haematoxylin (0.5 ), and afterwards treated with a solution of
potassium-bichromate (0.5 I %) as recommend by HEYDENHAIN
(vide p. 76). Now the structures are washed, again and hardened,
somewhat quickly in alcohol gradatim. Preparations treated in this
way can, as a rule, be very easily cut into thin sections, directly
enclosed in paraffin only (vide p. 76). Embedding in celloidin or
paraffin can of course also be employed; this can not, however, be
performed with too great care as we have here to deal with struc-
tures of the most delicate nature.
In preparations carefully treated in this way, the nerve-tubes,
with their contents, are fixed in their natural position and state, and
no visible change of the form has taken place.
If we now examine good and thin longitudinal sections, it will
already, under low powers of the microscope, be easy to observe
a distinct longitudinal striation in the larger as well as the smaller
nerve-tubes. The extremely slender longitudinal lines or fibrillae
have got a distinct blackish staining, and are situated with intervals
between them, just similar to what we have found in the fresh state.
The substance in these intervals is not stained, and has a homogeneous
or rather a slightly granular appearance. If now, however, the
stained fibrillae are real fibrillae swimming in this homogeneous un-
stained substance, it is evident that they, in thin transverse sections
of nerve-tubes, must appear as minute black dots or points, just
similar to what, VlGNAL for instance, has also really described and
But what do we find? - Instead of black dots, ^ve find in the
sections of the tubes a delicate reticulation, with minute circular
meshes, apparently formed by extremely slender filaments (fig. 2; fig. 5> t).
In these filaments we certainly find small dark granules (fig. 6, o)
which, occur however, only in the point or knots where several
filaments unite; they never occur in the centres of the meshes. And
the same reticulation is extended through the whole transverse sec-
tion of every nerve-tube, and fills the whole space inside the
If we, now, compare the impressions which transverse sections of
the nerve-tubes give, with those obtained by longitudinal sections, there
can, in my opinion, be no doubt left but, that the ,,fibrillce" of most
writers belong to a substance, spongioplasm, forming a bundle af
slender, cylindrical tubes or channels enclosed in the neur Hem- sheath
of each nerve-tube. These primitive tubes, if we may call them so,
are filled with the homogeneous, viscous substance, hyaloplasm, which
we already know from fresh nerve-tubes. 1 } The granules in the transverse
sections show themselves to be transsected thicker longitudinal fibres
of spongioplasm which, occur especially along the longitudinal edges
where several tubes, usually three, meet. If we imagine the spongio-
plasm as forming cylindrical tubes laid or pressed together, it will
be evident, that the spongioplasmic walls of the different tubes will
unite and form septa, as the spongioplasm cannot, of course, be
considered as a quite solid and unadherent substance; it will also be
evident that, in the corners where several tubes meet the spongio-
plasmic walls will be still thicker. 2 )
In this way, I think, we may easily understand why it was so
extremely difficult to isolate and get a clear idea of SCHULTZE's
Primitivfibrillen. As we have seen, they do not exist in the way
he has explained ; what he called fibrillae, are the spongioplasmic walls
between the real primitive fibrillae or primitive tubes as I have
The above description refers to the structure of nerve tubes,
which are, I think, in the most primary state.
There are, however, in the longitudinal commissures, as well as
also in the peripheral nerves a great many tubes exhibiting a kind of
concentration towards an axis in their centre. As described p. Si,
a more or less concentrated longitudinal striation was visible in the
a ) The primitive tubes have a diameter of about .0015 .0017 Mm.; their
size vary, however, and is very difficult to measure.
2 ) There occur, however, besides these granules larger dark granules, as well
in transverse sections as in longitudinal ones. They are usually very sparingly spread,
especially near the centres of the nerve-tubes, as will shortly be mentioned and
hey are always situated in the spongioplasmic walls of the primitive tubes.
- 8; -
centre of many large nerve-tubes. If we now examine similar nerve-
tubes in transverse and longitudinal sections we will find that, their
contents consist of a bundle of slender cylindrical primitive tubes,
quite in the same way as that just mentioned. The only difference is
that, their central primitive-tubes have a smaller diameter and thicker
walls, and are more deeply stained than the peripheral ones, it
seems, indeed, as if they may have been pressed more tightly
together and thus been obliged to occupy a smaller space, as they have
on the other hand got a firmer consistency, with thicker walls. In
this axis, and in its neighbourhood, larger dark granules, as mentioned
above, also occur more frequently than anywhere else, which
perhaps contributes somewhat to the darker staining. In some nerve-
tubes this concentration and forming of an axis is so far developed
that it, in transverse sections (fig. 3), appears as a, by osmic-acid
and haematoxylin, deeply stained spot in the centre of the tube.
