temporarily longer than the vertebral column. It seems probable, therefore, that in the wall of the atrophic portion of the cord the ependymal cells increase rapidly by proliferation, and perhaps also by the migration of other ependymal cells from the more caudal part of the tail, which is in process of regression. By reason of these two processes folds develop in this waU, such as are well shown in text-figure 1. In these embryos the caudal end of the chorda dorsalis seems to exert an attraction upon the caudal end of the medullary tube, thus drawing it into a more cranial position (fig. 39). In embryos of 30, 33, 35, and 37 mm the atrophic canal is longer and narrower than in the slightly younger specimens, but the caudal end is still dilated. At several points the canal has become so narrowed that its central cavity is obliterated and gradually becomes converted into a cell-strand, as shown in figure 44.


'''Text-Figure 1.''' Sagittal section through caudal end of a human embryo 18 mm long (Carnegie Collection, No. {{CE432}} slide 22, row 1, sec. 3), enlarged 50 diameters. The wall of the atrophic portion of the .spinal cord is thinner and represents a much younger stage of development than that of the main part of spinal cord. From this time on it undergoes gradual regressive changes with obliteration of the central canal and is eventually replaced by a simple fibrous strand.


At the stage where the embryo has entirely lost its external tail the sjMual cord is about the same length as the vertebral column, as shown clearly in the 30 antl 37 mm. specimens. From this time on the vertebral column increases relatively in length, although there is no cessation of growth of the spinal cord. In the 37 mm. embryo a bundle of nerve-fibers {i. e., marginal zone) is visible on the ventral side of the atrophic cord, as shown in figure 44 (fil. t.), and represents a primitive filum terminale. This structure extends caudalward from the apex of the primitive conns medullaris at a level between the twenty-ninth and thirtieth vertebrae. In the 33 mm. embryo the cranial portion of the atrophic canal and its ependymal lining disappear and the bundle of nerve-fibers remains in the sheath of the dura mater. The dilated portion of the atrophic canal remains as the coccygeal vestige. The process of dedifferentiation of the caudal end of the spinal cord, viz, the reduction of the cranial end of the atrophic canal, advances step by step, so that what appeared as a long, narrow tube in the 37 mm. embryo is divided into several parts in the 50 mm. specimen, each part containing a cavity, the more cranially situated part being joined to the ventriculus terminalis by a cellstrand which is destined also to disappear in the course of development (figs. 44, 45, and 46). The ventriculus terminalis, which had only begun in the 37 mm. specimen, has developed completely in one of 50 mm. Its formation is the result of the gradual constricting of the expanded area of the central canal which marks the division between its upper wider and lower atrophic portions, and a separate cavity is thus formed. Complete fusion of the margins, however, does not occur, a narrow channel being left which connects the two portions of the central canal. In their thesis Brugsch and Unger have described in detail this process of reduction of the central canal. In my specimens the phenomenon is first noted in the 39 mm. embryo. In the 39,, 46, 50. and 52 mm. specimens the conus medullaris, ventriculus terminalis, and filum terminale are quite distinct, and the remnant of the neural tube caudal to the filum terminale persists as the coccygeal medullary vestige. The form of the ventriculus terminalis varies in the different specimens, while that of the conus meduUaris is much the same in all. In the specimens above enumerated the ventriculus terminalis is situated at about the level of the twenty-ninth or thirtieth vertebra, the position of both it and the conus medullaris gradually becoming more cranial as the result of the fact that the growth of the vertebral column becomes progressively more rapid than that of the spinal cord. In these embryos, especially the 46 mm., specimen, the membranes of the spinal cord - the dura mater and pia mater - are visible. At a level with the upper border of the thirty-fnst vertebra, in the 46 mm. embryo, the dura mater may be seen to leave the wall of the vertebral canal for the caudal end of the conus medullaris, which marks the beginning of the filum terminale, thus forming a sheath for the latter. As a rule, the coccygeal meduUary vestige is on the dorsal side of the last two vertebrae. It is situated in the connective-tissue surrounding the vertebrae and does not adhere to the epidermis. Tourneux and Hermann discovered the caudal remnant of the spinal cord in a 37 nma. embryo and termed it the vestiges medullaires coccygiens. Tourneux advanced the theory that the sUghtly enlarged caudal tip of the neural tube is closely united in the deep layers of the skin. Toward the end of the third month the spinal column, developing more rapidly than the soft parts, draws along the part of the neural tube adherent to it, the extreme tip of which remains attached to the skin. As a result of this the terminal ojr coccygeal portion of the neural tube becomes bent in the form of a loop, the more deeply situated Umb being termed segment coccygien direct, and the more superficial one segment coccygien reflcche. In my specimens I did not find such to be the case, nor was it noted by Unger and Brugsch. I believe, therefore, that the condition noted by Tourneux is of rare occurrence. In the 39 mm. embryo may be seen a small papiUiform tail, at the root of which is a group of cells representing the remnant of the spinal cord. The caudal "end of the coccygeal medullary vestige appears to adhere to the epidermis, but in reality does not, although Tourneux and Hermann found that it did adhere in their case.


