Paper - The embryological development of the commissura posterior in the human spinal cord

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Hogg ID. The embryological development of the commissura posterior in the human spinal cord. (1945) J Comp. Neural. : 255-281.

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This historic paper published in 1945 by Hogg describes the development of the spinal cord. Our understanding of spinal cord development development has improved since this historic human study.

Also by this author: Hogg ID. Sensory nerves and associated structures in the skin of human fetuses of 8 to 14 weeks of menstrual age correlated with functional capability. (1941) J. Comp. Neural.

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The Embryological Development Of The Commissura Posterior In The Human Spinal Cord

Ira Dwight Hogg

Department Of Anatomy, School Of Medicine, University Of Pittsburgh, Pennsylvania

Snvnn Memes


During recent studies on the development of the nucleus dorsalis and the course of the axons from its cells the fibers which comprise the dorsal gray commissure were studied carefully because a number of investigators have declared that some of them come from the nucleus dorsalis. Oneeof these investigators (Valenza, 1897) used the Golgi method of silver impregnation and another (Pass, ’33) used the Marchi method of staining degenerated myelin sheaths after producing experimental lesions in the nucleus dorsalis.

During the course of the present investigation no fibers were discovered which arose from cells of the nucleus dorsalis and passed through the commissura posterior in human material but the steps in its formation and the sources of its component fibers were very clearly revealed.

The thirty—three fetuses studied were the same as those used in the study of the nucleus dorsalis (Hogg, "44, table 1, p. 74). The silver methods used were those recommended by Ranson (’11) and ‘Bodian (’36). A portion of the commissure probably was present in each of these fetuses although there is a question as to whether the decussating fibers in the very youngest members of the series belong to neuroblasts or spongioblasts. The fact that no particular change can be observed in the fibers from the earliest stages until some of the cells from which they arise are definitely neurons makes it seem highly probable that the first fibers are from neuroblasts, oriindifferent cells which soon can be classed as neuroblasts. The reason for doubt as to their nature arises from the fact that in the protargol series spongioblasts and ependymal cells and their processes often are as heavily impregnated with the silver as are those of the neuroblasts. This method, when it works well, tends to become.a universal stain which is excellent for certain cytological details but leaves the differentiation of types of cells to the keenness of observation and judgment of the observer.

1 Publication no. 14, Physiological and morphological studies on human prenatal development. Aided by grants to Davenport Hooker from the Penrose Fund of the American Philosophical Society, from the Carnegie Corporation of New York, and from the University of Pittsburgh.

In fetuses as young as those in which the commissura posterior begins to appear all stages in the differentiation of neuroblasts and spongioblasts are present. With the protargol method nuclear changes seem to be the most easy to recognize and there, are many shades of dilference between stages which cannot be recognized as difierent by the unpractised eye. In this investigation impregnated fibers in the commissure were not considered to be axons necessarily unless they could be traced from a cell which the author considered to be definitely neuroblastic in form.


The idea of the cellular structure of tissues had been making considerable progress during the half century ending -in 1838 when Schleiden and Schwann attracted the attention of scientists to it (Conklin, ’40 a, ’40 b). At that time Stilling (1842) was working on the physiology of the nervous system and in" order to correlate more accurately the functions of the spinal cord with its structure he and Wallach (Stilling and Wallach, 1842) prepared a description of its finer anatomy.

Considering the methods that they had to employ and the diificulties

of technique they had to solve, they gave a remarkably accurate de scription of the microscopic structure of the spinal cord. In this article they discuss the structure of the commissures on each side of the centra1_canal. They recognized the fibrous nature of both and state that fibers which pass through the dorsal commissure are distributed

to all parts of the gray matter on each side and can be followed almost‘

to the white matter. At that time they had no proof that there were any real connections between fibers of the gray matter and those of the whi.te matter but considered the cord to be composed of two systems of fibers, an outer longitudinal and _an inner transverse, with loops, continuous from the dorsal to the ventral roots, passing through the gray matter. They looked upon the transverse fibers of the gray matter as being a supporting element. They saw thecells in the gray matter but had not become familiar with the term or at least make no reference to the literature on that subject and do not use the term “cell”. In many respects their observations on the commissura posterior were not improved upon until after the discovery of the silver methods of staining nerve tissue had been applied to it 50 years later. They had not differentiated between myelinated and unmyelinated fibers as such but evidently saw both types.

Clarke (1851), in the same article in which he first described the nucleus Vdorsalis, described the cornmissura posterior briefly but he had not made as careful a study of it as Stilling had made in 1842. He considered it to consist of a continuation of the substantia gelatinosa from one side to the other and that all of the fibers‘that were visible around the central canal were part of the supporting tissue for the epithelium which lined it and that there were no commissural fibers in the spinal cord. V

Stilling (1859) published a monumental work on the structure of the spinal cord. This was accompanied by a large atlas of the cord in which every detail that could be recognized was illustrated. By that time the idea of cellular structure of tissues had become known to both Stlling and Clarke and the nerve cells were recognized as such although, because of the low powers of magnification and primitive methods of technique, the -idea of the syncytial nature of cells was firmly fixed in their minds.

Stilling (1859) and Clarke (1859) studied the cord in unstained sections and with such simple aids as acetic acid and iodine. The interpretation of some of their observations depended on the development of other histological techniques.

Kutschin (1863) described a commissura posterior in the spinal cord

of the river lamprey. According to him the fibers forming it arise from '1

small cells in the dorsal part of the gray matter and run obliquely through the commissure to a point near the entering dorsal root of the opposite side where they turn and are lost among the longitudinal fibers. This description of the dorsal commissure is one of the best that have been found prior to the works of Ramon y Cajal (1890) and van Grehuchten (1891).

The use of metallic impregnation for crude histological methods had been introduced soon after the microscope came into general use but prior to 1870 they had not been used extensively. A review of the development of the use of silver nitrate as a neurological stain was ‘made by Stadtmiiller (’21).

Palladium chloride and gold chloride were used by Schiefferdecker (1874). Apparently he was able to follow fibers in the nervous system fairly well but his descriptions indicate a lack of ability to recognize the cells from which the fibers originated. In discussing the fibers in the commissura posterior he states that some of them come from the medial part of the posterior funiculus and that some come from cells which lie in the medial portion of the dorsal group of ganglion cells. The author interprets this statement to mean cells of the dorsal cornu.

