(Fig. 7) and the proximal (smaller) complex eye (Fig. 13) are so named
to distinguish them from the lateral simple eyes of the clubs. The
distal complex eye consists of the following parts: a cellular cornea,
continuous with the epithelium of the sensory club; a cellular lens
(externally cellular and internally often quite homogeneous) immediately
beneath the cornea; a homogeneous capsule just internal from the lens,
and evidently a secretion from the lens cells; a vitreous body composed
primarily of prisms and pyramids just beneath the capsule; and a retina
of pigmented cells, with subretinal nerve tissue, ganglion cells and
fibers. To my knowledge all observers (except Carrière, who missed the
capsule) are quite agreed on the anatomical structure of the distal
complex eye as also on the proximal complex eye and the lateral simple
eyes.[d] It is on the histological structure of some of the various parts
that differences exist.
_Cornea._--Little need be said on the cornea except that it consists
of flattened cells applied to the outer surface of the lens. It is
continuous with the epithelium of the club and evidently a modified
portion of this epithelium (Fig. 7). All observers conform to this
statement.
_The Lens._--The lens is of cellular origin, but in its interior
the cells are often so changed--absence of nuclei, cell walls, and
protoplasmic structure--as to make a mass quite homogeneous and
structureless. While this internal mass sometimes shows practically no
structure, yet at other times it is found broken up into masses much
the size and shape of cells but without nuclei, while again, cells with
nuclei may be quite evident. This occasional breaking up of this mass is
evidently predetermined by its original cell structure. Iron-hæmatoxylin
stains this inner mass very dark and it is difficult to wash out the
stain. Borax carmine and Lyons blue give the best results on the lenses.
In figure 7 the lens of the distal complex eye is shown as quite
homogeneous internally, while in figure 13 (proximal complex eye) it is
drawn cellular. In this latter lens the inner cells are quite round and
nucleated as they may also appear in the distal eye. What I have said
applies equally to the lenses of both complex eyes, though the cellular
nature of the inside of the lens is more readily demonstrated in the
proximal eye.
It appears that it is in younger specimens that the central mass of
the lens shows the cellular structure best, and that as the animal
grows older this structure is more and more lost until no trace of it
remains. As concerns most of my series I could not well determine which
were from younger and which from older individuals, yet, several series
of quite small (5 mm.) and therefore young animals, in which the eyes
were so small that the lenses were compassed into less than half a dozen
sections, the cellular structure of the lens was very evident.
The external cells of the lens form a spherical shell (both complex eyes)
which, in section, shows as a hollow ring (Figs. 7, 13). The thicker ends
of these cells lie at the inner (toward the capsule) half of the sphere
and the cells taper toward the corneal surface, dovetailing laterally
with their immediate neighbors as also distally with those from the
opposite side of the sphere. The thicker inner ends of the cells contain
the large nuclei with nucleoli. At a point (* Figs. 7 and 13) on the inner
(next the capsule) surface of the lens the cells only approximate each
other and thus leave a place which is easily broken through, as is shown
by portions (drops, probably representing cells or portions of cells)
of the mass within the lens becoming squeezed out into the substance of
the capsule and the vitreous body, and found occasionally also among the
cells of the retina. A considerable portion of the inside of the lens
may be found thus squeezed out, and its path can often be traced. This
phenomenon is evidently brought about by a contraction of the shell of
the lens during fixation and before the inside of the lens has become
hardened.
In origin the lens is evidently ectodermal, originating from an
ectodermal invagination which becomes pinched off as a hollow sphere,
the outer (_i. e._ next the cornea) half of which becomes the lens, the
inner half the retina (_i. e._ vitreous body plus the so called retina).
(See Retina.) The transition from retinal to lens cells is quite readily
made out at the lower side of Fig. 7, but the corresponding structure on
the upper left side is not so manifest. It is further evident that the
lens is again an invagination into this sphere, and the point at which
the lens cells approximate (where the central mass of the lens may be
squeezed out as above described) represents the place of pinching off
of the original lens-retina sphere. It appears, then, that the lens is
formed in the lens-retina sphere in the following manner: The cells of
the secondary invagination going to form the lens begin to lengthen
distally (_i. e._ toward the cornea) during their invagination to form a
hollow sphere, at the same time dovetailing with each other and budding
off cells to form the inside of the lens (Figs. 7, 13).