Concerning the development of the coccygeal medullary vestige from the remnant of the neural tube, I am led to the following conclusions :

# The expanded caudal end of the neural tube in an embryo in which the tail has disappeared is the primordium of the coccygeal vestige (figs. 40, 42, and 43).
# In addition to the coccygeal vestige there frequently occurs a similarly formed epithelial sac situated in a more cranial position.
# The caudal end of the coccygeal vestige merges into the caudal ligament, as believed by Brugsch.
# In the younger specimens the fibers which persist as the filum terminale always lie ventral to the ependymal cells, which become the coccygeal vestige.
# The middle sacral artery and vein extend to and curve around the apex of the coccygeal vestige.
# Only in rare cases is the coccygeal vestige lacking. In my entire series of specimens, from 4 to 125 nam., in only one did I actually fail to find it (fig. 24).
# In specimens from 4 to 100 mm. the coccygeal vestige is not adherent to the epidermal layer of the skin.
# The coccygeal vestige continues to grow after the 100 mm. stage.


In the 67 mm. embrjo, as can be seen in figure 46, the ventriculus terminalis occupies a more cranial position than in the younger specimens; the conus medullaris has become relatively more slender and the filum terminale longer, the latter disappearing caudal to the thirty-second vertebra, two remanants of the neural tube being left. The membranes of the spinal cord are here also considerably further developed than in the younger specimens. At this stage the arachnoid membrane hes between the dura mater and pia mater. At the upper border of the twenty-seventh vertebra the dura mater leaves the wall of the vertebral canal for the filum terminale, forming a .sheath and reaching the filum terminale at a level between the twenty-eighth and twenty-ninth vertebra\ In younger specimens, for example in the 46 mm. embryo, this separation occurs at a level with the thirty-first vertebra. Therefore, the caudal end of the dural sac, as well as the spinal cord, recedes cranialward. This ])henomenon is an evidence of the relatively more rai)id growth of the vertebral column. What, then, is the cause f)f the lengthening of the filum terminale? In the 33 nun. embryo I could recognize distinctly, below the conus medullaris, a bundle of fibers rejjresenting a jmmitive filum terminale. In the 37, 39, and 50 mm. embryos this reaches almost to the caudal end of the neural tube or the coccygeal medullary vestige. In the 37 and 39 mm. sj)ecimens it extends farther caudalward than in any of the others (fig. 44). It is my opinion, therefore, that the filum terminale consists at an early stage of nerve-fibers, especially those from the ventral portion of the spinal cord, although von Kolliker does not hold this view. After the development of the ventriculus terminalis the caudal portion of the conus medullaris is converted into the filum terminale by the ventriculus terminalis and conus medullaris movdng cranialward. This is due to the fact that the gray substance which Ues next to the ventriculus terminalis is undergoing degeneration and the caudal end of the central canal is gradually excavated, while the caudal end of the ventriculus terminalis narrows by degrees, losing its cellular substance. The relative lengthening of the filum terminale, therefore, is due to the growth of the nerve-fibers with their sheath of dura mater and pia mater, and in part also to the gradual addition of tissue from the caudal portion of the conus medullaris, which has become converted into the tissue of the filum terminale.