Between 1870 and 1890 new histological methods appeared frequently. Rudolph Arndt (1875) used indigo, carmine, aniline, haematoxylin, iodine, gold chloride, palladium chloride, silver nitrate, and osmic acid as stains while Fischer ( 1876) introduced the newly synthesized eosin and Unna (1876) described the use of picro—carmine. Denissenko (1877) advocated the use of the combination of haematoxylin and eosin. Vlleigert (1884, 1885) discovered a method of staining myelin sheaths with haematoxylin. For some years thereafter most of the neurological invesigators chose this method or some modification of it, particularly one introduced by Pal (1886). Since then several other modifications of this method have appeared (Lee, ’21, pp. 442-448). Marchi (1886) introduced his method for staining degenerated myelin sheaths with osmic acid and since then many investigators have almost ignored the unmyelinated fibers which these methods do not stain. Because most of the fibers of the gray commissure of the spinal cord are unmyelinated, investigators, who confined their observations to material prepared by the methods mentioned above, paid very little attention to its structure.

Henle (1879) recognized the presence of both myelinated and unmyelinated fibers in the commissura posterior. He describes the commissure in the adult cord as being thicker in the cervical and lumbar enlargements than it is in the thoracic region.

Waldeyer (1889) made a comparative study of the spinal cord of the gorilla and man. He confined his investigation to a study of the groups of cells in the cords and the distribution of the myelinated fibers. He

(described the commissura posterior as being of about the same size

throughout the whole length of the spinal cord‘. According to his observation the myelinated fibers in it arise from the posterior funiculus and are lost in the gray matter of the opposite side.

Fortunately a small group of investigators, consisting at first chiefly of Golgi, Ramon y Cajal, and van Gehuchten, continued to use and improve the use of the more erratic silver nitrate which did reveal the unmyelinated fibers.

Ramon y Cajal (1890) described the formation of the dorsal commissure in the chick as occurring on the tenth day of incubation. At that time it is composed of a dorsal and a ventral portion, the former being collaterals from the posterior funiculi and the latter being collaterals from the lateral and ventral funiculi. DEVELOPMENT or COMMISSURA POSTERIOR 259

At about the same time he discovered that the commissura posterior in a. newborn dog was composed of three fascicles, a dorsal_one composed of collaterals from the posterior funiculus, a middle component composed of fibers which traverse the nucleus dorsalis and end in the plexus in the cervix of the posterior cornu, and an anterior component which lies close to the central canal. The middle component was recognized as being formed in part by collaterals from the lateral funiculus and in part from a doubtful origin. _The origin of fibers of the anterior component was said to be uncertain. Later (’09) he corroborates his earlier discoveries and adds some of his later observations concerning the termination of collaterals from the funiculi which form a part of it and the location of cells in the gray matter which send their axons into

it. He also discovered a commissure of dendrites which is a part of the dorsal commissure.

Van Gehuchten (1891) found similar components in the thoracic portion of the spinal cord of a 55-cm. calf embryo. He corroborated the findings of Ramon y Cajal in general but declared that the middle component arose in part from the lateral funiculus and in part from that part of the posterior funiculus called the Randzone by Kiilliker or what is. now generally known as the fasciculus dorsolateralis or fascicu— his of Lissauer.

Hatschek (1896) made a comparative study of the spinal cords in the seal and dog. He seems to have been interested in the comparative size of the dorsal commissure rather than in its composition or the distribution of its fibers. He recognized two types of fibers, one of which was thin and myelinated and followed the edge of the posterior: funiculus and one of which lay a little nearer the central canal and was lost in the middle of the gray matter of the posterior cornu.

Kiilliker (1896) states that in the newborn cat there is a distinct dorsal commissure and that if one examines it carefully he will recognize that the fibers which form it arise from the border of the gelatinous substance and include other elements which disappear in the neuropil at the border between the substantia gelatinosa and spongiosa. In a 4-day dog he found a dorsal commissure in the thoracic portion of the spinal cord. From his description this consisted chiefly of the dorsal component with possibly some fibers of the middle component. He did not recognize the ventral component. In this animal he could not find any commissure in the lumbar segments of the cord and very few commissural fibers in the cervical region although he found it well developed at the beginning of the decussation of the pyramids.

Bechterew (1899) describes the commissura posterior as appearing in three fascicles in Golgi preparations but notes that in man it is relatively much more Weakly developed than in other animals.

Schacherl (’02) found in exceptional cases in man that the nucleus

dorsalis more or less invaded the commissura posterior but, in his ex perience, never completely.

Van Grehuchten (’06) does not describe the gray commissure in great detail but barely mentions it as being separated by the central canal into dorsal and ventral portions.

Biach (’07) found that fibers of the commissura posterior pass on the dorsal side of the nucleus dorsalis in the antelope and among them small stellate cells are scattered which are similar to those in the gray matter of the oolumna posterior. From his description theylseem to correspond to the nucleus cornucommissuralis posterior of Marie (1894), Kuntz (’42), which is also described as the tractus cellularum mediodorsalis by J acobsohn (’08), zona medio-dorsale del cordone posteriore by Massazza ( ’22, ’24), and medioposterior cell column by Larsell (’42_). Biach states that some commissural fibers separate this nucleus from the posterior funiculus and that it is separated from Clarke ’s column by coarse bundles of fibers from the posterior roots, some of which seem to cross in the commissura posterior.

Medially, nearer the raphe and close to the central canal, he observed another nucleus of small cells which lie in the broadened commissure. His description corresponds to the position of a group of cells described by Ramon y Cajal (’O9) as the nucleus of the intermediate gray substance (Larsell, ’42).

Edinger .(’08, ’11) states that myelinated fibers from the posterior funiculus cross through the commissura posterior to terminate in the gray matter of the opposite side and that in fetuses its development varies in difierent animals being found to consist of three parts in the

dog, two in the calf, etc. He also states that the strength of the com- ,

missure depends on the size of the entering dorsal root and the nearness of the section to it.

Gregenbauer (’10) states that the dorsal gray commissure is best developed in the lumbar region and is weakest in the thoracic portion of the spinal cord. M l ‘

Obersteiner (’12) recognized" the presence of myelinated and unmyelinated fibers in the commissura posterior. He states that while the small white fibers of the dorsal commissure are probably not directly from the posterior roots, still this does not exclude the probability that they form a subordinate secondary path which may give greater meanDEVELOPMENT or COMMISSURA POSTERIOR 261

ing to the commissure. He further states that in the guinea pig there is- a strong dorsal bundle of fibers, a less developed middle component which seems to arise from the lateral funiculus at the base of the columna posterior and a very feebly developed ventral component which lies close to the central canal. .

