DEVELOPMENT OF THE SPINAL NERVES AND OF THE SYMPATHETIC NERVOUS SYSTEM.
_The spinal nerves._
The development of the spinal nerves has been already treated by me at
considerable length in a paper read before the Royal Society in December,
1875[247], and I have but little fresh matter to add to the facts narrated
in that paper. The succeeding account, though fairly complete, is much less
full than the previous one in the _Philosophical Transactions_, but a
number of morphological considerations bearing on this subject are
discussed.
Footnote 247: _Phil. Trans._ Vol. 166, p. 175. [This Edition,
No. VIII.]
The rudiments of the posterior roots make their appearance considerably
before those of the anterior roots. They arise during stage I, as
outgrowths from the spinal cord, at a time when the muscle-plates do not
extend beyond a third of the way up the sides of the spinal cord, and in a
part where no scattered mesoblast-cells are present. They are formed first
in the anterior part of the body and successively in the posterior parts,
in the following way. At a point where a spinal nerve is about to arise,
the cells of the dorsal part of the cord begin to proliferate, and the
uniform outline of the cord becomes broken (Pl. 14, fig. 3). There is
formed in this way a small prominence of cells springing from the summit of
the spinal cord, and constituting a rudiment of a pair of posterior roots.
In sections anterior to the point where a nerve is about to appear, the
nerve-rudiments are always very distinctly formed. Such a section is shewn
in Pl. 14, fig. 2, and the rudiments may there be seen as two club-shaped
masses of cells, which have grown outwards and downwards from the extreme
dorsal summit of the neural canal and in contact with its walls. The
rudiments of the two sides meet at their point of origin at the dorsal
median line, and are dorsally perfectly continuous with the walls of the
canal.
It is a remarkable fact that rudiments of posterior roots are to be seen in
every section. This may be interpreted as meaning that the rudiments are in
very close contact with each other, but more probably means, as I hope to
shew in the sequel, that there arises from the spinal cord a continuous
outgrowth from which discontinuous processes (the rudiments of posterior
roots) grow out.
After their first formation these rudiments grow rapidly ventralwards in
close contact with the spinal cord (vide Pl. 14, fig. 1, and Pl. 11, figs.
6 and 7), but soon meet with and become partially enclosed in the
mesoblastic tissue (Pl. 11, fig. 7). The similarity of the mesoblast and
nerve-tissue in Scyllium and Pristiurus embryos hardened in picric or
chromic acid, render the nerves in these genera, at the stage when they
first become enveloped in mesoblast, difficult objects to observe; but no
similar difficulty is encountered in the case of Torpedo embryos.
While the rudiments of the posterior roots are still quite short, those of
the anterior roots make their first appearance. Each of these (Pl. 14,
fig. 4, _a.r._) arises as a very small but distinct conical outgrowth from
a ventral corner of the spinal cord. From the very first the rudiments of
the anterior roots have an indistinct form of peripheral termination and
somewhat fibrous appearance, while the protoplasm of which they are
composed becomes attenuated towards its end. The points of origin of the
anterior roots from the spinal cord are separated by considerable
intervals. In this fact, and also in the fact of the nerves of the two
sides never being united with each other in the median line, the anterior
roots exhibit a marked contrast to the posterior. There are thus
constituted, before the close of stage I, the rudiments of both the
anterior and posterior roots of the spinal nerves. The rudiments of both of
these take their origin from the involuted epiblast of the neural canal,
and the two roots of each spinal nerve are at first quite unconnected with
each other. It is scarcely necessary to state that the pairs of roots
correspond in number with the muscle-plates.
It is not my intention to enter with any detail into the subsequent changes
of the rudiments whose origin has been described, but a few points
especially connected with their early development are sufficiently
important to call for attention.
One feature of the posterior roots at their first formation is the fact
that they appear as processes of a continuous outgrowth of the spinal cord.
This state of affairs is not of long continuance, and before the close of
stage I each posterior root has a separate junction with the spinal cord.
What then becomes of the originally continuous outgrowth? It has not been
possible for me to trace the fate of this step by step; but the discovery
that at a slightly later period (stage K) there is present a continuous
commissure independent of the spinal cord connecting the dorsal and central
extremities of all the spinal nerves, renders it very probable that the
original continuous outgrowth becomes converted into this commissure. Like
all the other nervous structures, this commissure is far more easily seen
in embryos hardened in a mixture of osmic and chromic acids or osmic acid,
than in those hardened in picric acid. Its existence must be regarded as
one of the most remarkable results of my researches upon the Elasmobranch
nervous system. At stage K it is fairly thick, though it becomes much
thinner at a slightly later period. Its condition during stage K is shewn
in Pl. 12, fig. 18, _com_. What it has been possible for me to make out of
its eventual fate is mentioned subsequently[248].
