fiber such as I have described for the prism and pyramid cells. I find no
definite arrangement of the nuclei of the retina into definite layers,
but the nuclei of the three kinds of cells lie quite mixed, sometimes one
kind lying deeper than the other as can be seen in the figures. Again,
they may lie quite at the same level. (This point will be referred to
later.)
It is these long pigment cells that I believe retract their pigmented
part from between the prisms and pyramids when the medusæ are placed in
the dark, protruding with their pigment when placed in the light. Fig.
5 is a section from a slightly pigmented retina killed in the dark. The
parts of the cells projecting beyond the pigmented zone, and which would
lie between the prisms and pyramids (here not shown) of the vitreous body
are seen to be narrower than in sections from retinas killed in the light
(Figs. 1, 3, 4, 7) and the cells themselves appear in a condition of
retraction as is shown by their large centrad portions with the nuclei,
which latter, also, here lie at quite a lower level than the other
nuclei. (The pyramid cells were not shown in this series.) I occasionally
found appearances like Fig. 5 in retinas killed in the dark (indeed,
in some the pigmented portions in the vitreous body were much thinner
and more retracted than in Fig. 5). Yet this appearance was not of
sufficiently general occurrence to leave no doubt as to its significance.
As positive evidence, however, I cannot give it any other interpretation
than the one given--that the cells retract themselves with their pigment
when in the dark. Again, it must be added that the nuclei of these cells
may occasionally lie quite deep even in retinas killed in the light.
Indeed, like structures in different retinas may vary considerably in
size and shape. None of my darkness retinas, however, showed such a large
proportion of the pigmented parts of the long pigment cells projected
between the prisms and pyramids as did the light retinas. I examined
and tabulated all my series with respect to the extent the long pigment
cells were projected into the vitreous body, and I found that those which
showed these cells with their pigment least projected between the prisms
and pyramids to be those that had been killed in the dark. I thus feel
satisfied that the pigmented parts of these cells become in part or quite
completely retracted from between the prisms and pyramids of the vitreous
body when in the dark, but just how this is accomplished--whether
the whole cell with its nucleus takes up a deeper position, the cell
substance at the same time collecting in the region about the nucleus,
as shown in Fig. 5 and the diagram (Fig. 22), I cannot with certainty
state. It would seem, too, as though the pigment became less in the cells
exposed to darkness, for I rarely, even in the most retracted heavily
pigmented series, saw the pigment to extend farther towards the nucleus
than commonly. The time of keeping in the dark, prior to fixing, varied
from three-fourths of an hour to one and one-half hours. I could not
bring the amount of retraction into relation with the time of exposure,
except that in general the retinas longest exposed showed the greater
amount of retraction.
(d) The tissue underlying the retina is described by former observers
(Claus, Schewiakoff, Conant) as composed of nerve-fibers and ganglion
cells. I cannot give it any other interpretation, but I must add that
the supposed ganglion cells are seen only as nuclei, no cell bodies ever
being demonstrable in any of my sections. Conant also recognized no cell
bodies. Occasionally, as in Fig. 7, long fibers could be traced for some
distance in this subretinal tissue, in some instances quite to or from
a visual cell. Pigment was not regularly observed in this tissue, as
Schewiakoff describes, and when present I believe it has been dissolved
in from the pigmented zone.
(e) Schewiakoff describes the retina (my pigmented and nuclear regions)
as composed of spindle-shaped visual cells (my pyramid cells?)
alternating with pigmented supporting cells (long pigment cells), with
the nuclei of the former lying more centrad than those of the latter.
The visual cells are pigmented only at their periphery, or surface,
leaving an unpigmented axis, while the supporting cells have pigment
throughout their whole substance within the pigmented zone. Distally,
the visual cells have hyaline rods, or fibers, which extend into spaces
in the vitreous body, and pass through this and the capsule to the lens.
The vitreous body is described as homogeneous, except the spaces for the
visual rods, and a secretion from the retinal cells.