Pass (’33) describes fibers belonging to the dorsal and ventral spinocerebellar tracts of the cat which decussate through the commissura posterior. .

The recently published text-books of neuroanatomy place very little stress on the presence of the gray commissure of the spinal cord. Most of them mention it as being divided into dorsal and ventral portions by

_ the central canal. Keiller (’27) states that it is composed chiefly of

unmyelinated fibers among which fine myelinated "fibers course. Herrick (’31), Mettler (’42), Larsell (’42), Kuntz ( ’42), Ranson (’43), and Strong and Elwin (’43) mention the presence of a gray commissure which lies on both sides of the central canal but do not attempt to analyze it.


In the study of the development of the nucleus dorsalis (I-Iogg, ’44) the commissura posterior was observed to develop throughout the thoracic region almost concomitantly with it. In that article the figures show the state of development of the commissure as it was observed in each fetus from which an illustration was made, although very little reference is made to that fact. A '

In the youngest fetuses in which a commissura posterior has been found it exists only in the thoracic region‘ and begins at the level of the second thoracic segment. Inhfetus no. 18 it is beginning to develop in the second and third thoracic segments. Figure 1 was made from one of two sections, both in the second thoracic segment, which showed the commissural fibers well enough to be used for an illustration. Apparently the commissure develops throughout the thoracic region almost simultaneously for in all of therest of our fetuses of approximately the age of no. 18 it is either present throughout almost thewhole region or almost entirely absent. When present it occupies almost the same

linear extent as the nucleus dorsalis in fetuses of 9 to 11.5 weeks of I

menstrual age. It does not extend into the lower cervical and lumbar and sacral regions for several weeks after its appearance in the thoracic region. The. axons which compose it form three distinct components in

_ the thoracic region of young fetuses and these can be subdivided further

if one considers the source of the fibers which decussate in them. At least the dorsal and ventral components receive fibers from more than one source. The middle component is broken into segments and at first seems to receive all of its fibers from a segmented column of cells which lies in the cervix of the columna posterior.

ABBREVIATIONS CC, central canal NCP, nucleus cornucommissuralis posterior DC, dorsal component of commissura post- ND, nucleus dorsalis E en“ d NIL, nucleus inter-mediolateralis

n ma Flfifefasyciculus dorsolateralis NR’ nucleus reticularis 7 FL, funiculus lateralis R’ mphe FP, funieulus posterior SG, substantia gelatinosa MC, middle component of commissura post- VC, ventral component of commissura posterior erior‘

The first six figures are camera lucida drawings made from the region of the commissura posterior of fetuses in which significant stages in the formation of the commissure appear. Figure 7 was made by projecting a section at a relatively low magnification and later adding some details as to the distribution of fibers by studying the section under a higher magnification. Reference numbers in parentheses refer to slide number, row, section, and thoracic segment.

Fig. 1 Fetus 18 (26 -3 — 15, T 2). The very earliest indication of commissural processes occur at DC. '

In fetuses of approximately 9 to 9.5 weeks of menstrual age (30 mm. to 35 mm. c.r. length) the alar plates of the spinal cord fuse rapidly to form the dorsalraphe. During this time a great many of the cells in the mantle layer, which were derived from this part of the cord, differentiate into neuroblasts which find their permanent position near the dorsal edge of the base of the columna posterior. As the nucleus dorsalis develops in the thoracic and lumbar regions it migrates into a DEVELOPMENT or COMMISSURA POSTERIOR 263

position just ventrally of the majority of these neuroblasts. Marie (1894) gave the name of nucleus cornucommissuralis posterior to this mass of cells. In the fetus it is very clearly divided into subgroups at first, if the distribution of processes is considered as the criterion for such a classification. All the cells in each subgroup send their processes in the same direction as if they were all differentiating under the same influence. The majority of the cells send their axons dorsally into the posterior funiculus but here and. there small masses of cells are oriented toward the raphe to become commissural cells. Later in life the subgroups cannot be recognized easily, if at all. Those fibers which grow toward the raphe form the first indication of a commissura posterior and laterthey lie in its dorsal component.

Fig. 2 Fetus 33 (25—3 —-6, T 2). Commissural neuroblaats are migrating farther from the raphe than in figure 1.

In fetus no. 33 of this series, which was 9 to 9.5 weeks of menstrual age, the conimissura posterior is fairly distinct (fig. 2). This fetus seems to have a rather precocious dorsal commissure. There is a segment extending from the first to the fourth cervical roots inclusive in which a commissure formed by cells which lie close to the raphe occurs. The fifth cervical to second thoracic segments of the cord are almost free of dorsal commissural fibers. ‘It begins again in the caudal border of the first thoracic segments and is not only much better developed with respect to the dorsal component than any of the other fetuses of approximately the same age in our series but also, at intervals of approximately 35 u, small bundles of fibers arise from cells which lie near the middle of the cervix of the posterior cornu and pass through the raphe on the Ventral side of the dorsal component. Apparently these fibers belong to the middle component although this fetus is more than half a week, probably a whole week, younger than any other fetus in which this component has been recognized.

In the upper thoracic segments there are other fibers in small numbers which decussate through the dorsal gray commissure. Most of these seem to come from the region of the anterior cornua. Among them are processes of cells whose bodies lie at the side of the raphe and have a long process directed Ventrolaterally.

In the lumbar region the commissure gradually diminishes in size until it is represented by very few fibers per section and then the spaces between segments begin to be more than 10 u wide and some sections have no decussating fibers in them. At this level and nearer the most caudal part of the commissure many of the ependymal cells in the raphe seem to be differentiating into some kind of cell, possibly spongi« oblasts, and migrating out of it. Some of these cells send out lateral processes first. Sometimes there are two processes on a single cell but more often there is only one. On the side toward the raphe there may be one or two knobs which do not cross the midline. As they move out of the raphe many of them are bipolar and where there is a definite commissure some of these processes cross through it. It seems that this level may show the typical mod-e.of formation of the commissural cells.