Footnote 248: It is not by any means always possible to detect
this commissure in transverse sections. As I have suggested, in
connection with a similar commissure connecting the vagus
branches, it perhaps easily falls out of the section, and is
always so small that the hole left would certainly be
invisible.
A second feature of the earliest condition of the posterior roots is their
attachment to the extreme dorsal summit of the spinal cord--a point of
attachment very different from that which they eventually acquire. Before
the commencement of stage K this state of things has become altered; and
the posterior roots spring from the spinal cord in the position normal for
Vertebrates.
This apparent migration caused me at first great perplexity, and I do not
feel quite satisfied that I have yet got completely to the bottom of its
meaning. The explanation which appears to me most probable has suggested
itself in the course of some observations on the development of the thin
roof of the fourth ventricle. A growth of cells appears to take place in
the median dorsal line of the roof of the spinal cord. This growth tends to
divaricate the two lateral parts of the cord, which are originally
contiguous in the dorsal line, and causes therefore the posterior roots,
which at first spring from the dorsal summit, to assume an apparent
attachment to the side of the cord at some little distance from the summit.
If this is the true explanation of the change of position which takes
place, it must be regarded as due rather to peculiar growths in the spinal
cord, than to any alteration in the absolute attachment of the nerves.
By stage K the rudiment of the posterior root has become greatly elongated,
and exhibits a division into three distinct portions (Pl. 14, fig. 6):
(1) A proximal portion, in which is situated the pedicle of attachment to
the wall of the neural canal.
(2) An enlarged portion, which may conveniently from its future fate be
called the spinal ganglion.
(3) A distal portion beyond this.
The proximal portion presents a fairly uniform diameter, and ends dorsally
in a rounded expansion; it is attached, remarkably enough, _not by its
extremity, but by its side, to the spinal cord. The dorsal extremities of
the posterior roots are therefore free._ It seems almost certain that the
free dorsal extremities of these roots serve as the starting points for the
dorsal commissure before mentioned, which connects the roots together. The
attachment of the posterior nerve-root to the spinal cord is, on account of
its small size, very difficult to observe. In favourable specimens there
may however be seen a distinct cellular prominence from the spinal cord,
which becomes continuous with a small prominence on the lateral border of
the nerve-root near its distal extremity. The proximal extremity of the
rudiment is composed of cells, which, by their small size and circular
form, are easily distinguished from those which form the succeeding or
ganglionic portion of the nerve. This succeeding part has a swollen
configuration, and is composed of large elongated cells with oval nuclei.
The remainder of the rudiment forms the commencement of the true nerve.
The anterior root, which, at the close of stage I, formed a small and
inconspicuous prominence from the spinal cord, grows rapidly during the
succeeding stages, and soon forms an elongated cellular structure with a
wide attachment to the spinal cord (Pl. 14, fig. 5). At first it passes
obliquely and nearly horizontally outwards, but, before reaching the
muscle-plate of its side, takes a bend downwards (Pl. 14, fig. 7).
I have not definitely made out when the anterior and posterior roots unite,
but this may easily be seen to take place before the close of stage K
(Pl. 12, fig. 18).
One feature of some interest with reference to the anterior roots, is the
fact that they arise not vertically below, but alternately with the dorsal
roots, a condition which persists in the adult.
Although I have made some efforts to determine the eventual fate of the
commissure uniting the dorsal roots, these have not hitherto been crowned
with success. It grows thinner and thinner, becoming at the same time
composed of fibrous protoplasm with imbedded nuclei (Pl. 14, figs. 8 and
9). By stage M it is so small as to be quite indistinguishable in
transverse sections; and I have failed in stage P to recognize it at all. I
can only conclude that it gradually atrophies, and finally vanishes without
leaving a trace. Both its appearance and history are very remarkable, and
deserve the careful attention of future investigators.
There can be little doubt that it is some sort of remnant of an ancestral
structure in the nervous system; and it would appear to indicate that the
central nervous system must originally have been formed of a median and two
lateral strands. At the same time I very much doubt whether it can be
brought into relation with the three rows of ganglion-cells (a median and
two lateral) which are so frequently present on the ventral side of
annelidan nerve-cords.
_My results may be summarised as follows_:--Along the extreme dorsal summit
of the spinal cord there arises on each side a continuous outgrowth. From
each outgrowth processes corresponding in number to the muscle-plates grow
downwards. These are the rudiments of the posterior nerve-roots. The
outgrowths, though at first attached to the spinal cord throughout their
whole length, soon cease to be so, and remain in connection with it at
certain points only, which form the primitive junctions of the posterior
roots with the spinal cord. The original outgrowth on each side remains as
a bridge, uniting together the dorsal extremities of all the posterior
roots. The posterior roots, though primitively attached to the dorsal
summit of the spinal cord, eventually come to arise from its sides. The
original homogeneous rudiments before the close of stage K become
differentiated into a root, a ganglion, and a nerve.
