to suck on to fishes and so obtain nutriment. It is, however, undoubtedly a
free-swimming fish; and when we see it coming up the rivers in thousands to
reach the spawning-beds, and sucking on to the stones on the way in order
to anchor itself against the current, or holding on tightly during the
actual process of spawning, it does not seem justifiable to base a charge
of degeneration upon a parasitic habit, when such so-called habit simply
consists in holding on to its prey until its desires are satisfied. If, of
course, its suctorial mouth had arisen from an ancestral gnathostomatous
mouth, then the argument would have more force.
Dohrn, however, gives absolutely no evidence of a former gnathostomatous
condition either in Petromyzon or, in its larval state, Ammocoetes. He
simply assumes that the Cyclostomata are degenerated fishes and then
proceeds to point out the rudiments of skeleton, etc., which they still
possess. Every point that Dohrn makes can be turned round; and, with more
probability, it can be argued that the various structures are the
commencement of the skeletal and other structures in the higher fishes, and
not their degenerated remnants. Compare the life-history of the lamprey and
of the tunicate. In the latter case we look upon the animal as a degenerate
vertebrate, because the larval stage alone shows vertebrate
characteristics; when transformation has taken place, and the adult form is
reached, the vertebrate characteristics have vanished, and the animal,
instead of reaching a higher grade, has sunk lower in the scale, the
central nervous system especially having lost all {61}resemblance to that
of the vertebrate. In the former case a transformation also takes place, a
marvellous transformation, characterized by two most striking facts. On the
one hand, the resulting animal is more like a higher vertebrate, for, by
the formation of new cartilages, its cranial skeleton is now comparable
with that of the higher forms, and the beginnings of the spinal vertebræ
appear; by the increased formation of nervous material, its brain increases
in size and complexity, so as to compare more closely with higher
vertebrate brains; its eyes become functional, and its branchiæ are so
modified, simultaneously with the formation of the new alimentary canal in
the cranial region, that they now surround branchial pouches which are
directly comparable to those of higher vertebrates. On the other hand, the
transformation process is equally characterized by the throwing off of
tissues and organs, one and all of which are comparable in structure and
function with corresponding structures in the Arthropoda--the thyroid of
the Ammocoetes, the tentacles, the muco-cartilage, the tubular muscles, all
these structures, so striking in the Ammocoetes stage, are got rid of at
transformation. Here is the true clue. Here, in the throwing off of
invertebrate characters, and the taking on of a higher vertebrate form,
especially a higher brain, not a lower one, Petromyzon proclaims as clearly
as is possible that it is not a degenerate elasmobranch, but that it has
arisen from Ammocoetes-like ancestors, even though Myxine, Amphioxus, and
the tunicates be all stages on the downward grade from those same
Ammocoetes-like ancestors.
As to the eyes, they are functional in the adult form and as serviceable as
in any fish. There is no sign of degeneracy; it is only possible to speak
of a retarded development which lasts through the larval stage.
COMPARISON OF BRAIN OF AMMOCOETES WITH THAT OF AN ARTHROPOD.
Seeing that the steady progress of the development of the central nervous
system is the most important factor in the evolution of animals, it follows
that of all organs of the body, the central nervous system must be most
easily comparable with that of the supposed ancestor. I will, therefore,
start by comparing the brain of Ammocoetes with that of arthropods,
especially of Limulus and of the scorpion-group.
{62}The supra-infundibular portion of the brain in vertebrates corresponds
clearly to the supra-oesophageal portion of the invertebrate brain in so
far that in both cases here is the seat of the will. Voluntary action is as
impossible to the arthropod deprived of its supra-oesophageal ganglia as to
the vertebrate deprived of its cerebrum. It corresponds, also, in that from
it arise the nerves of sight and smell and no other nerves; this is also
the case with the supra-oesophageal ganglia, for from a portion of these
ganglia arise the nerves to the eyes and the nerves to the first antennæ,
of which the latter are olfactory in function. Thus, in the accompanying
figure, taken from Bellonci, it is seen that the supra-oesophageal ganglia
consist of a superior segment corresponding to the cerebrum, a middle
segment from which arise the nerves to the lateral eyes and to the
olfactory antennæ, corresponding to the basal ganglia of the brain and the
optic lobes, and, according to Bellonci, of an inferior segment from which
arise the nerves to the second pair of antennæ. This last segment is not
supra-oesophageal in position, but is situated on the oesophageal
commissures. It has been shown by Lankester and Brauer in Limulus and the
scorpion to be in reality the first ganglion of the infra-oesophageal
series, and not to belong to the supra-oesophageal group.