In the other two 9.5 weeks fetuses gttable 1) nothing could be observed as to the manner of the formation of the commissura posterior. The four varied considerably in the degree of fusion of the walls of the central canal to form the posterior raphe and apparently the method of closure varies in certain respects or is better revealed in some fetuses than in others. In no. 18 the ependymal cells of the approximated alar walls seem to be cemented together by a gelatinous substance and in the others the cells of the ependyma seem to have fused with no visible cementing substance. In no. 33 the ependymal cells along the alar plate generally have started to grow ventrally along the central canal (fig. 2, E). There are no cilia on these cells. Where the walls have fused a small number of cells have processes which pass dorsally to form the raphe but the rest of them appear to be indifferent in form with a small number of neuroblasts among them. In no. 23 there is a seam which lies along the line of fusion, formed by nuclei of ependymal cells in which mitotic figures are visible. These cells which recently belonged to the wall of the central canal have interdigitated and the cytoplasm of the cells has become. so thoroughly rearrangedthat the only recognizable

Table 1[edit] Summary of progress of development of the commissura_postem'or as observed in .91 human fetuses ranging in age from approximately 9 weeks to birth.

APPROXP OOMMISSURA POSTEBIOR . MATE ‘ . _ 5E:(I)‘_u‘ Lnggrn ~ s¥:I§‘;'L Cervical Thoracic Lumbar I Sacral AGE IN “’“”‘S 0 1-0 3 0 4-1‘ 1 De L 1-1. 21 L 3-1. 5 s 1—s 5 23 33 9.5 0 0 O 0 0 0 0 0 16 35 - 9.5 +1 0 + 1 ‘ 0 0 0 0 0 18 36 9.5 +1 0 + 0 0 0 0 0 33 32 9.5 + 0 + 1 9 +7 0 I 0 34 40 10 + 0 -|" +1 0 + 0 0 85 M 44 1 10.5 +1 0 + 9 0 +1 0 o 46 46.5 10.5 + 0‘! - + 9 0 + ,0 0 ‘26 43.5 11 - r* o * * * 1* 0 o 65 48.7 11 + 0 + 1* 1* 0 0 0 51 49 11 + 0? + + O * 3* 7 44 53 11.5 * ' 0 + +. +9 + 0 +1 8 54.5 11.5 1 0 0 + + + 9. +9! 0 0 I 17 56 11.5 + 10 + + 7* * * * 5 ‘ so 12 + 0 + + . 1 i 7 * * * 39 61.5 1 .12 9* ' * + + <9 4: 7* 96 66 12.5 + 1 + + + + + . 1! 37 85.5 14 +* * + + + , * * 9 1 2 A 86 14 +* 1* + + + + P‘ ?* 68 89.5 14.5 + + 1 + + + ‘' +1 +9 62 92.7 14.5 + +9 + + + + +9 1 52 95.5 14.5 + +1 + + + + +1 9 53 110.7 15.5 + + + + + + + +? 50 119.5 16 + +* + ‘ + + + + ‘ + 7 123.5 17 + + + + + +‘ + + 3 145 13-5 + + + + + + + + 82 168.2 20.5 + + + ‘ + + + + + 15 190 22 + + + + + + + + _ some 7 26? + + . + + + + +. + T V 32 * * + + + + + + U 1 - 35 + + + + + + + + S626 9 '40? +m +m ‘ +m ?m ‘!m +m +m +-m

0 — nerve fibers absent. ,

0! —very few processes present which were thought not to be nerve fibers. +1— processes present which were thought to be nerve fibers.

+ —flber present which were definitely nerve fibers.

9-flbers present in small numbers, the natureof which was indefinite.

— sections damaged or faulty technique’. In — myelinated fibers.

7m — presence of myelinated fibers doubtful.

sign of the former walls of the canal is the presence of the mitotic figures. This statement is not quite true for the whole length of the spinal cord for there are places where the fusion is not quite so complete and there are places where the sections are torn along this seam by manipulation in mounting. Such areas indicate that the line of fusion is not as strong as the adjacent tissue. These ependymal cells along the line of fusion quickly become a part of the mantle layer and no longer are recognizable as having been a part of the original ependymal lining of the central canal.

Fig. 3 Fetus 46 (15 — 3 —6, T 7). To cover a sufficiently large field a low magnification was used. Figure 4 was made from the area indicated by the rectangle.

At 10.5 weeks (fetus 46, figs. 3 and 4) the dorsal component of the commissura posterior is present in the first four cervical and all of the thoracic segments of the spinal. cord and possibly there is an occasional fiber in the four lower cervical segments. The presence of the middle

‘ component in the thoracic portion of the cord is extremely doubtful.

In this particular fetus the sections of the spinal cord were split along the dorsal raphe (fig. 3) and the commissural fibers are consequently much harder to recognize. However, the fibers which are present are so fine and have such a tendency to lie close together that the extent ‘of any single fiber is_very puzzling. Most of the cells which have a process that extends into the commissure are bipolar and the process at each end of the cell has such a tendency to branch thatit is often doubtful which process represents the axon and which the dendrite. It is also doubtful as to whether some of these cells are becoming neuroblasts or spongioblasts. In figure 4, 1, one of these bipolar cells is represented. At 2, a fiber is represented which seems to come from the opposite side but one of its slender processes comes to lie beside the medial process of the cell represented by 1 and how far it follows along this process is impossible to determine. I _ Some cells (fig. 4, 3) have dorsally directed processes which cannot be followed to the posterior funiculus in this fetus. However, in other fetuses, both older and younger, cells which lie in approximately the same position have axons which do pass dorsally to enter the posterior funiculus and it is presumed that these do, also. I

In 11-week fetuses the commissura posterior consists of a group of fibers in the upper two or three cervical segments which arise from cells that arerclosely associated with and may be the caudal end of the nucleus commissuralis of Cajal and, throughout the thoracic and up

Fig. 4 High magnification of area indicated in figure 3. 1. Typical commissural cell, the nature of which is indefinite. 2. Fiber which seems to come from the opposite side, join the medial proces of 1 and cannot be followed to its termination.

per two lumbar segments, of fibers which. later become a part of its,

dorsal component. Besides these fibers that are thought to belong to neurons or neuroblasts there are other transverse processes in the dorsal raphe, the greater number of which are probably from spongio ‘blasts. The presence of definite nerve fibersamong these processes

was not determined.

In like manner some transverse processes were seen in the lower cervical segments of one of these fetuses but they could not be identifieddefinitely as being axons of nerve cells. In this fetus, no. 51, some of the transverse processes at the level of the middle thoracic component of the commissure arise from cells with an oval nucleus which has

a very prominent nucleolus of the type the author believes to be indicative of a neuroblast (fig. 5, MC).