The anterior roots, like the posterior, are outgrowths from the spinal
cord, but are united independently with it, and the points from which they
spring originally, remain as those by which they are permanently attached.
The anterior roots arise, not vertically below, but in the intervals
between the posterior roots. They are at first quite separate from the
posterior roots; but before the close of stage K a junction is effected
between each posterior root and the corresponding anterior root. The
anterior root joins the posterior at some little distance below its
ganglion.
* * * * *
The results here arrived at are nearly in direct opposition to those of the
majority of investigators, though in accordance, at least so far as the
posterior roots are concerned, with the beautiful observations of Hensen
'on the Development of Mammalia[249].'
Footnote 249: _Zeit. f. Anat. u. Entwicklungsgeschichte_, Vol. I.
Mr Marshall[250] has more recently published a paper on the development of
the nerves in Birds, in which he shews in a most striking manner that the
observations recorded here for Elasmobranchii hold good for the posterior
roots of Birds. The similarity between his figures and my own is very
noticeable. A further discussion of the literature would be quite
unprofitable, and I proceed at once to certain considerations suggested by
the above observations.
Footnote 250: _Journal of Anatomy and Physiology_, Vol. XI.
April, 1877.
_General considerations._ One point of general anatomy upon which my
observations throw considerable light, is the _primitive origin of nerves_.
So long as it was admitted that the spinal and cerebral nerves developed in
the embryo independently of the central nervous system, their mode of
origin always presented to my mind considerable difficulties. It never
appeared clear how it was possible for a state of things to have arisen in
which the central nervous system as well as the peripheral terminations of
nerves, whether motor or sensory, were formed independently of each other;
while between them a third structure was developed, which, growing out
either towards the centre or towards the periphery, ultimately brought the
two into connection. That such a condition could be a primitive one seemed
scarcely possible.
Still more remarkable did it appear, on the supposition that the primitive
mode of formation of these parts was represented in the developmental
history of Vertebrates, that we should find similar structural elements in
the central and in the peripheral nervous systems. The central nervous
system arises from the epiblast, and yet contains precisely similar
nerve-cells and nerve-fibres to the peripheral nervous system, which, when
derived from the mesoblast, was necessarily supposed to have an origin
completely different from that of the central nervous system. Both of these
difficulties are to a great extent removed by the facts of the development
of these parts in Elasmobranchii.
It is possible to suppose that in their primitive differentiation
contractile and sensory systems may, as in Hydra[251], have been developed
from the protoplasm of even the same cell. As the sensory and motor systems
became more complicated, the sensory portion of a cell would become
separated by an increasing interval from the muscular part of a cell, and
the two parts of a cell would only be connected by a long protoplasmic
process. When such a condition as that was reached, the sensory portion of
the cell would be called a ganglion-cell or terminal sensory organ, the
connecting process a nerve, and the contractile portion of the cell a
muscle-cell. When these organs were in this condition, it might not
impossibly happen for the general developmental growth which tended to
separate the ganglion-cell and the muscle-cell to be so rapid as to render
it impossible for the growth of the connecting nerve to keep pace with it,
and that thus the process connecting the ganglion-cell and the muscle-cell
might become ruptured. Nevertheless the tendency of the process to grow
from the ganglion-cell to the muscle-cell, would remain, and when the rapid
developmental growth had ceased, the two would become united again by the
growth of the process which had previously been ruptured. It will be seen
that this hypothesis, which I have considered only with reference to a
single nerve and muscle-cell, might be extended so as to apply to a
complicated central nervous system and peripheral nerves and muscles, and
also could apply equally as well to the sensory as to the motor
terminations of a nerve. In the case of the sensory termination, we should
only have to suppose that the centre nervous cell became more and more
separated by the general growth from the recipient terminal sensory cell,
and that during the general growth the connection between the two was
mechanically ruptured but restored again on the termination of the more
rapid growth.
Footnote 251: Kleinenberg Hydra.
As the descendants of the animal in which the rupture occurred became
progressively more complicated, the two terminal cells must have become
widely separated at a continually earlier period, till finally they may
have been separated at a period of development when they were
indistinguishable from the surrounding embryonic cells; and since the
rupture would also occur at this period, the primitive junction between the
nerve-centre and termination would escape detection. The object of this
hypothesis is to explain the facts, so far as they are known, of the
development of the nervous system in Vertebrates.