[Illustration: FIG. 27.--THE BRAIN OF _Sphæroma serratum_. (After
BELLONCI.)
_Ant. I._ and _Ant. II._, nerves to 1st and 2nd antennæ. _f.br.r._,
terminal fibre layer of retina; _Op. g. I._, first optic ganglion; _Op. g.
II._, second optic ganglion; _O.n._, optic nerve-fibres forming an optic
chiasma.]
Further, in Limulus, in the scorpion-group, and in all the extinct
{63}Eurypteridæ--in fact, in the Palæostraca generally--there are two
median eyes in addition to the lateral eyes, which were innervated from
these ganglia.
In Ammocoetes, then, if the supra-infundibular portion of the brain really
corresponds to the supra-oesophageal of the palæostracan group, we ought to
find, as indeed is the case, an optic apparatus consisting of two lateral
eyes and two median eyes, innervated from the supra-infundibular
brain-mass, and an olfactory apparatus built up on the same lines as in the
scorpion-group, also innervated from this region. If, in addition, it be
found that those two median eyes are degenerate eyes of the same type as
the median eyes of Limulus and the scorpion-group, then the evidence is so
strong as to amount to a proof of the correctness of the theory. This
evidence is precisely what has been obtained in recent years, for the
vertebrate did possess two median eyes in addition to the two lateral ones,
and these two median eyes are degenerate eyes of the type found in the
median eyes of arthropods and are not of the vertebrate type. Moreover, as
ought also to be the case, they are most evident, and one of the pair is
most nearly functional in the lowest perfect vertebrate, Ammocoetes.
Of all the discoveries made in recent years, the discovery that the pineal
gland of the vertebrate brain was originally a pair of median eyes is by
far the most important clue to the ancestry of the vertebrate, for not only
do they correspond exactly in position with the median eyes of the
invertebrates, but, being already degenerate and functionless in the lowest
vertebrate, they must have been functional in a pre-vertebrate stage, thus
giving the most direct clue possible to the nature of the pre-vertebrate
stage. It is especially significant that in Limulus they are already
partially degenerated. What, then, ought to be the structure and relation
to the brain of the median and lateral eyes of the vertebrate if they
originated from the corresponding organs of some one or other member of the
palæostracan group?
This question will form the subject of the next chapter.
SUMMARY.
The object of this book is to attempt to find out from what group of
invertebrates the vertebrate arose; no attempt is made to speculate upon
the causes of variation by means of which evolution takes place.
{64}A review of the animal kingdom as a whole leads to the conclusion
that the upward development of animals from an original coelenterate
stock, in which the central nervous system consists of a ring of nervous
material surrounding the mouth, has led, in consequence of the
elaboration of the central nervous system, to a general plan among the
higher groups of invertebrates in the topographical arrangement of the
important organs. The mouth is situated ventrally, and leads by means of
the oesophagus into an alimentary canal which is situated dorsally to the
central nervous system. Thus the oesophagus pierces the central nervous
system and divides it into two parts, the supra-oesophageal ganglia and
the infra-oesophageal ganglia. This is an almost universal plan among
invertebrates, but apparently does not hold for vertebrates, for in them
the central nervous system is always situated dorsally and the alimentary
canal ventrally, and there is no piercing of the central nervous system
by an oesophagus.
Yet a remarkable resemblance exists between the central nervous system of
the vertebrate and that of the higher invertebrates, of so striking a
character as to compel one school of anatomists to attempt the derivation
of vertebrates from annelids. Now, the central nervous system of
vertebrates forms a hollow tube, and a diverticulum of this hollow tube,
known as the infundibulum, passes to the ventral surface of the brain in
the very position where the oesophagus would have been if that brain had
belonged to an annelid or an arthropod. This school of anatomists
therefore concluded that this infundibular tube represented the original
invertebrate oesophagus which had become closed and no longer opened into
the alimentary canal owing to the formation of a new mouth in the
vertebrate. As, however, the alimentary canal of the vertebrate is
ventral to the central nervous system, and not dorsal, as in the
invertebrate, it follows that the remains of the original invertebrate
mouth into which the oesophagus (in the vertebrate the infundibular tube)
must have opened must be searched for on the dorsal side of the
vertebrate; and so the theory was put forward that the vertebrate had
arisen from the annelid by the reversal of surfaces, the back of the one
animal becoming the front of the other.