Following this stage for a period of about 2 to 3 weeks (12 to 14 weeks certainly; probably from 11.5 to 14.5 weeks of menstrual age) there is a great deal of migration of indifferent cells from that part of the mantle layer which lies along the dorsal raphe (figs. 5 and 6; Hogg, ’44, figs. 6 and 7). In the thoracic segments of the cord many of these migrating cells seem to come to rest either in the cervix of the columna posterior or near the processus reticularis in the lateral part of its base. At short intervals, which correspond to the intervals between the short segments of the nucleus dorsalis, a number of these migrating cells form a transverse chain which reaches from the middle of the cervix on one side to a corresponding position on the other. In some of the fetuses of 11 to

Fig. 5 Fetus 44 (383 —— 1 — 1, T 6). This figure shows the earliest stages in the formation of the middle component of the commissura posterior (MC).

11.5 weeks of menstrual age a very small number of fibers have been found decussating in the raphe and following the course that these cells follow (fig. 5, MC). The youngest fetus in which some of these chains of cells have been found is no. 44. The interpretation of this observation is that the formation of the middle component of the thoracic portion of the commissura posterior is a progressive affair which extends over a period of more than a Week in any one individual. The bodies of these cells, which are transformed into neuroblasts and belong to the middle component of the commissura posterior come to rest in the Ventrolateral portion of the cervix of the columna posterior not far from the processus reticularis but remain somewhat separated from the nucleus reticularis while they remain in the neuroblastic state.

Above and below the thoracic part of the spinal cord in fetuses of’ 14 to 14.5 weeks (table 1) collaterals from the posterior funiculi begin to be added to the commissura posterior. Previous to this time, with the exception of the first two lumbar segments, those segments of the spinal cord from which the brachial, lumbar, and sacral plexuses arise have no decussating fibers in the posterior raphe which can be recognized definitely as axons. In fetuses no. 52 and 62 fibers are found in the two lowest cervical segments. In the sacral region there are many processes of cells which lie transversely in the raphe but so many of them are undoubtedly the processes of spongioblasts or young neuroglia that it is very hard to be sure that some of the fine processes are really

. cm Fig. 6 Fetus 37 (19—3—9, T 7). This section is cut at such an angle that the fibers of none of the structures can he followed far in the field chosen for the illustration but it was the only section which showed both the middle (MC) and ventral (V0) components in the same section in a somewhat atisfactory manner. This section cuts through an end of one of the segments of the nucleus dorsalis (ND) and does not show the fibers of the middle component of -the commissure (MC) as they pass between the segments of the nucleus. A small cluster of neuroblasts of the nucleus cornucommissuralis posterior (NCP), which send their axons to the posterior funiculus, can be seen. Below the nucleus dorsalis some of the neuroblasts which help to form the ventral component of the commissure (V0) can be een. This group is indicated by the numeral 3 in figure 7. They seem to come to rest eventually in the nucleus reticularis (fig. 7, NR).

nerve fibers. At this time the migration ‘of indiflerent cells is tremendous. Most of them have relatively rather long processes extending fromone and often from both poles. These processes generally lie in

bundles among which any nerve fibers which pass in that direction also lie. Therefore, in table 1 the presence of true commissural nerve fibers in the sacral segments has been indicated by an interrogation point as being doubtful in these fetuses .and in the lower cervical and lower lumbar regions where the commissure is evidently in the early stages of development the combination + ‘I has been used. '

During the course of this investigation nothing new has been discovered concerning the ultimate distribution of the axons which form the middle component of the commissure. However, no reference to the presence of a group of cells which contribute to its formation has been found in the literature. Their axons can be traced through the middle component into the cervix of the opposite side but are lost before they terminate. Sometimes they seem to blend with the fibrous mass in the center of the cervix of the columna posterior and some fibers have been traced from this component of the commissure into the dorsal part of the lateral funiculus but whether such fibers arise from cells of the opposite side or whether they are collaterals from the lateral funiculus which enter this component as Ramon y Cajal and van Gehuchten have described is not clear.

The ventral component of the commissura posterior has been de« scribed by several investigators as being composed largely of collaterals from the lateral and ventral funiculi in the forms studied. In human fetuses this component begins to appear about the fourteenth week. As in the formation of the middle component, there, is an interval about the time of its formation when the fibers which form it lie or pass among so many migrating indifferent cells that their origin is very confusing (fetus 37, fig. 6, VC; Hogg, ’44, fig. 7). By the seventeenth week it is becoming fairly well established. Many of the fibers which compose it then and in older fetuses are of indefinite origin and are probably collaterals from the ventral and lateral funiculi but in the thoracic portion of the cord some of them arise from cells in the gray matter. Some of these cells lie close tothe central canal but below the level of the commissure, but the greater number arise from cells which lie in a column close to the processus reticularis and which has been called the tractus cellularum intercornualis lateralis by J acobsohn ( ’08), the zona intercornuale del cordonne laterale by Massazza ( ’22, ’23), the lateral basal group by Larsell (’42), the nucleus reticularis by Kuntz (’42) and Ranson (’43), and the interstitial nucleus by Ramon y Cajal (’O9). The latter group of cells is very evidently a composite group functionally and to a certain extent morphologically. Some of the cells which compose it mature rapidly, develop coarse Nissl granules, and send their axons through the anterior commissure to the opposite side. These axons seem to reach the lateral funiculus near the position of the lateral spino—thalamic tract. Other cells are intermediate in size and evidently belong to the fasciculus proprius mechanism for some of them send their axons into the homolateral lateral funiculus while others send their axons out into the gray matter of the anterior horn and some of them may reach the anterior funiculus of the same or opposite sides of the cord. Some of these intermediate sized cells send their axons ventrally to reach the preganglionic sympathetic neurons of the intermedio-lateral cell column. They seem to_ be a link in the visceral mechanism (fig. 7, NB).