In Vertebrates we certainly appear to have an outgrowth from the nervous
system, which eventually becomes united with the muscle or sensory terminal
organs. The ingenious hypothetical scheme of development of the nerves
given by Hensen[252] would be far preferable to the one suggested if it
could be brought into conformity with the facts. There is, however, at
present no evidence for Hensen's view, as he himself admits, but
considering how little we know of the finer details of the development of
nerves, it seems not impossible that such evidence may be eventually
forthcoming. The evidence from my own observation is, so far as it goes,
against it. At a time anterior to the outgrowth of the spinal nerves, I
have shewn[253] that the spinal cord is completely invested by a delicate
hyaline membrane. It is difficult to believe that this is pierced by a
number of fine processes, which completely escape detection, but which
must, nevertheless, be present on the hypothesis of Hensen.
Footnote 252: Virchow's _Archiv_, Vol. XXXI. 1864.
Footnote 253: _Phil. Trans._, 1876. [This Edition, No. VIII.]
The facts of the development of nerves in Vertebrates are unquestionably
still involved in considerable doubt. It may, I think, be considered as
certain, that in Elasmobranchii the roots of the spinal and cranial nerves
are outgrowths of the central nervous system. How the final terminations of
the nerves are formed is, however, far from being settled. Götte[254],
whose account of the development of the spinal ganglia is completely in
accordance with the ordinary views, yet states[255] that the growth of the
nerve fibres themselves is a centrifugal one from the ganglia. My own
investigations prove that the ganglia have a centrifugal development, and
also appear to demonstrate that the nerves themselves near the ganglion
have a similar manner of growth. Moreover, the account given in the
preceding chapter of the manner in which the nerves become connected with
the mucous canals of the head, goes far to prove that the whole growth of
the nerves is a centrifugal one. The combination of all these converging
observations tells strongly in favour of this view.
Footnote 254: _Entwicklungsgeschichte der Unke._
Footnote 255: _Loc. cit._ p. 516.
On the other hand, Calberla[256] believes that in the tails of larval
Amphibians he has seen connective-tissue cells unite with nerve-processes,
and become converted into nerves, but he admits that he cannot definitely
prove that the axis-cylinder has not a centrifugal growth, while the
connective-tissue cells merely become converted into the sheath of the
nerve. If Calberla's view be adopted, that the nerves are developed
directly out of a chain of originally indifferent cells, each cell of the
chain being converted in turn into a section of the nerve, an altogether
different origin of nerves from that I have just suggested would seem to be
indicated.
Footnote 256: _Archiv für Micros. Anat._, Vol. XI. 1875.
The obvious difficulty, already alluded to, of understanding how it is,
according to the generally accepted mode of development of the spinal
nerves, that precisely similar nerve-cells and nerves should arise in
structures which have such different origins as the central nervous system
and the spinal nerves, is completely removed if my statements on the
development of the nerves in Elasmobranch represent the truth.
One point brought out in my investigations appears to me to have bearings
upon the origin of the central canal of the vertebrate nervous system, and
in consequence upon the origin of the vertebrate nervous system itself.
This point is, that the posterior nerve-rudiments make their first
appearance at the extreme dorsal summit of the spinal cord. The transverse
section of the ventral nervous cord of an ordinary segmented Annelid
consists of two symmetrical halves placed side by side. If by a mechanical
folding the two lateral halves of the nervous cord became bent towards each
other, while into the groove between the two the external skin became
pushed, we should have an approximation to the vertebrate nervous system.
Such a folding as this might take place to give extra rigidity to the body
in the absence of a vertebral column.
If this folding were then completed in such a way that the groove, lined by
external skin and situated between the two lateral columns of the nervous
system, became converted into a canal, above and below which the two
columns of the nervous system united, we should have in the transformed
nervous cord an organ strongly resembling the spinal cord of Vertebrates.
It is well known that the nerve-cells are always situated on the ventral
side of the abdominal nerve-cord of Annelids, either as a continuous layer,
or in the form of two, or more usually, three bands. The dorsal side of the
cord is composed of nerve-fibres or white matter. If the folding I have
supposed were to take place in the Annelid nervous cord, the grey and white
matters would have very nearly the same relative situations as they have in
the Vertebrate spinal cord. The grey matter would be situated in the
interior and line the central canal, and the white matter would nearly
surround the grey. The nerves would then arise, not from the sides of the
nervous cord as in existing Annelids, 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[257] arise, in the exact
manner which might have been anticipated, were the spinal canal due to such
a folding as I have suggested.
Footnote 257: There are strong reasons for regarding the
posterior roots as the primitive ones. These are spoken of
later, but I may state that they depend:
(1) On the fact that only _posterior_ roots exist in the brain.
(2) That only posterior roots exist in Amphioxus.
(3) That the posterior roots develop at an earlier period than
the anterior.