==Abnormalities of the Caudal End of the Spinal Cord==

(a) Embryo No. {{CE405}}, 26 mm.; {b) embryo No. {{CE145}}, 33 mm.; (c) embryo No. {{CE449}}, 36 mm.

In the first two specimens the caudal tip of the neural tube is spiral. It is probable that this part is covered with a layer of epidermis, although I could not discover it and therefore conclude that it was injured in the preparations. In the first specimen the caudal end of the spinal cord enters into the tail-bud. Two branches of the anterior spinal artery penetrate between the coils of the spinal cord. In the second specimen the coil of the caudal end of the spinal cord forms the summit of the papillary tail and terminates at the ventral side of its root. The third specimen has only 32 vertebrae and no remnant of the neural tube.

==Spinal Ganglia==

It is very difficult to locate the first cervical ganglion at a very early stage of embryonic development, particularly if the specimen is poorly preserved. This structure is frequently found in close apposition to the Froriep ganglion on the trunk of the spinal accessory nerve. Sometimes it is poorly developed and resembles a Froriep ganglion, except for the fact that it lies always on the ventral side of the trunk of the accessory nerve, while the Froriep ganglion lies on the dorsal side. The first cervical is smaller than the others, and the second in turn is smaller than the third. In embryo No. 991 (17 mm.) both first cervical gangUa are lacking. In embryos from 5 to 10 mm. there are in most cases 32 pairs of ganglia; from 12 to 14 mm. there may be 33 pairs. When the number is 33 the last caudal ganglion is usually very small and has no nerve. In embryos from 15 to 33 mm. the number is usually 31. I have frequently formed the thirty-second spinal nerve without a ganglion, the latter having degenerated. In embryos from 35 to 67 mm. long, and older, there are usually only 30 ganglia; occasionally there may be 31, but the last is usually undergoing degeneration.

==Sympathetic Ganglia==

In embryos 33 mm. long, and older, the caudal ends of the paired sympathetic ganglionated nerve-trunks join together at the upper plane of the ventral side of the thirtieth vertebra, as shown in text-figure 2. At the point of union there is usually found a ganglion; another occur.s apijroxiniately at a point between the thirty-first and thirty-second vertebra. From the latter ganglion the single nerve trunk follows the course of the middle sacral artery and vein, running between them, and emerges dorsally from the caudal end of the vertebral cohmin, where the coccygeal medullary vestige and caudal ligament curve about the apex of the column. This nerve consists of a large bundle of non-medullated nervefibers, but the structure of its caudal end can not at this stage be made out distinctly. At a level between the thirty-third and thirty-fourth vertebra?, or perhaps a little above, there is a small group of cells representing a sympathetic ganglion. At this point are frequently found numerous plexiform branches of blood-vessels enmeshing this group of cells. This richly vascularized cell-group may be the primordium of the glandula sacralis.



'''Text-Figure 2.''' Ventral view nf caudal i>nrtioii of vertebral column, showingfharacteristic arrangement of sympathetic ganglion-strands in human embryos between 46 and 67 mm. long. Cart, inlv., intervertebral fibrocartilage; Gang. symp., ganglionic node; Virl., body of vertebra.