The difficulties in the way of accepting such reversal of surfaces have
proved insuperable, and another school has arisen which suggests that
evolution has throughout proceeded on two lines, the one forming groups
of animals in which the central nervous system is pierced by the
food-channel and the gut therefore lies dorsally to it, the other in
which the central nervous system always lies dorsally to the alimentary
canal and is not pierced by it. In both cases the highest products of the
evolution have become markedly segmented animals, in the former, annelids
and arthropods; in the latter, vertebrates. The only evidence on which
such theory is based is the existence of low forms of animals, known as
the _Enteropneusta_, the best known example of which is called
_Balanoglossus_; they are looked upon as aberrant annelid forms by many
observers.
This theory does not attempt to explain the peculiarities of the tube of
the vertebrate central nervous system, or to account for the
extraordinary resemblance between the structure and arrangement of the
central nervous systems of vertebrates and of the highest invertebrate
group.
Neither of these theories is satisfactory or has secured universal
acceptance. The problem must be considered entirely anew. What are the
guiding principles in this investigation?
{65}The evolution of animal life on this earth can clearly, on the whole,
be described as a process of upward progress culminating in the highest
form--man; but it must always be remembered that whole groups of animals
such as the Tunicata have been able to survive owing to a reverse process
of degeneration.
If there is one organ more than another which increases in complexity as
evolution proceeds, which is the most essential organ for upward
progress, surely it is the central nervous system, especially that
portion of it called the brain. This consideration points directly to the
origin of vertebrates from the most highly organized invertebrate
group--the Arthropoda--for among all the groups of animals living on the
earth in the present day they alone possess a central nervous system
closely comparable with that of vertebrates. Not only has an upward
progress taken place in animals as a whole, but also in the tissues
themselves a similar evolution is apparent, and the evidence shows that
the vertebrate tissues resemble more closely those of the arthropod than
of any other invertebrate group.
The evidence of geology points to the same conclusion, for the evidence
of the rocks shows that before the highest mammal--man--appeared, the
dominant race was the mammalian quadruped, from whom the highest mammal
of all--man--sprung; then comes, in Mesozoic times, the age of reptiles
which were dominant when the mammal arose from them. Preceding this era
we find in Carboniferous times that the amphibian was dominant, and from
them the next higher group--the reptiles--arose. Below the Carboniferous
come the Devonian strata with their evidence of the dominance of the
fish, from whom the amphibian was directly evolved. The evidence is so
clear that each succeeding higher form of vertebrate arose from the
highest stage reached at the time, as to compel one to the conclusion
that the fishes arose from the race which was dominant at the time when
the fishes first appeared. This brings us to the Silurian age, in which
the evidence of the rocks points unmistakably to the sea-scorpions,
king-crabs, and trilobites as being the dominant race. It was preceded by
the great trilobite age, and the whole period, from the first appearance
of the trilobite to the time of dwindling away of the sea-scorpions, may
be designated the Palæostracan age, using the term Palæostraca to include
both trilobites and the higher scorpion and king-crab forms evolved from
them. The evidence of geology then points directly and strongly to the
origin of vertebrates from the Palæostraca--arthropod forms which were
not crustacean and not arachnid, but gave origin both to the modern-day
crustaceans and arachnids. The history of the rocks further shows that
these ancient fishes, when they first appeared, resembled in a remarkable
manner members of the palæostracan group, so that again and again
palæontologists have found great difficulty in determining whether a
fossil is a fish or an arthropod. Fortunately, there is still alive on
the earth one member of this remarkable group--the Limulus, or King-Crab.
On the vertebrate side the lowest non-degenerate vertebrate is the
lamprey, or Petromyzon, which spends a large portion of its existence in
a larval stage, known as the Ammocoetes stage of the lamprey, because it
was formerly considered to be a separate species and received the name of
Ammocoetes. The larval stages of any animal are most valuable for the
study of ancestral problems, so that it is most fortunate for the
solution of the ancestry of vertebrates that Limulus on the one side and
Ammocoetes on the other are {66}available for thorough investigation and
comparison. There are no trilobites still alive, but in Branchipus and
Apus we possess the nearest approach to the trilobite organization among
living crustaceans.
