Chapter X: Introduction: 1 (9)

The grey matter would be situated in the interior and surround the epithelium of the central canal, and the white matter would nearly surround the grey and form the anterior white commissure. The nerves would then arise, not from the sides of the nervous cord as in existing Vertebrates, but from its extreme ventral summit.

One of the most striking features which I have brought to light with reference to the development of the posterior roots, is the fact of their growing out from the extreme dorsal summit of the neural canal--a position analogous to the ventral summit of the Annelidan nervous cord. Thus the posterior roots of the nerves in Elasmobranchii arise in the exact manner which might have been anticipated were the spinal cord due to such a folding as I have suggested. The argument from the nerves becomes the stronger, from the great peculiarity in the position of the outgrowth, a feature which would be most perplexing without some such explanation as I have proposed. The central epithelium of the neural canal according to this view represents the external skin; and its ciliation is to be explained as a remnant of the ciliation of the external skin now found amongst many of the lower Annelids.

I have, however, employed the comparison of the Vertebrate and Annelidan nervous cords, not so much to prove a genetic relation between the two as to shew the _à priori_ possibility of the formation of a _spinal canal_ and the _à posteriori_ evidence we have of the Vertebrate spinal canal having been formed in the way indicated.

I have not made use of what is really the strongest argument for my view, viz. that the embryonic mode of formation of the spinal canal, by a folding in of the external epiblast, is the very method by which I have supposed the spinal canal to have been formed in the ancestors of Vertebrates.

My object has been to suggest a meaning for the peculiar primitive position of the posterior roots, rather than to attempt to explain in full the origin of the spinal canal.

EXPLANATION OF THE PLATES[60].

Footnote 60: The figures on these Plates give a fair general
idea of the appearance presented by the developing spinal
nerves; but the finer details of the original drawings have in
several cases become lost in the process of copying.

The figures which are tinted represent sections of embryos
hardened in osmic acid; those without colour sections of
embryos hardened in chromic acid.

PLATE 22.

Fig. A. Section through the dorsal region of an embryo of _Scyllium stellare_, with the rudiments of two visceral clefts. The section illustrates the general features at a period anterior to the appearance of the posterior nerve-roots.

_nc._ neural canal. _mp._ muscle-plate. _ch._ notochord. _x._ subnotochordal rod. _ao._ rudiment of dorsal aorta. _so._ somatopleure. _sp._ splanchnopleure. _al._ alimentary tract. All the parts of the section except the spinal cord are drawn somewhat diagrammatically.

Figs. B I, B II, B III. Three sections of a _Pristiurus_-embryo. B I is through the heart, B II through the anterior part of the dorsal region, and B III through a point slightly behind this. Drawn with a camera. (Zeiss CC ocul. 2.)

In B III there is visible a slight proliferation of cells from the dorsal summit of the neural canal.

In B II this proliferation definitely constitutes two club-shaped masses of cells (_pr_), both attached to the dorsal summit of the neural canal. The masses are the rudiments of the posterior nerve-roots.

In B I the rudiments of the posterior roots are of considerable length.

_pr._ rudiment of posterior roots. _nc._ neural canal. _mp._ muscle-plate. _ch._ notochord. _x._ subnotochordal rod. _ao._ dorsal aorta. _so._ somatopleure. _sp._ splanchnopleure. _al._ alimentary canal. _ht._ heart.

Fig. C. Section from a _Pristiurus_-embryo, slightly older than B. Camera. (Zeiss CC ocul. 2.) The embryo from which this figure was taken was slightly distorted in the process of removal from the blastoderm.

_vr._ rudiment of vertebral body. Other reference letters as in previous figures.

Fig. D a. Section through the dorsal region of a _Torpedo_-embryo with three visceral clefts. (Zeiss CC ocul. 2.) The section shews the formation of the dorsal nerve-rudiments (_pr_) and of a ventral anterior nerve-rudiment (_ar_), which at this early stage is not distinctly cellular.

_ar._ rudiment of an anterior nerve-root. _y._ cells left behind on the separation of the external skin from the spinal cord. _c._ connective-tissue cells springing from the summit of the muscle-plates. Other reference letters as above.