It will thus be seen that my results are quite different from those
just described. I find the vitreous body to be composed of prisms and
pyramids with axial fibers, while the long pigment cells (supporting
cells of Schewiakoff) are continued into the vitreous body, and becoming
narrowed into a non-pigmented fiber, extend to the lens as described.
The prisms and pyramids are, further, the distal continuations of cells
whose pigmented and nuclear parts lie in the so-called retina, but which,
together with the vitreous body, I have named the retina proper. Conant
has so summarily disposed of Schewiakoff’s distinction between retinal
cells based on pigmentation and location of nuclei, that I need not say
more. Schewiakoff’s Fig. 18 corresponds to my Fig. 1. In this figure he
shows the vitreous body as homogeneous with pigmented areas (my long
pigment cells) and with spaces with his visual rods. It is quite evident
that his spaces with the visual rods correspond to my lighter areas with
central dots; _i. e._ my pyramids of the vitreous body are the same as
the spaces shown in his Fig. 18. It is quite evident that Schewiakoff
mistook the lighter areas for spaces. That they are not spaces can
readily be seen by comparing them with real spaces. It is, of course,
possible, too, that the reagents had dissolved the pyramids, leaving
only the axial fibers with a little pyramid substance about them, and
that this is what Schewiakoff saw. I often found small circular spaces
in the centers of the pyramid areas, as also in the prism areas (Fig.
3), which might be taken for hyaline visual rods, fibers, in transverse
section, but in such spaces I could usually see a small dot to one side
of the space that I take to be the rod (fiber) proper. Fig. 14 also
shows such small circular spaces that have very much the semblance of
hyaline rods. This figure is a transverse section of the vitreous body
of the proximal complex eye, in which no long pigment cells or pyramid
cells are present, but it serves well to illustrate the point. The above
explanation also accounts for the large size of the visual rods (fibers)
in Schewiakoff’s figures. That the fibers of the pyramid cells (visual
rods of Schewiakoff) do not extend to the lens is quite evident in my
Figs. 4 and 7.
Again, since the long pigment cells are often not seen to terminate in
a fiber, but a part of the fiber can often be seen in the distal part
of the vitreous body and in the capsule, it will be quite readily seen
how Schewiakoff should associate his visual rods, or fibers, with these
distal parts of the fibers of the long pigment cells and suppose his
visual rods to extend to the lens.
Again, since the long pigment cells sometimes cannot be seen to terminate
distally in a fiber, while the vitreous body at the same time may be
broken away from the pigmented zone (Fig. 4), it is quite evident how
Schewiakoff should have interpreted the parts of the long pigment cells
in the vitreous body as conical pigmented caps placed opposite his
supporting cells (long pigment cells).
Finally, since Schewiakoff had only twelve marginal bodies to study, and
since this tissue is difficult to preserve properly, I do not believe
that I am doing Schewiakoff any injustice by explaining away his results
as I have done. This fact remains, that Conant and myself agree in all
points in which we differ from Schewiakoff.
To Conant belongs the credit of having first demonstrated the prismatic
structure of the vitreous body, and he also regarded the prisms as a
part of the retinal cells. H. V. Wilson[15, 8b] suggested, however, some
years prior to Conant, that the vitreous body might be of a prismatic
structure. Conant had evidence also of both the prism and pyramid fibers,
as is well shown in his figures of transverse sections but he found
his evidence too meager to make any very definite statements. Indeed,
Conant concludes that there are three kinds of fibers in the vitreous
body and complains of finding but two kinds of cells in the so-called
retina (pigmented and nuclear zones) to which to refer them. He saw the
pyramids with their axial fibers as lighter areas in transverse sections
of the vitreous body (his Figs. 64 and 68, and my Figs. 1, 4 and 7), but
suggests that they may be the same as the long pigment cells, the cells
having only to project themselves or their pigment in order to become
long pigment cells. This suggested to him to preserve material both in
the light and in the dark. I do not think Conant’s supposition to be a
fact, for I find the pyramids in specimens preserved in the light as well
as in the dark. It is, of course, possible that the pyramid cells are in
a stage of structural transition to the long pigment cells, for, besides
their pigmentation, they also have like nuclei. Furthermore, I held for
a long time with Conant that there may be only two kinds of cells in the
retina, but I soon found the pyramids so definitely shown as to leave no
doubt but that they represented a third kind of cell. For me it remained
to first definitely see all the fibers in the vitreous body as also the
pyramids in sagittal sections.