The argument from the position of the outgrowth of nerves becomes the more
striking from its great peculiarity, and forms 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 in certain cases may, perhaps, be
explained as a remnant of the ciliation of the external skin still found
amongst many of the lower Annelids.
I have 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 cord, and the _à
posteriori_ evidence we have of the vertebrate canal having been formed in
the way indicated. I have not made use of what is really my strongest
argument, viz. that the embryological mode of formation of the spinal canal
by a folding in of the external epiblast is the very method by which I
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.
Although the homologies between the Vertebrate and the Annelidan nervous
systems are not necessarily involved in the questions which arise with
reference to the formation of the spinal canal, they have nevertheless
considerable bearings on it.
Two views have recently been put forward on this subject. Professor
Gegenbaur[258] looks upon the central nervous system of Vertebrates as
equivalent to the superior oesophageal ganglia of Annelids and Arthropods
only, while Professors Leydig[259] and Semper[260] and Dr Dohrn[261]
compare it with the whole Annelidan nervous system.
Footnote 258: _Grundriss d. vergleichenden Anat._ p. 264.
Footnote 259: _Bau des thierischen Körpers._
Footnote 260: _Stammesverwandtschaft d. Wirbelthiere u.
Wirbellosen_ and _Die Verwandtschaftsbeziehungen d.
gegliederten Thiere_. This latter work, for a copy of which I
return my best thanks to the author, came into my hands after
what follows was written, and I much regret only to have been
able to make one or two passing allusions to it. The work is a
most important contribution to the questions about to be
discussed, and contains a great deal that is very suggestive;
some of the conclusions with reference to the Nervous System
appear to me however to be directly opposed to the observations
on Spinal Nerves above recorded.
Footnote 261: _Ursprung d. Wirbelthiere u. Princip des
Functionswechsels._
The first of these two views is only possible on the supposition that
Vertebrates are descended from unsegmented ancestors, and even then
presents considerable difficulties. If the ancestors of Vertebrates were
segmented animals, and several of the recent researches tend to shew that
they were, they must almost certainly have possessed a nervous cord like
that of existing Annelids. If such were the case, it is almost
inconceivable that the greater portion of the nervous system which forms
the ventral cord can have become lost, and the system reduced to the
superior oesophageal ganglia. Dr Dohrn[262], who has speculated very
profoundly on this matter, has attempted to explain and remove some of the
difficulties which arise in comparing the nervous systems of Vertebrates
and Annelids. He supposes that the segmented Annelids, from which
Vertebrates are descended, were swimming animals. He further supposes that
their alimentary canal was pierced by a number of gill-slits, and that the
anterior amongst these served for the introduction of nutriment into the
alimentary canal, in fact as supplementary mouths as well as for
respiration. Eventually the old mouth and throat atrophied, and one pair of
coalesced gill-slits came to serve as the sole mouth. Thus it came about
that on the disappearance of that portion of the alimentary canal, which
penetrated the oesophageal nervous ring, the latter structure ceased to be
visible as such, and no part of the alimentary canal was any longer
enclosed by a commissure of the central nervous system. With the change of
mouth Dr Dohrn also supposes that there took place a change, which would
for a swimming animal be one of no great difficulty, of the ventral for the
dorsal surface. This general explanation of Dr Dohrn's, apart from the
considerable difficulty of the fresh mouth, appears to me to be fairly
satisfactory. Dr Dohrn has not however in my opinion satisfactorily dealt
with the questions of detail which arise in connection with this
comparison. One of the most important points for his theory is to settle
the position where the nervous system was formerly pierced by the
oesophagus. This position he fixes in the fourth ventricle, and supports
his hypothesis by the thinness of the roof of the spinal canal in this
place, and the absence (?) of nervous structures in it.
Footnote 262: _Loc. cit._
It appears to me that this thinness cannot be used as an argument. In the
first place, if the hypothesis I have suggested as to the formation of the
spinal canal be accepted, the formation of the canal must be supposed to
have occurred in point of time either after or before the loss of the
primitive mouth. If, on the one hand, the spinal canal made its appearance
before the atrophy of the primitive mouth, the folding to form it must
necessarily have ceased behind the mouth; and, on the supposition of the
oesophageal ring having been situated in the region of the fourth
ventricle, a continuation of the spinal canal could not be present in front
of this part. If, on the other hand, the cerebro-spinal canal appeared
after the disappearance of the primitive mouth, its roof must necessarily
also be a formation subsequent to the atrophy of the mouth, and varieties
of structure in it can have no bearing upon the previous position of the
mouth.