The cells which send axons through the ventral portion of the dorsal commissure are the smallest in size of any of the cells of this complex group in these fetuses and they are the last'to difierentiate from the indiflerent cells. As indifferent cells they seem to migrate from the

Fig. 7 Fetus 37 (18——5—-12, T 7). One fiber in each component of the commissura posterior has been drawn in more fully than could be observed in this section to show its coure from its cell body to the raphe. Some collaterals from the fasciculus dorso-lateralis (FDL) join the ventral component of the commissure. The left side of this section is cut almost exactly between segments of the-nucleus dorsalis and shows the relation of the ‘middle and ventral components of the commissure to these intersegmental nodes very clearly.‘ The numerals 1, 2, and 3 indicate the positions of cells whose axons pass through the commissura posterior.

mantle layer near the point where their axons cross the dorsal raphe (fig. 7, 3). They begin to migrate to their permanent position at about the fourteenth week and still greatly resemble neuroblasts at the seventeenth vyeek. The Whole column of cells of which they become a part is divided into segments which correspond in number approximately to the number of fascicles of the dorsal root fibers, i.e., about seven or more per vertebral segment. Some cells of each class mentioned above seem to be represented in each of these short segments. The axons of the small cells cross near the spaces which lie between segments of the nucleus dorsalis but not quite so exactly at those spaces as the fibers of the middle component of the dorsal commissure (fig. 7, 2 and MC).

The fibers which compose the dorsal gray commissure seem to be most abundant per unit of linear measure at about 20 weeks. In the older fetuses it is evident that new fibers are not added to the commissure as rapidly as they are separated from each other by the increase in length of the spinal cord. Consequently the‘number of decussating fibers per section actually decreases in older specimens and the relative, if not the actual, distance between the central canal and the dorsal longitudinal fibers at the raphe is reduced as the fetus increases in size. Thus the difficulty of an anlysis of the sources of the commissural fibers in— creases with age. However, the immaturity of the cells and fibers in— volved in the formation of the commissure in young fetuses increases the difficulty an observer encounters in trying to identify and correlate the structures found with those described in the literature, most of which are from more mature specimens than those used in this study.

In fetuses over 20 weeks of menstrual age indifferent cells near the dorsal raphe differentiate in situ to form the posterior commissural nucleus of Ramon y Cajal and as they develop long dendrites some of these cross the raphe to form a commissure of dendrites as described by him (Ramon y Cajal, ’09, p. 408) and illustrated (’09, p. 322, fig. 117).

By 20.5 weeks, no. 82, the commissure has developed throughout the whole length of the spinal cord and all of the components which have been described in the literature are established. In the cervical region the space between the central canal and the posterior funiculi is filled with commissural fibers but the various components which appear in the thoracic region are not recognized. The collaterals from dorsal root fibers are numerous and some are collaterals from the long collaterals which run to the anterior horns. The latter have their origin far out in the gray matter and run through the middle or anterior parts of the commissure. Some of the collaterals from the region of the processus reticularis run almost directly medially across the cervix of the pos terior cornu, join those dorsal root collaterals which skirt the border

of the posterior funiculus, and decussate in the dorsal part of the commissure. These fibers seem to represent the middle component as it is found in the thoracic region but none of them were recognized as aris— ing from cells of the cervix. In no. 82, particularly in the thoracic region, collaterals seem to arise from fibers which belong to the dorsal part of the lateral funiculus. Fibers from this region were studied by Ranson ( ’13, ’14) who thought that some of them belonged to the visceral mechanism. Still further evidence of the conduction of visceral sensory impulses from one posterior cornu to the other was obtained by Ranson and Billingsley (’16) when they discovered that afferent pressor impulses travel up the spinal cord in the apex of the posterior cornu on both sides butmore readily on the homolateral side. In this fetus some of the fibers from the area under discussion sweep around the lateral margin of the substantia gelatinosa and turn back into it while others pass through the commissura posterior. Ramon y Cajal (’09, vol. 1, pp. 325 and 326, and fig. 119) describes the distribution of collaterals from the posterior portion of the lateral funiculus almost exactly as they have been recognized in fetus no. 82. A composite drawing of these collaterals, made from the works of Ramon y Cajal, has been published by Krieg (’42, p. 31, fig. 13).

The results of this investigation seem to indicate that in the first three cervical segments of the spinal cord and throughout the thoracic and first two lumbar segments a part of the commissura posterior is formed by the axons of cells which lie in some part of the posterior cornu and near the processus reticularis. As the fetuses mature, collaterals from the sensory collaterals of the posterior funiculi and collaterals from the lateral and possibly the Ventral funiculi are added to the commissura posterior. These grow through the commissure in

' considerable numbers by the twenty—first week. Later the number of

decussating fibers-seen in a single section decreases. It is thought that the reason for this decrease is a slowing up of the addition of new fibers whilethe increasing length of the cord separates those already present by wider and wider intervals. 1

In the anencephalic fetuses, T and U, both of which are over 20 weeks of menstrual age, the number of commisural fibers per 10 u section is very much reduced but in the "thoracic portion of the cords some cells were found which gave off commissural fibers and which lie in the positions of the three groups. described as contributing to the three components of the commissura posterior. Because of the greatly reduced number of fibers in the lateral and Ventral funiculi of the two fetuses (Hogg, ’44, fig. 11) it is quite probable that the number of collaterals from those regions to the commissure is reduced but because of the lack of prepared material from normal fetuses of approximately the same age as the two anencephalic ‘fetuses no positive statement concerning that probability can be made at this time.


A search of the literature reveals the fact that the commissura posterior was recognized by the first investigators of the structure of the spinal cord and that after the discovery of a method for staining the myelin sheaths of nerve fibersthe majority of investigators neglected it because it contains so few myelinated fibers. Those investigators who resorted to silver preparations, Ramon y Cajal (1890, ’09), van Gehuchten (1891, 1895, ’06), Valenza (1897), particularly the first two mentioned, gave detailed descriptions of the structure of this commissure as they found it in young animals. .

They did not attempt to follow its development. They had some difficulty in reconciling their earlier observations but later each discovered at least some indication of what the other had described.

From a study of the development of the human spinal cord it seems that some of the difiiculties in harmonizing the observations of the earlier investigations probably resulted from studying the cords of individuals that were near term or past the time of birth. In such material the increased length of the cord has separated the segments of some of the components by such long intervalspthat many serial sections have to be studied before the complete picture can be seen. The author is convinced that none of the earlier investigators ever made a careful study of the commissura posterior throughout the whole length of a spinal cord. Their descriptions indicate,_ rather, that they studied what they considered to be representative sections, probably made from rather short blocks removed from different levels of the spinal cord. Thus they secured all or almost all of thevanatomical details without, apparently, recognizing the orderly, segmental arrangement of the components of the commissure. Most of the fibers they saw were collaterals from one or other of the marginal funiculi of fibers. A very small number of fibers in the commissura posterior were recognized as coming from cells in the gray matter (Ramon y Cajal, ’09, fig. 139) and Valenza (1897) described some of them as coming from cells of the nucleus dorsalis.