Conant describes the long pigment cells with their fibers extending
between the prisms of the vitreous body quite as I have described, and
in this my work is only confirmatory of his. Conant does not, however,
describe the several centrad processes of these cells, nor is he clear
that their distad processes extend to the lens, though he speaks of
fibers within the capsule.
(f) What, now, is the function of these three varieties of cells of the
retina? Schewiakoff regards his visual cells (pyramid cells), as the
name implies, as having a visual function. That they have such it seems
reasonable to suppose, since they have an axial fiber in their pyramids.
If the pyramid cells are visual cells, it appears that the prism cells
also are such. Indeed, since these are the only ones present in the
proximal eye and the more numerous ones in the distal eye, and like the
pyramid cells have an axial fiber in their prisms, it seems that they
are the visual cells _par excellence_ of the Cubomedusan eye. Also, the
analogy between the prisms and pyramids on the one hand, and the rods and
cones of the vertebrate eye on the other hand, does not seem to be so far
fetched. It may be of interest, here, to briefly consider Patten’s theory
of color vision.[5b]
The gist of Patten’s theory is this: In the eyes of certain molluscs
and arthropods, in the parts of the retinal cells corresponding to my
prisms and pyramids, he not only finds an axial fiber (or fibers) but
finer fibrils that extend at right angles from these axial fibers to the
surface of the rods (I shall here, for convenience, call the prisms,
pyramids, etc., rods) where they probably become continuous with other
fibrils in the surface of the rods. These fibrils from the axial fibers
are arranged in superimposed planes, and if I understand rightly, an
axial fiber with its radiating fibrils may be compared to the axial
wire with its radiating bristles of a brush used for cleaning bottles,
provided the bristles of such a brush be arranged in superimposed
planes. The lateral arrangement of the fibrils will, of course, be
modified according whether a rod is circular, hexagonal, square, etc.,
in transverse section. It will also be remembered (p. 49) that Patten
describes the retinal cells studied by him as composed of twin cells,
and he gives the name _retinophora_ to a pair. The system of fibers and
fibrils in the rods he names a _retinidium_. Centrad the axial fibers
are continued past the nucleus as a nerve fiber. The fibrils extending
laterally in superimposed planes from the axial fiber of a rod, Patten
supposes to be the ones stimulated by the incoming rays of light, the
retinophora being so arranged that the light rays entering them are
parallel to the axial fibers or perpendicular to the lateral fibrils of
the retinidium. Again, since the rods are usually the shape of truncated
pyramids or cones the lateral fibrils, which are perpendicular to the
axial fibers, are of different lengths accordingly as they are situated
at the larger or smaller end of a rod. Patten assumes similar fibrils to
exist in the rods and cones (particularly the cones) of the vertebrate
eye, and he thus makes a general application of his theory. He supports
himself in this rather sweeping generalization by the claim to have
demonstrated the twin-cell nature of the cones in amphibia and fishes.
For illustration, Patten supposes that if red light only were admitted
to the retinophora this would stimulate the fibrils near the broader
end of the cone (but that all the fibrils of the retinidium would be
stimulated a little) and that we would thus have the sensation of red
light. Likewise, if violet light only were admitted, the fibrils at the
narrower end of the cone would be stimulated, and we should have violet
light. Similarly, if light including all the different wave lengths of
the spectrum were admitted, all the lateral fibrils would be stimulated
and the sensation of white light produced. The method of stimulation need
not be that of a vibration of the fibrils.