At the lower side of the lens, near the margin of the retina, the cells
of the lens are slightly indented or pushed inwards (Fig. 7, ind.). I
believe this to be due to the weight of the lens in the normal position
of the club, when the lens rests against the margin of the retina and the
capsule and adjacent tissue.
Anticipating the description of the retina, it may here be added, that
the retina is formed from the inner half of the lens-retina sphere. The
cells of this portion of the sphere become differentiated into prism
cells, pyramid cells, and long pigment cells, while laterally, beyond the
margin of the vitreous body, they are differentiated into pigmented iris
cells (Figs. 7, 6a).
Above are my results on the lens. Haake[2] speaks of the lens as
consisting of a cellular “Kern” with a covering of lamellated cells.
Carrière describes it as cellular and filled internally with a
“Gerinsel,” or coagulation. Carrière and Haake are each in part right.
Claus describes it as wholly cellular. Schewiakoff regards the lens as
wholly cellular, and like Claus has not noted that internally this cell
structure may be quite obliterated. Schewiakoff regards the lens and
retina as formed from an invaginated sphere, and shows the transition
from the lens cells into retinal cells as I have figured. Conant also
gives the structure of the lens for the complex eyes as cellular but
missed the change of structure that the interior of the lens may undergo.
_The Capsule._--The capsule of the lens (Figs. 4, 7) lies immediately
below (inward from) the lens. In structure it is homogeneous, except for
certain fibers from the long pigment cells of the retina that traverse
it, while sometimes also other fibers can be seen which, possibly, are
branches from the fibers just mentioned or continuations from the fine
fibers of the prism cells of the retina soon to be described. I have,
however, no evidence that the fibers from the prism cells extend beyond
the prisms in whose axis they lie. The capsule lies very closely applied
to the lens, never becoming separated from it in sections, and is, hence,
regarded as a secretion from the lens cells. Just what its function
may be is difficult to surmise. The proximal complex eye possesses no
capsule. I have thought, however, that if the lens should be adjustable,
the capsule might serve as a protection to the prisms of the vitreous
portion of the retina during the adjusting movements of the lens. (But
more on this below.) To my knowledge all previous observers are quite
agreed on the structure of the capsule. Carrière and Haake, however,
missed it altogether.
_Retina._--While I have enumerated (following previous observers) the
vitreous body and the so-called retina as distinct parts, yet, as the
sequel will show, they are, histologically, different parts of the same
thing--namely the sensorium proper of the eye--and I propose to use
the term retina for both taken together, while I retain the expression
vitreous body (as hitherto used) for the vitreous portion of the retina.
This simplifies matters; and using a word that is already used for
analogous structures of other eyes (vertebrates, anthropods, molluscs)
is conducive to clearness. I have been tempted, furthermore, to use the
words _rods_ and _cones_ for the prisms and pyramids that I find in
the vitreous bodies of the retinas of the complex eyes. But since the
prisms in reality approximate prisms and the pyramids pyramids, in their
shape, I have decided to retain the words prism and pyramid for these
structures. The former of these terms (prism) was first used by Conant in
his description of the complex eyes.
What I shall call the retina, then, in the distal and proximal complex
eyes of Charybdea, consists of three kinds of elements: the prism cells,
the pyramid cells, and the long pigment cells. (Figs. 4, 7, 22, prc,
pyrc, lp.) We may also describe the retina as composed of three zones:
the vitreous zone (vitreous body of authors), the pigmented zone, and the
nuclear zone. (Figs. 4, 7, 22, vb, pz, nz.)
The cells composing the retina form a single layer in the shape of a
hollow cup, into which cup the lens with its capsule fits. (Fig. 7.) This
single layer of cells takes in the thickness of the vitreous zone, the
pigmented zone, and the nuclear zone. Indeed, the distinctions vitreous
zone (vitreous body), pigmented zone, and nuclear zone characterize three
topographical regions of the retinal cells.