In this investigation at least two very definite groups of cells have been recognized as contributing to the formation of the commissura posterior in very young fetuses. One of these groups is somewhat subdivided by the position of the cell bodies but more distinctly separated

by the course of migration from the mantle layer and the position of their axons after migration. One of these subdivisions is formed by those cells which lie dorsomedially of the processus reticularis, the axons of which help to form the middle component of the commissura posterior (Hogg, ’44, fig. 6, MC)- The other subdivision is formed by those cells which lie almost at the apex of the processus reticularis, the axons of which pass medially to enter the ventral component of the commissura posterior (Hogg, ’44, fig. 7 and fig.) 9, VC).

None of the fibers which the author has traced across the dorsal gray commissure have come from the nucleus dorsalis. As referred. to-above and as described on p. 268, some cells which liein the gray matter near the processus reticularis have axons which pass between the segments of the nucleus dorsalis in the very young fetuses. In the older fetuses some of them are so deeply buried in the nucleus that it is very diflicult to be sure that they are not joined by other fibers which do arise from it but I have never succeeded in finding an axon that could be traced from a cell of the nucleus to join one of these bundles. Moreover, these

‘bundles which join the middle and ventral portions of the commissure lie at first between the segments of the nucleus dorsalis and throughout this series they retain that segmental arrangement although they are overgrown by the enlarging nucleus dorsalis. It is altogether possible that an investigator could confuse these fibers with others which do arise from cells of the nucleus dorsalis. It may be that some of the small stellate cells, which lie around the borders of and occasionally within the nucleus dorsalis, send axons through the dorsal gray comrnissure. Valenza (1897) is the only investigator who used the Golgi method who claims to have seen axons from the nucleus dorsalis pass through "it. Ramon y Cajal (1890) described the dorsal gray commissure in chick embryos but did not describe any axons as coming from the nucleus dorsalis. In his later work (’09) he still did not describe axons from it which passed through the dorsal gray commissure.

Consequently, it seems from the evidence at hand that considerable doubt is cast on the interpretation of the findings described by Pass (’33). It might be that the axons of cells which lie in the cervix of the posterior cornu and which pass through the commissura posterior eventually reach the cerebellum through the dorsal spino-cerebellar tract. If they do, a lesion of any considerable length placed in the nucleus dorsalis should cause the degeneration of some of these fibers. The fact that McNalty and Horsley (’09) did not discover degenerated fibers in the contralateral dorsal spindcerebellaritract after carefully placing lesions in the nucleus dorsalis, using a fine bent.needle for an electrode and a Weak faradic current to produce the lesion, is further indication that the results obtained by Pass may depend‘ on injury to fibers other than those arising from it.

Some unpublished work by the author on degenerating fibers after avulsion of the facial nerve as practiced by vanGehuc_hten (,’03), the results of Nittono (’23_) on section of the branches of the trigeminal nerve in the rat and those of Friihlich (’04) raise the question of the trustworthiness of degenerative changes following operative proce276 IRA DWIGHT HOGG

dures. Nittono interpreted his results as indicating that toxic substances Were elaborated by the degenerating fibers in one nerve which acted specifically upon the blood vessels and possibly the fibers of the corresponding nerve of the opposite side with suificient intensity to produce lesions in it. Frohlich severed the dorsal roots of some of the cervical spinal nerves and obtained degeneration in the dorsal spinecerebellar tract which he attributed to traction on the fibers of the tract. In the case of the rats in which the facial nerve was avulsed on one side, the damage, which was manifested in the opposite facial nerve to some extent but which was much more grave in the fibers of the trapezoid body, has been attributed to traction transmitted to them before the fibers of the operated nerve broke. Because of the indirect course of the facial nerve through the skull, such traction must have been extremely mild. Consequently, it seems that the passage of an electrocautery knife through the overlying tissue to the nucleus dorsalis or lengthwise of the cord in the raphe may produce enough traction on the fibers which do decussate there to traumatize both them and those among which they lie in the lateral funiculi.

A. second reason for ,doubting the presence of fibers from the nucleus dorsalis in the dorsal gray eommissure is that those fibers which pass laterally to the homolateral spino—cerebellar tract assume a heavy myelin sheath in fetuses near term. This sheath begins near the point of emergence of the fibers from the nucleus while the fibers of the dorsal gray commissure are mostly unmyelinated and fibers with thin myelin sheaths, even in the adult. Strong (’36) expresses a similar opinion.

Although fibers which arise from cells in the cervix of the posterior cornua and just ventral to them in the neighborhood of the reticular nucleus cross through the dorsal commissure and some of them enter the dorsal portion of the lateral funiculus, this investigation does not prove that they enter the dorsal spino-cerebellar tract. Throughout the stages of this investigation they have remained unmyelinated. In the anencephalic fetuses there is a reduction in the number of fibers in all of the components of the dorsal commissure, but I have not recognized the complete loss of any component. Furthermore, because of the great reduction of longitudinal fibers in the lateral funiculi there probably is a great reduction in the number of collaterals from this region to the commissura posterior. Consequently it seems that the reduction in number of fibers is probably the result of a general underdevelopment of the intrinsic association mechanism in the dorsal half of the cords from the two anencephalic fetuses.

The fact that the cells of origin of some of the fibers which pass through both the middle and ventral components of the commissure in the thoracic and upper lumbar portions of the spinal cord lie very near the processus reticularis- and adjacent to or among the cells and terminal fibers which are, as nearly as can be determined by anatomical methods, a part of the visceral mechanism, it seems to be highly probable that they too, belong to the visceral mechanism, at least in part. Massazza (’22) describes some of these cells as having the anatomical structure of visceral cells although he did not trace the course of their axons. Some of them may belong to the somatic reflex mechanism of the fasciculus proprius. and may be activated by pain and temperature impulses (Ranson, ’13, ’14).