So strongly do all these different lines of argument point to the origin
of vertebrates from arthropods as to make it imperative to reconsider the
position of that school of anatomists who derived vertebrates from
annelids by reversing the back and front of the animal. Let us not turn
the animal over, but re-consider the position, the infundibular tube of
the vertebrate still representing the oesophagus of the invertebrate, the
cerebral hemispheres and basal ganglia the supra-oesophageal ganglia, the
_crura cerebri_ the oesophageal commissures, and the infra-infundibular
part of the brain the infra-oesophageal ganglia. It is immediately
apparent that just as the invertebrate oesophagus leads into the large
cephalic stomach, so the infundibular tube leads into the large cavity of
the brain known as the third ventricle, which, together with the other
ventricles, forms in the embryo a large anterior dilated part of the
neural tube. In the arthropod this cephalic stomach leads into the
straight narrow intestine; in the vertebrate the fourth ventricle leads
into the straight narrow canal of the spinal cord. In the arthropod the
intestine terminates in the anus; in the vertebrate embryo the canal of
the spinal cord terminates in the anus by way of the neurenteric canal.
Keep the animal unreversed, and immediately the whole mystery of the
tubular nature of the central nervous system is revealed, for it is seen
that the nervous matter, which corresponds bit by bit with that of the
arthropod, has surrounded to a greater or less extent and amalgamated
with the tube of the arthropod alimentary canal, and thus formed the
so-called central nervous system of the vertebrate.
The manner in which the nervous material has invaded the walls of the
tube is clearly shown both by the study of the comparative anatomy of the
central nervous system in the vertebrate and also by its development in
the embryo.
This theory implies that the vertebrate alimentary canal is a new
formation necessitated by the urgency of the case, and, indeed, there was
cause for urgency, for the general plan of the evolution of the
invertebrate from the coelenterate involved the piercing of the anterior
portion of the central nervous system by the oesophagus, while, at the
same time, upward progress meant brain-development; brain-development
meant concentration of nervous matter at the anterior end of the animal,
with the result that in the highest scorpion and spider-like animals the
brain-mass has so grown round and compressed the food-tube that nothing
but fluid pabulum can pass through into the stomach; the whole group have
become blood-suckers. These kinds of animals--the sea-scorpions--were the
dominant race when the vertebrates first appeared: here in the natural
competition among members of the dominant race the difficulty must have
become acute. Further upward evolution demanded a larger and larger brain
with the ensuing consequence of a greater and greater difficulty of
food-supply. Nature's mistake was rectified and further evolution
secured, not by degeneration in the brain-region, for that means
degradation not upward progress, but by the formation of a new
food-channel, in consequence of which the brain was free to develop to
its fullest extent. Thus the great and mighty kingdom of the Vertebrata
was evolved with its culminating organism--man--whose massive brain with
all its possibilities could never have been evolved if he had still been
{67}compelled to pass the whole of his food through the narrow
oesophageal tube, still existent in him as the infundibular tube. This,
then, is the working hypothesis upon which this book is written. If this
view is right, that the Vertebrate was formed from the Palæostracan
without any reversal of surfaces, but by the amalgamation of the central
nervous system and alimentary canal, then it follows that we have various
fixed points of comparison in the central nervous systems of the two
groups of animals from which to search for further clues. It further
follows that from such starting-point every organ of importance in the
body of the arthropod ought to be visible in the corresponding position
in the vertebrate, either as a functional or rudimentary organ. The
subsequent chapters will deal with this detailed comparison of organs in
the arthropod and vertebrate respectively.
{68}CHAPTER II
_THE EVIDENCE OF THE ORGANS OF VISION_
Different kinds of eye.--Simple and compound retinas.--Upright and
inverted retinas.--Median eyes.--Median or pineal eyes of Ammocoetes and
their optic ganglia.--Comparison with other median eyes.--Lateral eyes of
vertebrates compared with lateral eyes of crustaceans.--Peculiarities of
the lateral eye of the lamprey.--Meaning of the optic
diverticula.--Evolution of vertebrate eyes.--Summary.
THE DIFFERENT KINDS OF EYE.