That the retina is made up of three kinds of cells is most readily
demonstrated in transverse sections through the vitreous body. Fig. 1 is
such a section, taken quite near the pigmented zone (at about the level
x, Fig. 4). Three different kinds of areas are readily made out in such
a section. The more numerous areas (pr) are transverse sections of the
distal prisms of the prism cells, the less numerous and lighter areas
(pyr) are transverse sections of the pyramids of the pyramid cells, and
the large oval heavily pigmented areas (lp) are the transverse sections
of the long pigment cells. The dots within the two first named areas
represent fine fibers in the axes of the prism and pyramid cells, to
be described below. The presence of three kinds of cells can again
be readily seen in such Figs. as 4 and 7, in which the elements of
the retina are cut parallel to their long axis. (Fig. 22.) Again, a
transverse section through the most distal part of the pigmented zone of
a slightly pigmented retina (Fig. 2) also shows us the presence of three
kinds of elements. The larger and more heavily pigmented areas (lp) are
the long pigment cells; the smaller, lighter areas (pyrc) with a central
dot are the pyramid cells, and the more numerous dots, with no definite
polygonal areas outlined about them (prc), belong to the prism cells.
Thus, I believe, we have conclusive evidence of the existence of three
kinds of cells in the retina of the distal complex eye.
(a) The prism cells are the more numerous, and, as the name implies,
end distally in a vitreous polygonal prism (Figs. 4, 7, 22, pr). The
prismatic structure of the vitreous body is also shown in Figs. 10 and
11, which are drawn from a macerated preparation of Conant’s. (See the
descriptions of these figures.)
In Figs. 4 and 7 the prism cells correspond to the cells with the darker
nuclei (npr); in Fig. 2 they are represented by the dots without defined
polygonal areas about them (prc), and in Fig. 1 by the most numerous
areas (pr). These cells, then, consist of a centrad portion with nucleus,
a pigmented portion with granules of a dark-brown pigment, distal from
the nucleus, and a distal vitreous prism which extends to the capsule of
the lens.
In the axis of each prism is a fine darkly-staining fibril extending
the entire length of the prism. I found no good evidence that this
fiber extends into the capsule. Centrad this fiber is continued through
the pigmented part of its cell and approaches to or near the nucleus
(Fig. 2, dots without defined polygonal areas; Fig. 7, part of retina
left unpigmented). In some instances I could trace this fiber quite to
the nucleus, while in others it ended before reaching the nucleus or a
little to one side of it. I am inclined to believe, however, that it
extends past the nucleus and is continued as a nerve fiber. I believe
this to be so because the fiber is evidently sensory, and _a priori_ we
should expect it to be so continued. Further, I find decided evidence in
sections of the simple eyes to show that the fibers there extend past the
nucleus into the subretinal tissue where I could not trace them farther.
(Fig. 16.) Again, that the flagella of the epithelial cells of the club
are also continued into the cells, in some instances could be traced past
the nuclei (Figs. 12 and 26), and the fact, too, that the retinal cups of
the eyes represent invaginated epithelium (the axial fibers of the prisms
are hence cilia?)--all this leads me to believe that the axial fibers
of the prism-cells extend centrad past the nuclei through their cells
and are continued as nerve-fibers. (See below under pyramid-cells and
under epithelium). Immediately upon entering the pigmented part of its
cell the axial fiber of a prism-cell has a dumbbell-shaped enlargement
which lies quite at the distal edge of the pigmented part of the cell
(Fig. 7, unpigmented part of figure). This, of course, can be seen only
in unpigmented retinas. This dumbbell-shaped body, (Basalkörperchen of
Apathy), which name I give it, since it evidently is homologous to the
basal bodies described by others for the cilia of epithelia, can be most
beautifully seen as two minute spheres lying close together and in line
with the nucleus. These two little spheres of the basal bodies put to the
test the highest powers of the microscope; but, when, after a prolonged
and careful study, one satisfies himself of their existence and exact
shape, the very difficulty with which they are resolved adds a zest to be
appreciated. The length of a basal body is about one-fifth to one-fourth
that of the nuclei of the prism-cells.