==Summary==

# The human embryo possesses a true tail composed of primitive vertebra? and the caudal ends of the spinal cord, chorda dorsalis, and middle sacral artery and vein.
# The longest and most completely developed tail among the specimens examined by me was found in a 7.5 mm embryo. This was 1.2 mm in length.
# The human embryo does not possess a caudal filament homologous with that of other mammals.
# The reduction of the tail, especially of the primitive vertebrae, begins when the embryo has reached a length of about 8 or 9 mm.
# Prior to this the tail consists of two parts : a proximal longer part (the vertebrated tail), which has well-formed somites, and a caudal shorter part which contains only a mesodermie remnant.
# In embryos from 25 to 27 mm the tail is reduced to a small papilla, in which are contained the caudal ends of the spinal cord and the middle sacral artery and vein, and into which the end of the caudal ligament enters. As a rule this tail-bud is not situated directly at the caudal extremity of the vertebral column, but slightly dorsal to it. The vertebrated portion of the external tail has retracted into the soft tissues and has thus become an intoi-nal tail, whereas the lost-vertebra tail projects temporarily and finally it also disappears.
# At the time the division of the external tail takes place two eminences appear at the caudal region; one ventral (coccygeal tubercle or Steisshcicker) . the other dorsal (caudal tubercle or Kaudalhocker) . The first is due to the pushing up of the caudal end of the internal-tail vertebral formerly situated in the root of the external tail and constituting the vertebral portion of it in vounger embryos. The second is usually shaped like a small papilla and by some "authors is called the tail-bud or caudal filament, although the latter, as stated above, is entirely a misnomer. Sometimes it appears as a rounded eminence and was therefore termed caudal tubercle by Unger and Brugsch.
# The tail-Uke appendage that occasionally persists in adults may possibly be explained as a persistent caudal tubercle that did not undergo the normal reduction. It must be granted, however, that in none of the cases reported has osseous or cartilaginous tissue been found.
# When the embryo reaches 30 to 35 mm the tail has usually entirely disappeared, although the time of disappearance is quite variable. Thus, the tail was found to have disappeared in the 24 mm embryo, while in one 39 mm long it still persisted.
# In embryos above 40 mm in length that have lost the external tail I have designated as the internal tail the portion caudal to the thirtieth vertebra, for three reasons: (a) the curve of the vertebral column occurs at the thirtyfirst and thirty-second vertebrae; (b) below the twenty-ninth vertebra the spinal ganglia disappear at about the same time as the external tail; (c) the sympathetic ganglion strands unite between the thirtieth and thirty-first vertebrae.
# The disappearance of the canal of the caudal gut had already begun in a 5.5 mm embryo and in a 6.5 mm specimen the caudal gut had become converted into a long ceil-strand, except for a short caudal portion. In embryos 7.5, 8, and 9 mm. the remnant of the caudal gut, inclosing a small cavity, was still found in the end of the tail. In those 10 mm. and older the caudal gut had entirely disappeared.
# In the very youngest specimens the medullary tube reaches to the extreme tip of the tail.
# In those 11, 12, and 15.5 mm long the medullary tube can be divided into two parts at the level of the thirty-second vertebra: a cranial part, having a wide central canal, and an atrophic caudal part with a narrow canal. This distinction becomes quite marked in the 15.5 mm. specimen. The canal at the junction of these two parts is slightly enlarged transversely and constitutes the primordium of the ventriculus terminalis.
# The atrophic portion of the spinal cord gradually becomes more slender, although its caudal end remains unchanged for some time or in some instances shows temporary enlargement. It later subdivides, the cranial end forming the cell-strand of the filum terminale and the caudal end developing mto the coccygeal medullary vestige.
# The caudal end of the wider part of the spinal cord develops into the conus meduUaris and its lumen constitutes the ventriculus terminalis.
# In the 46 mm. embryo the ventriculus terminalis is perfectly developed.
# The conus medullaris and the ventriculus terminalis recede cranialward as the result of two processes: (a) the growth of the vertebral column, which is more rapid than that of the spinal cord; (b) the degeneration of the gray substance which forms the upper wall of the ventriculus, thus causmg the cavity to enlarge and gradually move upward while its caudal end narrows.
# The extent of the coiling of the chorda dorsaUs in its various stages indicates the extent of fusion of the last primitive vertebrae.
# In embryos from 12 to 14 mm the spinal ganglia are 33 in number, Init at about this period reduction begins in the more caudally situated ones. Thus in an embryo of 67 mm there are but 29 ganglia.


In conclusion, I take this opportunity to acknowledge my indebtedness to the late [[Embryology_History_-_Franklin_Mall|Professor Franklin P. Mall]] for the privilege of using the valuable material in the collection of human embryos belonging to the Carnegie Institution of Washington. I also wish to thank [[Embryology History - George Streeter|Dr. George L. Streeter]] for his kind assistance in the preparation of this paper.