In all animals the eyes are composed of two parts. 1. A set of special
sensory cells called the retina. 2. A dioptric apparatus for the purpose of
forming an image on the sensory cells. The simplest eye is formed from a
modified patch of the surface-epithelium; certain of the hypodermal cells,
as they are called, elongate, and their cuticular surface becomes bulged to
form a simple lens. These elongated cells form the retinal cells, and are
connected with the central nervous system by nerve-fibres which constitute
an optic nerve; the cells themselves may contain pigment.
The more complicated eyes are modifications of this type for the purpose of
making both the retina and the dioptric apparatus more perfect. According
to a very prevalent view, these modifications have been brought about by
invaginations of the surface-epithelium. Thus if ABCD (Fig. 28) represents
a portion of the surface-epithelium, the chitinous cuticle being
represented by the dark line, with the hypodermal cells beneath, and if the
part C is modified to form an optic sense-plate, then an invagination
occurring between A and B will throw the retinal sense-cells with the optic
nerve further from the surface, and the layers B and A between the retina
and the source of light will be available for the formation of the dioptric
apparatus. The lens is now formed from the cuticular surface of A, and the
{69}hypodermal cells of A elongate to form the layer known by the name of
corneagen, or vitreogen, the cells of B remaining small and forming the
pre-retinal layer of cells. The large optic nerve end-cells of the retinal
layer, C, take up the position shown in the figure, and their cuticular
surface becomes modified to form rods of varying shape called rhabdites,
which are attached to the retinal cells. Frequently the rhabdites of
neighbouring cells form definite groups, each group being called a
rhabdome. Whatever shape they take it is invariably found that these little
rods (bacilli), or rhabdites, are modifications of the cuticular surface of
the cells which form the retinal layer. Also, as must necessarily be the
case from the method of formation, the optic nerve arises from the nuclear
end of the retinal cells, never from the bacillary end. As in the case
first mentioned, so in this case, the light strikes direct upon the
bacillary end of the retinal cells; such eyes, therefore, are eyes with an
_upright retina_.
[Illustration: FIG. 28.--DIAGRAM OF FORMATION OF AN UPRIGHT SIMPLE RETINA.]
It may happen that the part invaginated is the optic sense-plate itself, as
would be the case if in the former figure, instead of C, the part B was
modified to form a sense-plate. This will give rise to an eye of a
character different from the former (Fig. 29). The optic nerve-fibres now
lie between the source of light and the retinal end-cells, the layer A as
before forms the cuticular lens, and its hypodermal cells elongate to form
the corneagen; there is no pre-retinal layer, but, on the contrary, a
post-retinal layer, C, called the tapetum, and, as is seen, the light
passes through the retinal layer to the {70}tapetum. The cuticular surface
of the retinal cells forming the rods or bacilli is directed towards the
tapetal layer away from the source of light, and the nuclei of the retinal
cells are pre-bacillary in position, in contradistinction to the upright
eye, where they are post-bacillary. The retinal end-cells are devoid of
pigment, the pigment being in the tapetal layer.
Such an eye, in contradistinction to the former type, is an eye with an
_inverted retina_; but still the same law holds as in the former case--the
optic nerve-fibres enter at the nuclear ends of the cells, and the rods are
formed from the cuticular surface.
In these eyes the pigmented tapetal layer is believed to act as a
looking-glass; the dioptric apparatus throws the image on to its shiny
surface, from whence it is reflected directly on to the rods, which are in
close contact with the tapetum. A similar process has been suggested in the
case of the mammalian lateral eye, with its inverted retina. Johnson
describes the post-retinal pigmented layer as being frequently coloured and
shiny, and imagines that it reflects the image directly back on to the
rods.
[Illustration: FIG. 29.--DIAGRAM OF FORMATION OF AN INVERTED SIMPLE RETINA.
The arrow shows the direction of the source of light in this as in the
preceding figure. In both figures the cuticular rhabdites are represented
by thick black lines.]
Thus we see that eyes can be placed in different categories, _e.g._ those
with an upright retina and those with an inverted retina; also, according
to the presence or absence of a tapetum, eyes have been grouped as tapetal
or non-tapetal. All the eyes considered so far are called simple eyes, or
ocelli; and a number of ocelli may be {71}contiguous though separate, as in
the lateral eyes of the scorpion. They may, however, come into close
contact and form one single, large, compound eye. Such ocelli, in a very
large number of cases, retain each its own dioptric apparatus, and
therefore the external appearance of the compound eye represents not a
single lens, but a large number of facets, as is seen in the eyes of
insects. Owing to these differences, eyes have been divided into simple and
compound, and into facetted and non-facetted.