In this spot it is not easy to distinguish any structure or primitive
tubes, but in extremely thin sections, and by very high powers, it
is, however possible to see slender tubes with thick, deeply stained,
walls or membranes in which granules occur. In longitudinal sections
a longitudinal striation appears as in fig. 4. In one end (fig. 4, a) this
section has passed through the periphery of the axis and, here, a
striation is distinctly visible; in the other end (fig. 4, a 1 ) the section
has, however, passed more through the centre of the axis and, here,
the staining is so deep that almost no striation is visible, all the less
from the section being somewhat thick.
Usually however this concentration is not so distinctly developed
as here. Nay, we can indeed find every degree of development,
from the primary state where no concentration at all can possibly
be traced to have taken place, up to an axis as described.
Fig. 2, t, t', t" thus represent sections of fibres with different degrees
of concentration, from a very slight one where we can only see
that the primitive tubes are somewhat deeper stained in this central
part than in the rest of the tube and in some nervn-tubes even this
is not visible (*).
It is the nerve-tubes with such an axis that, previous writers
have called myeloid fibres; with what right we will, on a later
occasion, have an opportunity of examining.
However we consider this formation of an axis ; in one thing
we cannot be in doubt, viz., that the whole contents of the
nerve-tubes, wether they have an axis or not, is of real nervous
nature, because the constructing element in the ivhole contents through-
out is, as we have seen, the primitive tube"; that is the thread o
which the whole rope is woven.
In transverse sections of some nerve-tubes, largish vacuoli or,
as it were, primitive tubes, can be seen (fig. 5, t\ t" , t"'\ fig. 2, d).
These vacuoli often occur in a peripheral layer just inside the
sheath of the nerve-tubes (fig. 5, t\ "). I suppose, however, this
appearance to be to a great extent artificially produced. In some
cases the whole contents of the nerve-tubes has the same appearance
(fig. 5, ^) and looks as if it were principally constituted of large pri-
mitive tubes, amongst which only some few primitive tubes of the
common size are seen. I do not feel disposed to suppose this
appearance to be only an artificial and postmortem product, though
I cannot with certainty account for its nature at present.
We have, hitherto, only mentioned nerve-tubes of a relatively
large diameter; these are especially numerous in the longitudinal and
oesophageal commissures, as will be seen from fig. I, which re-
presents a transverse section of an oesophageal commissure. We
find, there, transsected nerve-tubes of very varying diameter. l ) In a
great many large tubes (t, t\ #,) a deeply stained axis is visible; in
other large tubes (") no axis is visible, n.t represent somewhat
smaller nerve-tubes running in a thick bundle along the centre of
the commissure, s.nt represent very small nerve-tubes situated more
Large nerve-tubes, usually having an axis, occur very frequently
also in the peripheral nerves, vide fig. 7, t\ they have generally, as
will be seen, very stout sheaths, and are prominent.
On the other hand there are, especially in the peripheral nerves,
a great multitude of extremely slender nerve-tubes; indeed, the
peripheral nerves principally consist of such nerve-tubes (fig. 7, nt).
These nerve-tubes have so small a diameter that, I have, usually,
only observed some few primitive tubes inside their sheaths (vide
fig. 8, nt)\ in a large number of them I have even detected no pri-
mitive tubes, and I believe they are partly constructed of only one
primitive tube, the sheath of which is, however, much stouter than
the spongioplasmic sheaths inside the larger nerve-tubes. 2 )
These slender nerve-tubes are, in most nerves, usually arranged
or united in bundles, and are enclosed in neurilem-sheaths, larger
fagots of these primitive bundles are again enclosed in larger and
] ) We can find nerve-tubes with a diameter of more than 0.140 Mm. ; and
we can find nerve-tubes with a diameters of less than 0.003 Mm.