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{{Ref-Bardeen1905}}

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==Description of Plates==

(The description of Plate 1 will be found on the plate itself.)

The figures on Plates 2 to 4 represent profile reconstructions of the caudal region in a series of human embryos selected from the [[Carnegie Collection]]. The different structures are indicated by the following abbreviations:

{{Kunitomo1920 abbreviations}}

===Plate 1===

The figures on Plate 1 are designed to show diagrammatically the relations of the caudal end of the spinal cord and the vertebral column in a series of human embryos varying from 4 mm to 67 mm long, and are so arranged that the segmental levels, as indicated at the left, correspond throughout, the thirty-fourth segment being emphasised by a heavy line. In the two youiiger stages the segments were determined by the myotomes; the remainder were determined by the sclerotomes, or the bodies of the vertebra. In making the diagram it was found necessary to make all the segments of the same width; in reality the more caudal ones are relatively much narrower. Also, the individual segments become wider in the older stages, whereas in the diagram they are kept at the same width. There is thus introduced an axial distortion which should be kept in mmd in studying the figures. In figures 1 to 15 the surface profile of the caudal region is indicated and the cloacal membrane la shown by a wider line. In figures 1 to 5 the caudal gut is shown by a broken line. In figures 4 and 5 it will be noted that a remnant of the gut is still present, though its communication with the cloaca is interrupted. Early stages in the foimation of the filum terminale are shown in figures 25 to 30. The figures in this plate are all based upon profile reconstructions made from the following embryos:


===Plate 2===

Fig. 31. Embryo No. {{CE810}}, 5.5 mm enlarged 31.5 diameters. The caudal gut already shows a constriction separating it from the cloaca. The lines along the dorsal margin of the spinal cord represent the boundaries of the myotomes.

Fig. 32. Embryo No. {{CE371}}, 6.6 mm enlarged 31 diameters. The segmental levels in this specimen are determined by the sclerotomes. It will be noted that the tail has attained nearly its maximum development, and as compared with the more cranial parts it will hereafter gradually take on a more atrophic appearance.

Fig. 33. Embryo No. {{CE389}}, 8 mm enlarged 31..5 diameters. The coccygeal portion of the spinal cord is already distinctly narrower than the main cord.

Fig. 34. embryo No. {{CE544}}, 11 mm enlarged 31.5 diameters. In this specimen the caudal gut has disappeared. The vertebrated and non-vert ebrated portions of the tail are clearly demarcated.

Fig. 3.5. Embryo No. {{CE852}}, 12 mm enlarged 31.5 diameters. The non-vertebratcd portion of the tail is here relatively much shorter than in the previous specimen.

Fig. 36. Embryo No. {{CE390}}. 15.5 mm enlarged 31.5 diameters. A rudiment of the tail persists as a small elevation (rud. cau.). The caudal end of the vertebral column terminates in a fibrous strand of cells. The terminal three sclerotomes may be regarded as having been converted into this strand, or they may have fused into one irregular vertebra.

===Plate 3===

Fig. 37. Embryo No. {{CE406}}, 16 mm crown-rump length, enlarged 31.5 diameters. The slender atrophic portion of the spinal cord is clearly demarcated from the remainder of the cord owing to the fact that it retains its earlier embryonic form. The point at which its narrow canal opens into the main canal corresponds to the future terminal ventricle.

Fig. 38. Embryo No. {{CE576}}, 17 mm crown-rump length, enlarged 22.5 diameters. Is compared with the last specimen, the caudal region has undergone marked reduction and resembles the condition that will be seen in embryos about 20 mm. long.

Fig. 39. Embryo No. {{CE432}}, 18 mm crown-rump length, enlarged 22.5 diameters. It will be noted that embryos of about this size show the tendency toward a sharp dorsal retroflexion of the caudal rudiment. The characteristic thinness and wrinkling of the wall of the atrophic portion of the spinal cord is also well represented in this embryo.

Fig. 40. embryo No. {{CE453}}, 23 mm crown-rump length, enlarged 22.5 diameters. In this specimen, just ventral to the junction of the atrophic part with the remainder of the cord, is a mass of cells which appeared to form a diverticulum, although a communication between its lumen and the central canal of the cord could not be clearly made out.