Certain grave objections may be raised against such a theory, the most
serious, perhaps, being the fact that no such fibrils as Patten has
described have as yet been demonstrated for the eyes of those animals
that we know have color vision. Yet, as a whole, the objections are
perhaps no more serious than any that can be brought against other
theories of color vision. What Patten’s theory does do,--it gives us
a definite mechanical basis to work from, and if these fibrils should
be demonstrated for the rods and cones of vertebrates, physiologists
would then have a mechanical basis for color vision quite as they now
have for hearing. As Patten says, the problem is primarily a mechanical
one. However, the theory cannot well pass for more than a suggestion, a
stimulus for future work, and in this lies its present value.
It is quite evident that my results for the retinal cells of Charybdea
are, if any thing, a support to Patten’s theory. While I have not been
able to demonstrate the fibrils that are the essential to Patten’s
theory, yet I have demonstrated the axial fibers of the rods, and if
these fibers should be continued as a nerve fiber to some central
ganglion (as I believe is reasonable to suppose, see p. 47), I do not
see how we can avoid the conclusion that these axial fibers of the prism
and pyramid cells are somehow concerned in vision. In Patten’s theory
these fibers would represent a conducting element, the real sensory
element (fibrils perpendicular to these axial fibers) not having been
demonstrated by me.
I have recently read in a short review of Patten’s theory[9] that the
evidence we at present have points to the tips of the cones (vertebrate
eye) as being the seat of the sensation of red. This would be exactly the
converse of what Patten’s theory supposes. Whether or not this objection
is a real one, future investigation only can determine.
Hesse[13] regards the axial fibers that he describes for the rods in
worms as the primitive fibers of Apathy. In this I agree with him,
regarding the axial fibers I have described as “Primitivfibrillen.”
Further, I believe, if I understand Apathy rightly, that the fibrils
described by Patten as extending laterally from the axial fibers
correspond to Apathy’s “Elementarfibrillen.”
It is the long pigment cells that are the puzzling element. Since there
can be little doubt but that these cells can project and retract their
pigmented parts (as already described), it would seem that a part of
their function is to check the diffusion of light in the vitreous body
when exposed to strong light. This function would be quite analogous to
that of the pigmented cells of the vertebrate retina, which in light
become projected between the rods and cones. Similar observations have
also been made on the compound eyes of arthropods by Herrick[10] and by
Parker[7], who find that the distal retinula cells of Palæmonites project
themselves distad in the dark, thus surrounding the vitreous cones with a
cylinder of pigment, while (Parker) the pigment of the proximal retinula
cells migrates centrad and the accessory cells move distad; in light the
reverse takes place. Other observations of this kind are not wanting for
crustacea, insects and arachnids. To my knowledge, the pigment changes
that I have described are the first of their kind for medusæ.
I suggested while describing the capsule, that the lens might be
adjustable. That the fibers of the long pigment cells extend to the lens
is my principal reason for this. May these cells not represent ganglion
cells and their distad fibers nerve fibers? That they are not sensory
(_i. e._ are stimulated by light waves) seems to be suggested by their
not having any axial fiber and in having several centrad processes.
These facts suggest that they are not sensory but the center of a reflex
mechanism.[h] When the sensory cells proper are stimulated, the impulses
are conducted centrad into some nerve center (it may be the nerve tissue
underlying the retina, or other nerve centers such as the two groups of
ganglion cells in the upper part of the club, or the radial ganglia)
from which center, again, impulses return over fibers leading to the
long pigment cells causing them to project their pigment, and conducting
the impulse to the lens, to produce a change in its adjustment. Since
these cells are not so numerous as the prism and pyramid cells taken
together, but in turn have a number of processes continued centrad (the
sum of which processes approximates the number of sensory cells, prism
and pyramid cells) it appears that these cells are admirably adapted to
function in just such a mechanism as I have described,--each long pigment
cell serving a number of its immediate neighbors.