'*) The diameter of these smallest nerve-tubes measures about .0017 Mm.
stouter neurilem-sheaths, and so on, concentrically, untill at last the
whole nerve is enclosed in one common external neurilem-sheath. 1 )
It is, indeed, very difficult to tell here where the separation of indi-
vidual nerve-tubes really begins, if we do not, as I have done, con-
sider the smallest tubes with distinct stout sheaths as representing,
^each of them, a nerve-tube, consisting of some few primitive tubes
only, or, in a great many cases, of one primitive tube only.
When we examine a transverse section of a nerve, we will, as
a rule, observe a great many dark granules or dots; these dots,
appear, however, on closer examination, to be thickenings in the
tube-sheaths along the longitudinal edges where several tubes meet
(vide fig. 8, p).
The sheaths of the nerve-tubes. Our attention has
hitherto been directed to the contents of the nerve-tubes only. We
will, now, before we leave the nerve-tubes of the lobster, pay a
little attention to the sheath which envelopes the contents. This
sheath consists of a connective substance which is, in my opinion,
the same substance as the neuroglia, or connnctive-tissue, as many
authors call it, which extends through the whole central nervous
system of every animal I have had under investigation. This
sheath of connective substance, or for the sake of brevity we
will call it neuroglia- sheath, has, also, a great resemblance to the
spongioplasm separating the primitive tubes, and it is, in fact, very
often, extremely difficult to distinguish the two substances from each
other viz. the spongioplasm occurring inside the nerve-tubes and the
ganglion cells as will subsequently be described and the
neuroglia enveloping the nerve-tubes and the ganglion cells with
sheaths or membranes; the two substances are often so intimately
united that it is really impossible to decide where the line of de-
marcation can be drawn. There is one difference, however, viz. that
in the spongioplasm no nuclei occur, whilst nuclei occur in the
neuroglia. LEYDIG'S view that it is the same substance, spongioplasm,
which penetrates from whithout into the nerve-system and even into
the nervous elements, is, in my opinion, not yet sufficiently well sub-
stantiated. If I had to choose, I would, however, much prefer that
theory to VEJDOVSKY'S, according to which the inner connective
substance (i. e. neuroglia) is a product originating in the ganglion
cells; I really do not understand how, in that case, to account for
T ) This separation into bundles and fagots is, however, les prominent or not
present at all near the origin of the nerves in the central nerve-system.
the neuroglia-cells and nuclei, and how they can be produced from
In my opinion there can be no doubt, but that the neuroglia is
a separate tissue composed of cells springing from the ectoderm,
just as the ganglion cells spring from other ectodermal cells.
The doubly marked outlines which are visible in nerve-tubes
isolated in the fresh state, but which are specially distinct in macerated
nerve-tubes, are produced by the sheath which, as a cylinder, enve-
lopes every nerve-tube and forms its outer isolating layer. It con-
sists of one or several concentric layers of connective substance or,
as we just above called it, neuroglia, and is easily seen in transverse
as well as in longitudinal sections; the sheaths of large nerve-tubes
are especially very prominent and stout. When there are several
layers; which is generally the case in large nerve-tubes, especially
those of the peripheral nerves; then the innermost layer is the
strongest, most differentiated, and refractive one (vide fig. 15). These
concentric layers of the tube-sheaths are seen, in fig. 7, round the
large nerve-tubes (), and in some peripheral nerves they are very
In the sheaths, nuclei occur. These nuclei are, as mentioned,
quite identical with the usual nuclei of the neuroglia; they have an
oblong form, with a granular appearance, and are usually situated on
the outer side of the sheaths (vide fig. 2, k\ fig. 5, k\ fig. 8); they
occur, however, also on the inside of the sheath, consequently in
the tube itself (vide fig. 4, &; fig. 5, &'; fig.' 13, &)
We obtain very similar results as to the structure of the nerve-
tubes on examining Nephrops norvegicus; in this respect Nephrops
so very much resembles Homarus that it is really unnecessary to
give any special description of it. We can, in fact, observe, in the
large nerve-tubes, the same tendency towards a concentration of a
sort of axis in their centre, though it is not so prominent in Nephrops
as in Homarus, and neither have I observed so narrow and deeply
stained axes in the former as I have in the latter.
In the transverse sections of many nerve-tubes, I have observed
similar large meshes as those. I have mentioned in the nerve-tubes
of Homarus (cmfr. fig. 5, ^). They had often a very regular ap-
pearance and looked as if they really were large transsected primi-
In the Annelids we find a structure of the nerve-tubes very
similar to what is described in Homarus.