Fig. D b. Section from dorsal region of a _Torpedo_-embryo somewhat older than D a. Camera. (Zeiss CC ocul. 2.) The posterior nerve-rudiment is considerably longer than in fig. Da, and its pedicle of attachment to the spinal cord is thinner. The anterior nerve-rudiment, of which only the edge is present in the section, is distinctly cellular.

_m._ mesoblast growing up from vertebral rudiment. _sd._ segmental duct.

Fig. D c. Section from a still older _Torpedo_-embryo. Camera. (Zeiss CC ocul. 2.) The connective-tissue cells are omitted. The rudiment of the ganglion (_g_) on the posterior root has appeared. The rudiment of the posterior nerve is much longer than before, and its junction with the spinal cord is difficult to detect. The anterior root is now an elongated cellular structure.

_g._ ganglion.

Fig. D d. Longitudinal and vertical section through a _Torpedo_-embryo of the same age as D c.

The section shews the commissures (_x_) uniting the posterior roots.

Fig. E a. Section of a _Pristiurus_-embryo belonging to the second stage. Camera. (Zeiss CC ocul. 2.) The section shews the constriction of the pedicle which attaches the posterior nerve-rudiments to the spinal cord.

_pr._ rudiment of posterior nerve-root. _nc._ neural canal. _mp._ muscle-plate. _vr._ vertebral rudiment. _sd._ segmental duct. _ch._ notochord. _so._ somatopleure. _sp._ splanchnopleure. _ao._ aorta. _al._ alimentary canal.

Fig. E b. Section of a _Pristiurus_-embryo slightly older than Ea. Camera. (Zeiss CC ocul. 2.) The section shews the formation of the anterior nerve-root (_ar_).

_ar._ rudiment of the anterior nerve-root.

Fig. F. Section of a _Pristiurus_-embryo with the rudiments of five visceral clefts. Camera. (Zeiss CC ocul. 2.)

The rudiment of the posterior root is seen surrounded by connective-tissue, from which it cannot easily be distinguished. The artist has not been very successful in rendering this figure.

Figs. G1, G2, G3. Three longitudinal and horizontal sections of an embryo somewhat older than F. The embryo from which these sections were taken was hardened in osmic acid, but the sections have been represented without tinting. G1 is most dorsal of the three sections. Camera. (Zeiss CC ocul. 1.)

_nc._ neural canal. _sp.c._ spinal cord. _pr._ rudiment of posterior root. _ar._ rudiment of anterior root. _mp._ muscle-plate. _c._ connective-tissue cells. _ch._ notochord.

PLATE 23.

Fig. H I. Section through the dorsal region of a _Pristiurus_-embryo in which the rudimentary external gills are present as very small knobs. Camera. (Zeiss CC ocul. 2.)

The section shews the commencing differentiation of the posterior nerve-rudiment into root (_pr_), ganglion (_sp.g_), and nerve (_n_), and also the attachment of the nerve-root to the spinal cord (_x_). The variations in the size and shape of the cells in the different parts of the nerve-rudiment are completely lost in the figure.

_pr._ posterior nerve-root. _sp.g._ ganglion of posterior root. _n._ nerve of posterior root. _x._ attachment of posterior root to spinal cord. _w._ white matter of spinal cord. _i._ mesoblastic investment to the spinal cord.

Fig. H II. Section through the same embryo as H I. (Zeiss CC ocul. 1.)

The section contains an anterior root, which takes its origin at a point opposite the interval between two posterior roots.

The white matter has not been very satisfactorily represented by the artist.

Figs. I I, I II. Two sections of a _Pristiurus_-embryo somewhat older than H. Camera. (Zeiss CC ocul. 1.)

The connective-tissue cells are omitted.

Figs. I a, I b, I c. Three isolated cells from the ganglion of one of the posterior roots of the same embryo.

Figs. K I, K II. Two horizontal longitudinal sections through an embryo in which the external gills have just appeared. K I is the most dorsal of the two sections. Camera. (Zeiss CC ocul. 1.)

The sections shew the relative positions of the anterior and posterior roots at different levels.

_pr._ posterior nerve-rudiment. _ar._ anterior nerve-rudiment. _sp.c._ spinal cord. _n.c._ neural canal. _mp._ muscle-plate. _mp´._ first-formed muscles.

Fig. L. Longitudinal and vertical section through the trunk of a _Scyllium_-embryo after the external gills have attained their full development. Camera. (Zeiss CC ocul. 1.)