But apart from speculations upon the origin of the spinal cord, there are
strong arguments against Dr Dohrn's view about the fourth ventricle. In the
first place, were the fourth ventricle to be the part of the nervous system
which previously formed the oesophageal commissures, we should expect to
find the opening in the nervous system at this point to be visible at an
early period of development, and at a later period to cease to be so. The
reverse is however the case. In early embryonic life the roof of the fourth
ventricle is indistinguishable from other parts of the nervous system, and
only thins out at a later period. Further than this, any explanation of the
thin roof of the fourth ventricle ought also to elucidate the nearly
similar structure in the sinus rhomboidalis, and cannot be considered
satisfactory unless it does so.
The peculiarities of the cerebro-spinal canal in the region of the brain
appear to me to present considerable difficulties in the way of comparing
the central nervous system of Vertebrates and segmented Annelids. The
manner in which the cerebro-spinal canal is prolonged into the optic
vesicles, the cerebral and the optic lobes is certainly opposed both to an
intelligible explanation of the spinal canal itself, and also to a
comparison of the two nervous systems under consideration.
Its continuation into the cerebral hemispheres and into the optic lobes
(mid-brain) may perhaps be looked upon as due to peculiar secondary growths
of those two ganglia, but it is very difficult to understand its
continuation into the optic vesicles.
If it be granted that the spinal canal has arisen from a folding in of the
external skin, then the present inner surface of the optic vesicle must
also have been its original outer surface, and it follows as a necessary
consequence that the present position of the rods and cones behind and not
in front of the nervous structures of the retina was not the primitive one.
The rods and cones arise, as is well known, from the inner surface of the
outer portion of the optic vesicle, and must, according to the above view,
be supposed originally to have been situated on the external surface, and
have only come to occupy their present position during the folding in,
which resulted in the spinal canal. On _à priori_ grounds we should
certainly expect the rods and cones to have resulted from the
differentiation of a layer of cells external to the conducting nervous
structures. The position of the rods and cones posterior to these suggests
therefore that some peculiar infolding has occurred, and may be used as an
argument to prove that the medullary groove is no mere embryonic structure,
but the embryonic repetition of an ancestral change. The supposition of
such a change of position in the rods and cones necessarily implies that
the folding in to form the spinal canal must have been a very slow one. It
must have given time to the refracting media of the eye gradually to travel
round, so as still to maintain their primitive position, while in
successive generations a rudimentary spinal furrow carrying with it the
retina became gradually converted into a canal[263].
Footnote 263: Professor Huxley informs me that he has for many
years entertained somewhat similar views to those in the text
about the position of the rods and cones, and has been
accustomed to teach them in his lectures.
If Dr Dohrn's comparison of the vertebrate nervous system with that of
segmented Annelids be accepted, the following two points must in my opinion
be admitted:--
(1) That the formation of the cerebro-spinal canal was subsequent to the
loss of the old mouth.
(2) That the position of the old mouth is still unknown.
The well-known view of looking at the pituitary and pineal growths as the
remnants of the primitive oesophagus, has no doubt some features to
recommend it. Nearly conclusive against it is the fact that the pituitary
involution is not, as used to be supposed, a growth towards the
infundibulum of the hypoblast of the oesophagus, but of the epiblast of the
mouth. It is almost inconceivable that an involution from the present mouth
can have assisted in forming part of the old oesophagus.
There is a view not involving the difficulty of the oesophageal ring, fresh
mouth[264], and of the change of the ventral to the dorsal surface, which,
though so far unsupported by any firm basis of observed facts, nevertheless
appears to me worth suggesting. It assumes that Vertebrates are descended
_not_ through the present line of segmented Vermes, but through some other
line which has now, so far as is known, completely vanished. This line must
be supposed to have originated from the same _unsegmented Vermes_ as the
present segmented Annelids. They therefore acquired fundamentally similar
segmental and other Annelidan organs.
Footnote 264: Professor Semper ("Die
Verwandtschaftsbeziehungen d. gegliederten Thiere," _Arbeiten
aus d. Zool.-zoot. Institut_, Würzburg, 1876) has some
interesting speculations on the difficult question of the
vertebrate mouth, which have unfortunately come to my knowledge
too late to be either fully discussed or incorporated in the
text. These speculations are founded on a comparison of the
condition of the mouth in Turbellarians and Nemertines. He
comes to the conclusion that there was a primitive mouth on the
cardiac side of the supra-oesophageal ganglion, which is the
existing mouth of Turbellarians and Vertebrates and the opening
of the proboscis of Nemertines, but which has been replaced by
a fresh mouth on the neural side in Annelids and Nemertines. In
Nemertines however the two mouths co-exist--the vertebrate
mouth as the opening of the proboscis, and the Annelid mouth as
the opening for the alimentary tract. This ingenious hypothesis
is supported by certain anatomical facts, which do not appear
to me of great weight, but for which the reader must refer to
the original paper. It no doubt avoids the difficulty of the
present position of the vertebrate mouth, but unfortunately at
the same time substitutes an equal difficulty in the origin of
the Annelidan mouth. This Professor Semper attempts to get over
by an hypothesis which to my mind is not very satisfactory (p.