As representative of the Polyclicetes I have examined several
species of Nereis (N. virens, N. pelagica and others) and also,
occasionally, Neplitys, Leanira etc. and can only say that, in them
all, I have found the same structure of the nerve-tubes repeating itself.
In the Nereida, to which I have especially paid attention, we
find nerve-tubes of every thickness, ranging from the two gigantic
nerve-tubes running along the centre of the ventral nerve-cord (vide
fig. 14 and fig. 10) down to nerve-tubes consisting of only one pri-
mitive tube, like what is described in Homarus. 1 )
The minute structure is, of course, easiest to study in the larger
nerve-tubes. If we examine them under high powers of the micro-
scope, in good preparations, 2 ) we find that quite similar primitive
tubes constitute their contents, in the same way as described in re-
spect of the nerve-tubes of Homarus. The differentiation in primi-
tive tubes is, however, not so distinctly marked nor so easily seen
as it is in Homarus, perhaps, partly, because the primitive tubes
have a smaller diameter, 3 ) partly, because their spongioplasmic
sheaths or membranes are thinner and less distinct. In fact, it often
requires the best lenses, and very carefully prepared preparations,
to see any structure in the contents, and this is evidently the reason
why so many authors have not understood these structures, and
have supposed these large nervetubes to be lymphoid vessels etc.
Of a concentration towards an axis, there are only very slight indi-
cations in some rather slender nerve-tubes. The large nerve-tubes
( gigantic fibres ) have always primitive tubes, only slightly marked,
and of the same size and appearance throughout. Fig. 10 represents
the central part af a transverse section through the ventral nerve-
cord, where one of the two central largest nerve-tubes (t) (of the
other only a small part is seen), also some more ventrally situated,
not so large nerve- tubes (t lt t 2 , t$) and a great deal of the surround-
ing small nerve-tubes (nt) are seen. Fig. 9, b represents a longi-
tudinal section of one of the largish nerve-tubes (cmfr. fig. 10, 2 );
T ) In my preparations I have found nerve-tubes with diameters measuring
from .030 Mm. down to .0018 Mm.
2 ) For this purpose the same method of fixing etc. can be recommended as
we have recommended for the neve-tubes of Homarus (vide p. 85).
3 ) In my preparations I have generally found the diameter of the primitive
tubes to be about .0012 Mm. or less.
nt represents smaller surrounding nerve-tubes. As will be seen,
there is, in these tubes, a longitudinal striation quite similar to what
we already have experienced in Homarus.
The nerve-tubes of the peripheral nerves have quite a similar
structure to those of the ventral nerve-cord.
The sheaths of the nerve-tubes are formed by the neuroglia,
in the same way as they are in Homarus (cmfr. fig. 10); they are,
however, not so stout, and nuclei occur very sparingly in them.
In Lumbricus, the nerve-tubes in the ventral nerve-cord have a
more uniform and relatively smaller size (the three gigantic nerve-
tubes excepted [fig. n, t, t, *,]) than they have in the ventral nerve-
cord of the Polychaetes examined; the nerve-tubes have, however,
a quite similar structure, their contents being composed of the same
elementary constituents, viz. the primitive tubes, which certainly have
a small diameter, but are, in the small nerve-tubes, only present in
limited number; in some cases nerve-tubes consist of apparently
only one primitive tube; it is consequently somewhat similar to what
is described in Nereis and Crustaceans.
The nerve-tubes have neuroglia-sheaths which are very prominent
in osmic-haematoxylin preparations, vide fig. n, nt.
The nerve-tubes of the peripheral nerves have a quite similar
structure to those of the ventral nerve-cord.
In the ventral nerve-cord of Lumbricus, there are, as is well
known, three large tubes running, dorsally, through the whole length
of the nerve-cord. These tubes, which are generally called CLAPA-
REDE'S gigantic nerve-fib res , have given rise to much dispute and in
the views of the various writers very different functions have been
attributed to them. Some writers have called them supporting rods,
the function of which is to give support and rigidity to the ventral
nerve-cord under the many movements of the animal; that is, for
instance, VEJDOVSKY'S view; other writers, again, consider them as
homologous with the corda spinalis of the vertebrates; some writers
call them vessels, etc. etc., and finally a great many writers call
them nerve-tubes. Lately, LEYDIG has published a paper on this
subject (1. c. 1886) in which he very emphatically maintains their
exclusively nervous nature.