The structure of the nuclei of the prism-cells is that of a dense network
(Figs. 4, 7, npr) which stains dark with hæmatoxylin. A nucleolus can
often be seen in these nuclei. In some few series, again, these nuclei
did not show a network-like structure, but the chromatin was arranged in
masses (Fig. 5, npr). These nuclei can usually be distinguished from
those of the other cells of the retina by their denser, darker-staining
network (Figs. 4, 7, npr), or as shown in Fig. 5 (npr). Their denser
structure and staining capacity are a distinguishing characteristic of
the nuclei of the prism-cells. I must add, however, that not in every
series is this apparent.
That portion of a prism-cell that contains the nucleus rarely contains
any pigment; and when pigment is present, I believe that it has been
dissolved in from the pigmented zone. The nucleus, again, lies a little
centrad from the pigmented part of its cell, so that an unpigmented zone
is seen in the retina between the pigmented zone and the row of nuclei
(Figs. 4, 7, 22).
Centrad the prism-cells are continued as a single process (Figs. 6, b,
c, d, and 8a, b, c, d). In some sections I thought I could trace these
processes to the basement membrane, but I could not satisfy myself that
such appearances were not due to artificial splitting in the tissue.
Schewiakoff makes a similar remark about his supporting cells, which
cells I believe are the same as my long pigment cells, but these do not
extend to the supporting lamella.
At the margin of the retina the cells do not develop prisms but remain
pigmented and form an iris (Fig. 7), which was so named by Claus and also
described by Schewiakoff. These cells also assume a somewhat different
shape (Fig. 6a). This cell (Fig. 6a) is seen from its broader side with
which it is applied to the capsule or the lens. Schewiakoff figures
similar cells. That the cells of the iris are prism cells without the
prisms does not necessarily follow. They simply represent cells of the
retinal cup that have become differentiated to serve as an iris.
As to the exact origin of the prisms, and pyramids (to be described
below), it is difficult to say anything definite. If the so-called basal
bodies of the axial fibers are really homologous with the basal bodies
of flagella, then it would seem that they (the prisms and pyramids) are
secretions comparable to cuticular secretions.
(b) The pyramid-cells, like the prism-cells, are differentiated into
three regions: a distal vitreous pyramid, a pigmented part, and a centrad
part with nucleus. The pyramids are seen in transverse section in Fig. 1
(pyr) and in longitudinal section in Figs. 4 and 7 (pyr).[e]
Each pyramid extends between the bases of the prism-cells about one-third
to one-half the depth of the vitreous body (Figs. 4, 7, 12 (pyr)). The
pyramids are also a shade lighter than the prisms, which fact is
characteristic. In the axis of each pyramid is a darkly-staining fiber
quite like the one described for the prism-cells (Figs. 1, 4, 7, 22).
That this fiber extends distally beyond the limits of the pyramids I
could not determine, but I do not think that it does. Centrad this fiber
extends into the pigmented portion of its cell quite to or near the
nucleus as was described for the fibers of the prism-cells (Figs. 7, 22).
Whether or not these fibers extend past the nucleus and become continued
as nerve fibers, the same course of reasoning holds as was given for the
fibers of the prism-cells. Each of these fibers possesses a basal body
just on its entrance into the pigmented part of the cell (Fig. 7), but
I could not determine that it was dumbbell-shape. In form it represents
an enlargement of the fiber itself, which gradually tapers again to its
normal size. The continuations of these fibers within the pigmented
parts of the pyramid-cells, as also the basal bodies, could only be
demonstrated in unpigmented series.
Patten[5] describes axial fibers extending centrad through the rods
(vitreous portions) of retinal cells (“retinophora”) into the region
of the nucleus and past the nucleus (arthropods and molluscs). My
retinal cells (prism and pyramid cells) evidently correspond to Patten’s
retinophora, but I find no evidence that one of my retinal cells
represents more than a single cell, while Patten gives evidence that
his retinophora are made up of two cells closely applied to each other
as twin cells. If this were also true for the retinal cells that I have
described, I believe my macerated preparations would have shown it.
Schreiner[12b] and Hesse[13] also figure and describe axial fibers for
the rods of the visual cells in polychætous annelids, and Schreiner[12a]
also for molluscs. Neither of these observers finds the fibers to extend
distally beyond the rods nor centrad toward the nucleus as Patten and
myself show. Neither Schreiner nor Hesse figures these cells as twin
cells as Patten does, so that to my knowing Patten stands alone in this
respect. Andrews[14] describes and figures rods for the visual cells of
polychæte annelids but no axial fibers. He was the first to describe
these rods in annelids.