Further, we may conceive each of the centrad processes of the long
pigment cells as receiving a fiber from one of the sensory cells directly
as well as indirectly, as just described. While I have been able to
demonstrate only a single centrad process for the sensory cells (prism
and pyramid cells), yet this does not exclude the possibility of a nerve
fibril passing out from such a centrad process to one of the processes
of the long pigment cells, and it seems possible that this constitutes
the reflex mechanism. That nerve fibrils ramify in ganglion and sensory
cells, and may even leave these cells to join those of other cells, has
been well demonstrated by Apathy,[6] so that my finding only a single
process of the visual cells leading centrad without giving off lateral
fibers cannot be a serious objection. Again, fine nerve fibers coming
off from the main centrad process of sensory cells in medusæ have been
figured by other observers, among whom I mention the Hertwigs. Careful
macerations at the seashore would probably demonstrate them for Charybdea.
Hesse thinks that the eyes of the Alciopidæ are adjustable. He describes
what he supposes to be muscle fibers just exterior (distal) to the lens,
and believes that a contraction of these fibers would have the effect of
forcing the lens nearer the retina, or _vice versa_. His supposition,
like mine, needs experimental verification. Hitherto the only instance
known of accommodation in the eyes of invertebrates was that described by
Beer[17] for Cephalopods.
_The Proximal Complex Eye._--With four exceptions, the description and
discussion given for the distal complex eye also holds good for the
proximal complex eye (Fig. 13). The four exceptions are: the absence of a
capsule to the lens; the absence of the long pigment cells; the absence
of the pyramid cells; and the different relative position of the lens
and retina. This eye, then, has a cornea continuous with the epithelium
of the sensory club, a lens, in structure and probable origin quite like
that described for the distal complex eye, and a retina of prism cells
with axial fibers for the prisms. Since Conant[8b] has described this eye
quite fully, and discussed Schewiakoff’s conclusions at length, I shall
be brief. Suffice it to say, that Schewiakoff describes two kinds of
cells (supporting cells and spindle-shaped visual cells) for the retina
of this eye just as he described for the distal complex eye. The vitreous
body he likewise describes as being homogeneous and with spaces for the
visual rods (fibers) of the visual cells. It is evident that Schewiakoff
has interpreted the structure of this eye from analogy with his results
on the distal complex eye. Claus likewise has described two kinds of
cells for the retina, and the vitreous body as homogeneous. Conant
and myself find only one kind of cells in the retina of this eye. The
pigmentation that Schewiakoff describes for the vitreous body I believe
to have been dissolved in from the pigmented zone of the retina, for I
find no regular pigmentation in the vitreous body. Haake’s observation,
previously noted (p. 42), applies also to the proximal complex eye.
Conant’s evidence for the axial fibers of the prisms was clearly
insufficient, so that he did not in this respect complete his Fig. 69. I
republish this figure with the prism fibers drawn (Fig. 13).
Since the long pigment cells are absent my reasons for supposing the lens
of this eye to be adjustable vanish.
Finally, a word on the origin of the lens and the relative position of
the lens and retina. The lens and retina in this eye are evidently
not developed from an outer and an inner half, respectively, of the
invaginated and pinched-off lens-retina sphere (as is true for the distal
complex eye) but from proximal and distal halves respectively. It is also
quite easy to understand the connection of the lens in this eye with the
supporting membrane. Since the cells of the ectoderm of the club can in
many instances be seen to extend to the basement membrane, or supporting
lamella, the cells of the lens, which arise from the ectoderm, simply
remain in connection with the basement membrane, this becoming thickened
to form a support for the lens. That the lens of the distal complex eye
has lost its connection with the basement membrane is evidently due to
the fact that the lens is formed from the outer half of the lens-retina
sphere. The cells of the lens are by this so far separated from the
basement membrane as to lose their connection with it. Schewiakoff also
notes the fact that the lens and retina of the proximal complex eye are
developed from proximal and distal halves of the lens-retina sphere. He
further supposes that the portion of the basement membrane that acts
as a support to the lens takes the place of the capsule in the distal
complex eye. This latter supposition I do not think probable, since the
supporting lamella does not form a distinct covering to the lens on its
retinal side.