After having examined their structure, I do not think there can
be any doubt that LEYDIG is right, and that VEJDOVSKY, who ex-
presses himself with so much self-confidence, is entirely wrong. It
was indeed to be hoped that those organs might at last enjoy that
rest wich they so well deserved and that they might now be left in
peace in the rubric of nervous elements.
Seeing that, comparatively, few authors have been in doubt as to
the nervous nature of the large nerve-tubes of the Polychaetes, it is really
very strange that there has been so much dispute about these organs
in Lumbricus. If we examine them closely under high powers of
the microscope, in carefully prepared sections, we will, instead of the
homogeneous contents usually described in them, find a contents
with quite the same structure as above described in the large nerve-
tubes of Nereis, it being composed of a large bundle of primitive
tubes, the spongioplasmic sheaths of which are, however, very thin
and but little differentiated; the primitive tubes are thus extremely
difficult to observe, and this is naturally the reason why no author
has noticed any striation in these large tubes, and why their contents
has always been described as being homogeneous.
I have not been able to find any spongioplasmic reticulation or
septa, similar to what LEYDIG has described and illustrated (1. c.
1886 p. 594). I cannot therefore explain their absence in any other
way, than that his preparations have not been quite succesful, there
may, perhaps, have been some irregularities in them produced by
shrinking of the tube-contents, which indeed very often happens,
especially in those thick tubes. From my own sad experience, I can
testify that it sometimes happens in spite of an apparently very
careful preparation. Judging from LEYDIG'S illustration I should
also say that such has been the case.
The whole contents of the three nerve-tubes consist of primitive
tubes, having the same size and diameter throughout, as will be seen
in fig. n, and no concentration towards an axis is visible in the
centre of them. 1 )
The sheaths of the nerve-tubes. The three large nerve-
tubes are surrounded by very thick and prominent neuroglia-sheaths
consisting of many layers or membranes of connective substance.
When LEYDIG says that they are closely surrounded by several
slender nerve-tubes, he is scarcely quite correct. I have certainly
observed nerve-tubes between them, and in the neuroglia surround-
ing them, but they are scarce, and, in my opinion, the three large
nerve-tubes are principally surrounded by connective substance, or
neuroglia, forming thick sheaths round them. This neuroglia does
not, however, differ from the neuroglia of the rest of the nervous
system, in anything, else than that it occurs in thicker layers than is
l ) In my preparations the diameter of the primitive tube measured about
generally the case, and is more sparingly mixed with nervous ele-
ments. As will be seen in fig. n, it has, also, an appearance some-
what different from the rest of the fibrillar mass, exhibiting, as it
does, a prominent disposition to form concentric layers or mem-
branes round the tubes. Between the tubes may also be seen septa
(fig. n,s), apparently of the same substance, issuing from, or rather
adhering to, the neurilem-slieatli or perineurium, enveloping the
ventral nerve-cord inside the muscular layer (fig. n, m). In the
neuroglia-mass surrounding the three large nerve-tubes nuclei occur
very often (fig. n, Jc), they are oblong, have a granular appearance
and are quite similar to the common neuroglia-nuclei.
Judging from the latest descriptions of the nervous system ot
the Molluscs, we should expect, here, to find very extraordinary
conditions. HALLER assures us that there exists no connective-tissue
within the nervous system of the more primary Molluscs (Chiton,
Patella, Rhipidoglossa etc.) and that there really exists no nerve-fibres
or, as I call it, nerve-tubes only Primitivfibrillen which unite in
large bundles to form the peripheral nerves. The same statements
are quite recently made by RAWITZ (as mentioned p. 70) as to the
nervous system of the Acephales; this latter writer seems, however,
in these as well as other respects, to walk very closely in the
footsteps of HALLER.
It was therefore with no small interest that I began the investigation
of this group. I chose Patella vulgata for my purpose, because it
was on the one hand a large species, and a rather primary mollusc,
and on the other hand I could get plenty of it, here, in Bergen.
I must confess, I had some doubt as to the correctness of BELA
HALLER's statement that no connective-tissue (i. e. neuroglia) existed,
and my researches have not at all diminished my doubts; on the con-
trary, I found a well developed neuroglia, with neuroglia-nuclei of the