The pigmented zone of the pyramid cells, in heavily pigmented series, is
filled throughout with dark-brown pigment granules, and is quite like
that of the prism cells (Figs. 4, 7). In transverse sections, however,
through the most distal part of the pigmented zone, of unpigmented
series (Fig. 2), lighter areas with central dots could occasionally be
demonstrated, which areas are the pyramid cells. In Fig. 2, the more
definite polygonal outline as well as the lighter shade of these areas
was a distinguishing feature. The difference in shade was not wholly due
to a difference in pigmentation but to a structural difference.
The nuclei of these cells are usually a little larger than those of the
prism cells and are filled with a finer and less dense network (Figs. 4
and 7, npyr), in consequence of which they present a lighter appearance
in sections when examined with a high power. It will be seen in the
figures (4, 7) with what regularity these lighter nuclei lie opposite
the pyramids. Some few exceptions occur. These are probably due to the
fact that a nucleus or pyramid was not differentiated by the technique.
If this opposition between the pyramids and the lighter nuclei were all,
I believe it would be sufficient evidence for associating these lighter
nuclei with the pyramid cells.[f]
(c) The _long pigment cells_ are about as numerous as the pyramid cells.
In these cells, as in the prism and pyramid cells, three regions can be
distinguished: the region of the nucleus, a pigmented region (the distal
half of which extends between elements of the vitreous body), and a
distal rod-like portion, or fiber, which is continued between the prisms
into the capsule of the lens (Figs. 4, 7, 9). The pigmented portion is
about twice the length of that described for the other cells, and also
often of greater diameter, so that in transverse sections (Figs. 1, 2, 3)
these cell-areas are larger than those of the other cells. As nearly as
I could determine, these cells are pigmented just like the other retinal
cells described. In quite unpigmented series, however, they often contain
more pigment than the other cells do (Fig. 2). Distally, the pigmented
part becomes narrowed to a strong pigmentless fiber (Figs. 3, 4, 7). This
fiber stains quite dark with iron-hæmatoxylin and appears homogeneous. It
passes between the prisms into the capsule, where it usually bends in a
direction toward the margin of the capsule (Fig. 7) and passes diagonally
across this to the lens. In sections, a space is often seen about these
fibers in the vitreous body, which I regard as a shrinkage space (Figs.
3, 4), since it is not evident in all series (Fig. 1). In Fig. 7, I have
assumed that these spaces are due to shrinkage and have not indicated
them. Also, in this same figure I have assumed that the spiral appearance
of the fibers (Fig. 4) is due to a shortening of the prisms during
fixation, and have drawn them straight. At the lens these fibers seem
to end. In a few instances they were seen to branch upon reaching the
capsule (Fig. 4). In Fig. 9, also, which shows some of these cells from
a macerated preparation by Conant, the rods show evidence of branching
at their distal terminations. In the same preparation I thought I could
see that a fiber became expanded into a membrane spreading over one of
the lens-cells. I could not satisfy myself, however, that this was the
actual condition of things. Judging from Fig. 9, one might conclude that
all the fibers are branched distally; yet, if such were the case I should
have seen more of it in sections, but branching as seen in Fig. 4 is the
exception. Hence, if all these fibers do branch, I am inclined to believe
that it must be among the bases of the lens-cells. Or, if the fibers
do expand into membranes to cover the lens-cells (I could not explain
purpose), the evidence in Fig. 9 may be nothing more than fragments of
this membrane left attached to the ends of the fibers. As is seen in Fig.
7, most of these rods end opposite the cells of the lens, and not usually
between two adjacent cells as Schewiakoff has described for Charybdea
marsupialis. The nuclei of these cells are like the nuclei of the pyramid
cells (Figs. 4, 5, 7, 9) and often have a nucleolus.[g] Centrad these
cells are continued into a number of processes as is seen in Figs. 5, 7
and 9. How far the several centrad processes extend and where they end I
cannot say; but, as seen in Fig. 5, they soon taper to a thin end which I
suppose may be continuous with a nerve fiber. I believe Schewiakoff was
mistaken when he stated that these cells extend to the basement membrane.