The other group of cells which contributes to the formation of the commissura posterior is indistinguishable from the group which is

recognized as forming the nucleus cornucommissuralis posterior. In young fetuses the cells of this group have simple processes which pass either dorally into the posterior 3funiculus or medially across the commissure toward the corresponding group of the opposite side. These cells never become very large but the axons of some of them later branch so that they may entertwo or more of the funiculi (Ramon y Cajal, ’O9, fig. 141 E and fig. 146). ' Whether those which first enter‘ the commissure later send branches of their axons into the posterior funiculus of the homolateral side or not has not been determined definitely. These cells of the nucleus cornucommissuralis posterior or noyau interne de la corne postérieure of Ramon y Cajal ( ’O9, pp. 402-403) appear to belongto the fasciculus proprius and are primarily concerned with that part of it which lies in theposterior funiculus. According to Ramon y Cajalrit is best developed in the cervical and lumbar regions. According to Massazza (’22, ’24) the group is continuous throughout the whole length of the spinal cord but is considerably larger in the cervical, lumbar, and middle sacral regions than in the thoracic region.

He states also, that it shows no evidence of metamerism and that the type of cells varies very little.

According to Ramon y Cajal many very fine collaterals from the posterior funiculus terminate among these cells (’O9, fig. 126) and in one of his illustrations (’O9, fig. 117) he shows collaterals from the posterior funiculus crossing through the dorsal part of the commissura posterior and arborizing in the position occupied by the majority of the cells of this -group. A similar distribution of collaterals has been observed in the older fetuses of this series but the terminal arborization has not been recognized as being so complex. Perhaps these fetuses are not sufficiently mature or the silver may have failed to bring out the complexity of the terminal arborizations.

According to Marie (1894) this group of cells is damaged in pellagra and the fibers of what he named the zona cornucommissuralis, which seems to be the same as the ground bundle or fasciculus proprius of the posterior funiculus, were somewhat altered. In commenting on the observations of Marie, Ramon y Cajal (’09, p. 403) takes exception to the

- statement concerning the destruction of fibers in the zona cornuc0m missuralis and thinks the greater part of the degeneration should occur in the lateral funiculus. However, according to the observations made on this series of fetuses it seems that the observations of Marie should be valid and that perhaps Ramon y Cajal was mistaken in his interpretation of the extent of the lesion described by Marie.

If the anatomical connections and relations of the nucleus cornucommissuralis posterior and the posterior component of the dorsal commissure be considered sufficient evidence on which to hazard a conclusion as to the probable function of this mechanism, then it seems that it should be looked upon as a part of the intrinsic mechanism for controlling responses to somatic sensory impulses. This interpretation is somewhat heightened by the observations of Hoff and Hoff (’34) who observed terminal collaterals from the cortico—spinal tract entering the region of the nucleus cornucommissuralis posterior. According to their illustrations some of them terminate in the region of the lateral reticular nuclei, also, but which of the several kinds of cells in that mass receives these collaterals is not indicated.

The results of this investigation indicate that the commissura posterior is of much’ greater importance to the -physiological activities of the spinal cord than has been indicated by most investigators and probably serves to associate the two sides of the cord in both somatic and visceral responses to sensory stimuli. From the-point of view of Lorente de N6 (’35; of. Forbes, ’22, and Ranson and Hinsey, ’32), this part of the mechanism probably is concerned chiefly with subliminal stimuli that help to set the pattern of response Worked out in the more definitely motor mechanism of the anterior cornua.


The commissura posterior or dorsal gray commissure of the spinal cord begins to appear in human fetuses at about the same time that the nucleus dorsalis can first be recognized, i.e., somewhere near 9 to 9.5 weeks of menstrual age. There seems to be a little Variation in age and relative development, when other structures are considered, between individuals as to when the commissure first appears. The first indications of a dorsal commissure have been found in the first three thoracic segments of the cord with the strongest development in the second segment. At very nearly the same time that the commissure appears in the thoracic region, some commissural fibers can be found in the first four cervical segments. The fibers in the cervical segments

are closely associated with the caudal end of the nucleus commissuralis of Cajal. Those in the thoracic portion become a part of the dorsal component of the mature commissure. All of these early fibers arise from cells which lie close to the ventral sides of the posterior funiculi, a mass of cells which seems to be the anlage of the nucleus cornucommissuralis posterior. Not all of the cells in this mass are commissural in nature. Many send processes into the posterior funiculi.

A little later, 10 to 10.5 weeks, small groups of cells of the thoracic cord which lie in the cervix of the posterior cornua at the level of the intervals between the segments of the nucleus dorsalis send axons through the dorsal commissure to form the first indication of its middle component. At first these groups are separated from each other by an interval of 30 to 40 u. .

A third component of the dorsal commissure appears in fetuses of about 14 weeks of menstrual age. It is composed of fibers which are derived from cells which seem to migrate from the neighborhood of the posterior raphc to a point near the processus reticularis and there they form groups, separated.from each other by approximately the same intervals as those between segments of the nucleus dorsalis. These fibers form part of the ventral component of the mature commissure.

At about the same time, collaterals from the posterior and lateral funiculibegin to be added to the commissure. The number of fibers forming the commissure increase in number per linearunit until some time near the twenty-first week. After that the increase in length of the cord reduces the numberiof fibers per linear unit until the apparent number of fibers is greatly reduced. During this time the fibers of the middle and ventral components of the commissure become so much overgrown by the nucleus dorsalis that they have the superficial appearance of arising from these nuclei. Many of the collaterals forming the ventral component of the commissure terminate among cells in the base and middle of the anterior cornua (columna anterior).

Sometime during the sixteenth or seventeenthweek of menstrual age commissural fibers appear in the regions from which the brachial, lumbar, and sacral plexuses arise. As nearly as has been determined these portions of the dorsal commissure are composed entirely of collaterals from the different funiculi, chiefly the dorsal and lateral. In these regions the three components observed in the thoracic segments were not distinctly recognized although a majority of the collaterals arising from the posterior funiculus lie in the dorsal half of the commissure and a majority of these derived from the lateral and whatever fibers may come from the ventral funiculus seem to lie in its ventral half. There is some intermixture or crossing over of fibers from different funiculi, especially by medial collaterals from the long collaterals to the anterior cornua. Many of the latter spring from the long collaterals far out in the gray matter. What the significance, if any, of this observation may be is not evident.

In the anencephalic fetuses, both of which were 30 or more weeks of menstrual age, the number of collaterals from the lateral funiculi to the dorsal commissure seems to be greatly reduced but in the thoracic region cells were found in all three positions described for normal fetuses which have axons that pass through the dorsal commissure.

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Cite this page: Hill, M.A. (2018, December 10) Embryology Paper - The embryological development of the commissura posterior in the human spinal cord. Retrieved from

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