The embryo was hardened in a mixture of chromic acid and osmic acid.

The section shews the commissures which dorsally unite the posterior roots, and also the junction of the anterior and posterior roots. The commissures are unfortunately not represented in the figure with great accuracy; their outlines are in nature perfectly regular, and not, as in the figure, notched at the junctions of the cells composing them. Their cells are apparently more or less completely fused, and certainly not nearly so clearly marked as in the figure. The commissures stain very deeply with the mixture of osmic and chromic acid, and form one of the most conspicuous features in successful longitudinal sections of embryos so hardened. In sections hardened with chromic acid only they cannot be seen with the same facility.

_sp.c._ spinal cord. _gr._ grey matter. _w._ white matter. _ar._ anterior root. _pr._ posterior root. _x._ commissure uniting the posterior roots.

Figs. M I, M II. Two sections through the head of the same embryo as fig. B. M I, the foremost of the two, passes through the anterior part of the thickening of epiblast, which becomes involuted as the auditory vesicle. It contains the rudiment of the seventh nerve, VII. Camera. (Zeiss CC ocul. 2.)

VII. rudiment of seventh nerve. _au._ thickening of external epiblast, which becomes involuted as the auditory vesicle. _n.c._ neural canal. _ch._ notochord. _pp._ body-cavity in the head. _so._ somatopleure. _sp._ splanchnopleure. _al._ throat exhibiting an outgrowth to form the first visceral cleft.

IX. ON THE SPINAL NERVES OF AMPHIOXUS[61].

Footnote 61: From the _Journal of Anatomy and Physiology_,
Vol. X. 1876.

During a short visit to Naples in January last, I was enabled, through the kindness of Dr Dohrn, to make some observations on the spinal nerves of Amphioxus. These were commenced solely with the view of confirming the statements of Stieda on the anatomy of the spinal nerves, which, if correct, appeared to me to be of interest in connection with the observations I had made that, in Elasmobranchii, the anterior and posterior roots arise alternately and not in the same vertical plane. I have been led to conclusions on many points entirely opposed to those of Stieda, but, before recording these, I shall proceed briefly to state his results, and to examine how far they have been corroborated by subsequent observers.

Stieda[62], from an examination of sections and isolated spinal cords, has been led to the conclusion that, in Amphioxus, the nerves of the opposite sides arise alternately, except in the most anterior part of the body, where they arise opposite each other. He also states that the nerves of the same side issue alternately from the dorsal and ventral corners of the spinal cord. He regards two of these roots (dorsal and ventral) on the same side as together equivalent to a single spinal nerve of higher vertebrates formed by the coalescence of a dorsal and ventral root.

Footnote 62: _Mém. Acad. Pétersbourg_, Vol. XIX.

Langerhans[63] apparently agrees with Stieda as to the facts about the alternation of dorsal and ventral roots, but differs from him as to the conclusions to be drawn from those facts. He does not, for two reasons, believe that two nerves of Amphioxus can be equivalent to a single nerve in higher vertebrates: (1) Because he finds no connecting branch between two succeeding nerves, and no trace of an anastomosis. (2) Because he finds that each nerve in Amphioxus supplies a complete myotome, and he considers it inadmissible to regard the nerves, which in Amphioxus together supply _two myotomes_, as equivalent to those which in higher vertebrates supply a _single myotome only_.

Footnote 63: _Archiv f. mikr. Anatomie_, Vol. XII.

Although the agreement as to facts between Langerhans and Stieda is apparently a complete one, yet a critical examination of the statements of these two authors proves that their results, on one important point at least, are absolutely contradictory. Stieda, Pl. III. fig. 19, represents a longitudinal and horizontal section through the spinal cord which exhibits the nerves arising alternately on the two sides, and represents each myotome supplied by _one nerve_. In his explanation of the figure he expressly states that the nerves of one plane only (_i.e._ only those with dorsal or only those with ventral roots) are represented; so that if all the nerves which issue from the spinal cord had been represented double the number figured must have been present. But since each myotome is supplied by _one_ nerve in the figure, if all the nerves present were represented, each myotome would be supplied by two nerves.

Since Langerhans most emphatically states that only _one nerve_ is present for _each myotome_, it necessarily follows that he or Stieda has made an important error; and it is not too much to say that this error is more than sufficient to counterbalance the value of Langerhans' evidence as a confirmation of Stieda's statements.