378), which, however, and this Professor Semper does not appear
to have noticed, _could equally well be employed to explain the
origin of a Vertebrate mouth as a secondary formation
subsequent to the Annelidan mouth_. Under these circumstances
this fresh hypothesis does not bring us very much nearer to a
solution of the vertebrate-annelid mouth question, but merely
substitutes one difficulty for another; and does not appear to
me so satisfactory as the hypothesis suggested in the text.
At the same time Professor Semper's hypothesis suggests an
explanation of that curious organ the Nemertine proboscis. If
the order of changes suggested by him were altered it might be
possible to suppose that there never was more than one mouth
for all Vermes, but that the proboscis in Nemertines gradually
split itself off from the oesophagus to which it originally
belonged, and became quite free and provided with a separate
opening and perhaps carried with it the so-called vagus of
Professors Semper and Leydig.
The difference between the two branches of the Vermes lay in the nervous
system. The unsegmented ancestors of the _present_ Annelids seem to have
had a pair of super-oesophageal ganglia, from which two main nervous stems
extended backwards, one on each side of the body. Such a nervous system in
fact as is possessed by existing Nemertines or Turbellarians[265]. As the
Vermes became segmented and formed the Annelids, these side nerves seem to
have developed ganglia, corresponding in number with the segments, and
finally, approximating on the ventral surface, to have formed the ventral
cord[266].
Footnote 265: It is not of course to be supposed that the
primitive nervous system was pierced by a proboscis like that
of the Nemertines.
Footnote 266: This is Gegenbaur's view of the development of
the ventral cord, and I regard it in the meantime as the most
probable view which has been suggested.
The other branch of Vermes which I suppose to have been the ancestors of
Vertebrates started from the same stock as existing Annelids, but I
conceive the lateral nerve-cords, instead of approximating ventrally, to
have done so dorsally, and thus a dorsal cord to have become formed
analogous to the ventral cord of living Annelids, only without an
oesophageal nerve-ring[267].
Footnote 267: A dorsal instead of a ventral approximation of
the lateral nerve-cords would be possible in the descendants of
such living segmented Vermes as Saccocirrus and Polygordius.
It appears to me, (if the difficulties of comparing the Annelidan ventral
cord with the spinal cord of Vertebrates are found to be insurmountable),
that this hypothesis would involve far fewer improbabilities than one which
supposes the whole central nervous system of Vertebrates to be homologous
with the super-oesophageal ganglia. The mode of formation of a nervous
system presupposed in my hypothesis, well accords with what we know of the
formation of the ventral cord in existing Annelids.
The supposition of the existence of another branch of segmented Vermes is
not a very great difficulty. Even at the present day we have possibly more
than one branch of Vermes which have independently acquired segmentation.
viz.: the Chætopodous Annelids and the Hirudinea. If the latter is an
isolated branch, it is especially interesting from having independently
developed a series of segmental organs like those of Chætopodous Annelids,
which we must suppose the ancestors of Vertebrates also to have done if
they too form an independent branch.
In addition to the difficulty of imagining a fresh line of segmented
Vermes, there is another difficulty to my view, viz.: the fact that in
almost all Vermes, the blood flows forwards in the dorsal vessel, and
backwards in the ventral vessel. This condition of the circulation very
well suits the view of a change of the dorsal for the ventral surfaces, but
is opposed to these surfaces being the same for Vertebrates and Vermes. I
cannot however regard this point as a very serious difficulty to my view,
considering how undefined is the circulation in the unsegmented groups of
the Vermes.
_Sympathetic nervous system._
Between stages K and L there may be seen short branches from the spinal
nerves, which take a course towards the median line of the body, and
terminate in small irregular cellular masses immediately dorsal to the
cardinal veins (Pl. 18, fig. 1, _sy.g._). These form the first traces that
have come under my notice of the sympathetic nervous system. In the
youngest of my embryos in which I have detected these it has not been
possible for me either definitely to determine the antero-posterior limits
of the system, or to make certain whether the terminal masses of cells
which form the ganglia are connected by a longitudinal commissure. In a
stage slightly younger than L the ganglia are much more definite, the
anterior one is situated in the cardiac region close to the end of the
intestinal branch of the vagus, and the last of them quite at the posterior
end of the abdominal cavity. The anterior ganglia are the largest; the
commissural cord, if developed, is still very indistinct. In stage L the
commissural cord becomes definite, though not very easy to see even in
longitudinal sections, and the ganglia become so considerable as not to be
easily overlooked. They are represented in Pl. 13, fig. 1, _sy.g._ and in
Pl. 18, fig. 2, in the normal position immediately above the cardinal
veins. The branches connecting them with the trunks of the spinal nerves
may still be seen without difficulty. In later stages these branches cannot
so easily be made out in sections, but the ganglia themselves continue as
fairly conspicuous objects. The segmental arrangement of the ganglia is
shewn in Pl. 18, fig. 3, a longitudinal and vertical section of an embryo
between stages L and M with the junctions of the sympathetic ganglia and
spinal nerves. The ganglia occupy the intervals between the successive
segments of the kidneys.