Fig. 41. embryo {{CE382}}, 23 mm crown-rump length, enlarged 22.5 diameters. The relative narrowness of the lumen of the atrophic portion of the spinal cord is a characteristic preliminary to its transition into the filum terminale. Xear the caudal tip, at the point marked X, is the seat of fusion with the chorda dorsalis.

Fig. 42. Embryo No. {{CE584a}}, 25 mm crown-rump length, enlarged 22.5 diameters. A point of fusion with the chorda dorsalis, marked X, can be seen in this specimen somewhat similar to that in figure 41.

===Plate 4===

Fig. 43. Embryo No. {{CE75}}, 30 mm ciown-rump length, enlarged  22. diameters. The relative thinning-out of the walls of the conus meduUaris results in a tendency to their being thrown into large irregular folds. In (he re;iion of the conus medullaris the regressive tendency sets in after that region has attainrd proportion.s larger than those seen in the more caudal part. It consequently becomes copressed by a thinness of the walls, producing a transparent terminal ventricle in contrast to the obliteratinn seen in the filum terminale.

Fig. 44. Embryo No. {{CE972}}, 37 mm crown-rump length, enlarged 18 diameters. This specimen shows the transition of the atrophic spinal cord into a fibrous filum terminale. The terminal portion retains its lumen and persists as the coccygeal medullary vestige

Fig. 45. Embryo No. {{CE448}}, 52 mm crown-rump length, enlarged 13.5 diameters. At this time the terminal ventricle, the filum terminale, and the coccygeal medullary vestige are distinctly marked off from each other, and their general adult characteristics attained.

Fig. 46. Embryo No. 16.5t), 17 mm. crown-rump length, enlarged 9 diameters. The regressive condition of the walls of the terminal ventricle are expressed by their relative thinness and their irregularity. The filum terminale is almost entirely converted into a solid fibrous strand in which trace.s of ependymal masses can be found. The membranes of cord can be seen and present an arrangement that closely simulates that of the adult.


{{Historic Disclaimer}}


{{Glossary}}

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[[Category:Historic Embryology]] [[Category:Human]][[Category:Axial Skeleton]][[Category:Neural]][[Category:1920's]][[Category:Draft]]
[[Category:Carnegie Collection]]
[[Category:Carnegie Embryo 1008]]
[[Category:Carnegie Embryo 1197]]
[[Category:Carnegie Embryo 142]]
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[[Category:Carnegie Embryo 1656]]
[[Category:Carnegie Embryo 184]]
[[Category:Carnegie Embryo 199]]
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[[Category:Carnegie Embryo 371]]
[[Category:Carnegie Embryo 382]]
[[Category:Carnegie Embryo 389]]
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[[Category:Carnegie Embryo 405]]
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[[Category:Carnegie Embryo 422]]
[[Category:Carnegie Embryo 43]]
[[Category:Carnegie Embryo 431]]
[[Category:Carnegie Embryo 432]]
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[[Category:Carnegie Embryo 453]]
[[Category:Carnegie Embryo 485]]
[[Category:Carnegie Embryo 544]]
[[Category:Carnegie Embryo 576]]
[[Category:Carnegie Embryo 584a]]
[[Category:Carnegie Embryo 632]]
[[Category:Carnegie Embryo 643]]
[[Category:Carnegie Embryo 662]]
[[Category:Carnegie Embryo 721]]
[[Category:Carnegie Embryo 75]]
[[Category:Carnegie Embryo 786]]
[[Category:Carnegie Embryo 810]]
[[Category:Carnegie Embryo 836]]
[[Category:Carnegie Embryo 837]]
[[Category:Carnegie Embryo 852]]
[[Category:Carnegie Embryo 875]]
[[Category:Carnegie Embryo 928]]
[[Category:Carnegie Embryo 940]]
[[Category:Carnegie Embryo 95]]
[[Category:Carnegie Embryo 96]]
[[Category:Carnegie Embryo 972]]
[[Category:Carnegie Embryo 991]]