I commenced my investigations by completely isolating the nervous system of Amphioxus by maceration in nitric acid according to the method recommended by Langerhans[64]. On examining specimens so obtained it appeared that, for the greater length of the cord, the nerves arose alternately on the two sides, as was first stated by Owsjannikow, and subsequently by Stieda and Langerhans; but to my surprise not a trace could be seen of a difference of level in the origin of the nerves of the same side.

Footnote 64: _Loc. cit._

The more carefully the specimens were examined from all points of view, the more certainly was the conclusion forced upon me, that nerves issuing from the ventral corner of the spinal cord, as described by Stieda, had no existence.

Not satisfied by this examination, I also tested the point by means of sections. I carefully made transverse sections of a successfully hardened Amphioxus, through the whole length of the body. There was no difficulty in seeing the dorsal roots in every third section or so, but not a trace of a ventral root was to be seen. There can, I think, be no doubt, that, had ventral roots been present, they must, in some cases at least, have been visible in my sections.

In dealing with questions of this kind it is no doubt difficult to prove a negative; but, since the two methods of investigation employed by me both lead to the same result, I am able to state with considerable confidence that my observations lend no support to the view that the alternate spinal nerves of Amphioxus have their roots attached to the ventral corner of the spinal cord.

How a mistake on this point arose it is not easy to say. All who have worked with Amphioxus must be aware how difficult it is to conserve the animal in a satisfactory state for making sections. The spinal cord, especially, is apt to be distorted in shape, and one of its ventral corners is frequently produced into a horn-like projection terminating in close contact with the sheath. In such cases the connective tissue fibres of the sheath frequently present the appearance of a nerve-like prolongation of the cord; and for such they might be mistaken if the sections were examined in a superficial manner. It is not, however, easy to believe that, with well conserved specimens, a mistake could be made on this point by so careful and able an investigator as Stieda, especially considering that the histological structure of the spinal nerves is very different from that of the fibrous prolongations of the sheath of the spinal cord.

It only remains for me to suppose that the specimens which Stieda had at his disposal, were so shrunk as to render the origin of the nerves very difficult to determine.

The arrangement of the nerves of Amphioxus, according to my own observations, is as follows.

The anterior end of the central nervous system presents on its left and dorsal side a small pointed projection, into which is prolonged a diverticulum from the dilated anterior ventricle of the brain. This may perhaps be called the olfactory nerve, though clearly of a different character to the other nerves. It was first accurately described by Langerhans[65].

Footnote 65: _Loc. cit._

Vertically below the olfactory nerve there arise two nerves, which issue at the same level from the ventral side of the anterior extremity of the central nervous system. These form the first pair of nerves, and are the only pair which arise from the ventral portion of the cerebro-spinal cord. The two nerves, which form the second pair, arise also opposite each other but from the dorsal side of the cord. The first and second pair of nerves have both been accurately drawn and described by Langerhans: they, together with the olfactory nerve, can easily be seen in nervous systems which have been isolated by maceration.

In the case of the third pair of nerves, the nerve on the right-hand side is situated not quite opposite but slightly behind that on the left. The right nerve of the fourth pair is situated still more behind the left, and, in the case of the fifth pair, the nerve to the right is situated so far behind the left nerve that it occupies a position half-way between the left nerves of the fifth and sixth pairs. In all succeeding nerves the same arrangement holds good, so that they exactly alternate on two sides.

Such is the arrangement carefully determined by me from one specimen. It is possible that it may not be absolutely constant, but the following general statement almost certainly holds good.

All the nerves of Amphioxus, except the first pair, have their roots inserted in the dorsal part of the cord. In the case of the first two pairs the nerves of the two sides arise opposite each other; in the next few pairs, the nerves on the right-hand side gradually shift backwards: the remaining nerves spring alternately from the two sides of the cord.

For each myotome there is a single nerve, which enters, as in the case of other fishes, the intermuscular septum. This point may easily be determined by means of longitudinal sections, or less easily from an examination of macerated specimens. I agree with Langerhans in denying the existence of ganglia on the roots of the nerves.

X.

A MONOGRAPH

ON THE

DEVELOPMENT OF
ELASMOBRANCH FISHES.

PUBLISHED 1878.