The sympathetic system only came under my notice at a comparatively late
period in my investigations, and the above facts do not in all points clear
up its development[268]. My observations seem to point to the sympathetic
system arising as an off-shoot from the cerebrospinal system. Intestinal
branches would seem to be developed on the main nerve stems of this in the
thoracic and abdominal regions, each of these then develops a ganglion, and
the ganglia become connected by a longitudinal commissure. On this view a
typical spinal nerve has the following parts: two roots, a dorsal and
ventral, the dorsal one ganglionated, and three main branches, (1) a ramus
dorsalis, (2) a ramus ventralis, and (3) a ramus intestinalis. This scheme
may be advantageously compared with that of a typical cranial nerve
according to Gegenbaur. It may be noted that it brings the sympathetic
nervous system into accord with the other parts of the nervous system as a
product of the epiblast, and derived from outgrowths from the neural axis.
It is clear, however, that my investigations, though they may naturally be
interpreted in this way, do not definitely exclude a completely different
method of development for the sympathetic system.
Footnote 268: The formation out of the sympathetic ganglia of
the so-called paired suprarenal bodies is dealt with in
connection with the vascular system. The original views of
Leydig on these bodies are fully borne out by the facts of
their development.
EXPLANATION OF PLATE 14.
_This Plate illustrates the Formation of the Spinal Nerves._
COMPLETE LIST OF REFERENCE LETTERS.
_ar._ Anterior root of a spinal nerve. _ch._ Notochord. _com._ Commissure
connecting the posterior roots of the spinal nerves. _i._ Mesoblastic
investment of spinal cord. _mp._ Muscle-plate. _n._ Spinal nerve. _nc._
Neural canal. _pr._ Posterior root of a spinal nerve. _spg._ Ganglion on
posterior root of spinal nerve. _v.r._ Vertebral rudiment. _w._ White
matter of spinal cord. _y._ Point where the spinal cord became segmented
off from the superjacent epiblast.
Figs. 1, 2, and 3. Three sections of a Pristiurus embryo belonging to stage
I. Fig. 1 passes through the heart, fig. 2 through the anterior part of the
dorsal region, fig. 3 through a point slightly behind this. (Zeiss CC,
ocul. 2.) In fig. 3 there is visible a slight proliferation of cells from
the dorsal summit of the neural canal. In fig. 2 this proliferation
definitely constitutes two club-shaped masses of cells (_pr_)--the
rudiments of the posterior nerve-roots,--both attached to the dorsal summit
of the spinal cord. In fig. 1 the rudiments of the posterior roots are of
considerable length.
Fig. 4. Section through the dorsal region of a Torpedo embryo slightly
older than stage I, with three visceral clefts. (Zeiss CC, ocul. 2.) The
section shews the formation of a pair of dorsal nerve-rudiments (_pr_) and
a ventral nerve-rudiment (_ar_). The latter is shewn in its youngest
condition, and is not distinctly cellular.
Fig. 5. Section through the dorsal region of a Torpedo embryo slightly
younger than stage K. (Zeiss CC, ocul. 2.) The connective-tissue cells are
omitted. The rudiment of the ganglion (_spg_) on the posterior root has
appeared, and the junction of posterior root with the cord is difficult to
detect. The anterior root forms an elongated cellular structure.
Fig. 6. Section through the dorsal region of a Pristiurus embryo of stage
K. (Zeiss CC, ocul. 2.) The section especially illustrates the attachment
of the posterior root to the spinal cord.
Fig. 7. Section through the same embryo as fig. 6. (Zeiss CC, ocul. 1.) The
section contains an anterior root, which takes its origin at a point
opposite the interval between two posterior roots.
Fig. 8. A series of posterior roots with their central ends united by a
dorsal commissure, from a longitudinal and vertical section of a Scyllium
embryo belonging to a stage intermediate between L and M. The embryo was
hardened in a mixture of osmic and chromic acids.
Fig. 9. The central end of a posterior nerve-root from the same embryo,
with the commissure springing out from it on either side.