Yet another complication occurs in the formation of eyes, which is,
perhaps, the most important of all: the retinal portion of the eye, instead
of consisting of simple retinal cells, with their accompanying rhabdites,
may include within itself a portion of the central nervous system.
The rationale of such a formation is as follows: The external covering of
the body is formed by a layer of external epithelial cells--the ectodermal
cell-layer--and an underlying neural layer, of which the latter gives
origin to the central nervous system. As development proceeds, this central
nervous system sinks inwards, leaving as its connection with the ectoderm
the sensory nerves of the skin. That part of the neural layer which
underlies the optic plate forms the optic ganglion, and when the central
nervous system leaves the surface to take up its deeper position, the
strand of nerve-fibres known as the optic nerve, is left connecting it with
the retinal cells as seen in Figs. 28, 29. It may, however, happen that
part of the optic ganglion remains at the surface, in close connection with
the retinal end-cells, when the rest of the central nervous system sinks
inwards. The retina of such an eye is composed of the combined optic
ganglion and retinal end-cells; the strand of nerve-fibres which is left as
the connection between it and the rest of the brain, which is also called
the optic nerve, is not a true peripheral nerve, as in the first case, but
rather a tract of fibres connecting two parts of the brain, of which one
has remained at the periphery. Such a retina, in contradistinction to the
first kind, may be called a _compound retina_.
The optic ganglion, as seen in eyes with a simple retina, consists of a
cortical layer of small, round nerve-cells, and an internal medulla of fine
nerve-fibres, which form a thick network known as 'Punctsubstanz,' or in
modern terminology, 'Neuropil.' Fibres which pass into this 'neuropil' from
other parts of the brain connect them with the optic ganglion.
{72}At the present time, owing to the researches of Golgi, Ramón y Cajal,
and others, the nervous system is considered to be composed of a number of
separate nerve-units, called neurones, each neurone consisting of a
nerve-cell with its various processes; one of these--the
neuraxon--constitutes the nerve-fibre belonging to that nerve-cell, the
other processes--the dendrites--establish communication with other
neurones. The place where these processes come together is called a
synapse, and the tangle of fine fibres formed at a number of synapses forms
the 'neuropil.'
[Illustration: FIG. 30.--DIAGRAM OF FORMATION OF AN UPRIGHT COMPOUND
RETINA.
_ABCD_, as in Fig. 28. _Op. g. I._ and _Op. g. II._, two optic ganglia
which combine to form the retinal ganglion, _Rt. g._]
When, therefore, a compound retina is formed by the amalgamation of the
ectodermal part--the retinal cells proper--with the neurodermic part--to
which the name 'retinal ganglion' may be given,--such a retina consists of
neuropil substance and nerve-cells, as well as the retinal end-cells. In
all such compound retinas, the retinal ganglion is not single, but two
optic ganglia at least are included in it, so that there are two sets of
nerve-cells and two synapses are always formed; one between the retinal
end-cells and the neurones of the first optic ganglion, which may be called
the ganglion of the retina, the other between the first and second ganglia,
which, seeing that the neuraxons of its cells form the optic nerve, may be
called the ganglion of the optic nerve. The 'neuropil' formed by these
synapses forms the molecular layers of the compound retina, and the cells
themselves form the nuclear layers. Thus an upright compound retina, formed
in the same way as the upright simple retina, would be illustrated by Fig.
30.
{73}Further, in precisely the same way as in the case of the simple retina,
such a compound retina may be upright or inverted. Thus, in the lateral
eyes of crustaceans and insects, a compound retina of this kind is formed,
which is upright; while in the vertebrates the compound retina of the
lateral eyes is inverted.
The compound retina of vertebrates is usually described as composed of a
series of layers, which may be analyzed into their several components as
follows:--
Layer of rods and cones }
External nuclear layer } retina proper } Ectodermic part
External molecular layer }}
Internal nuclear layer } ganglion of retina }
Internal molecular layer }} } retinal } neurodermic
Optic nerve-cell layer } ganglion of optic nerve} ganglion} part
Layer of optic nerve }
fibres
The difference between the development of these two types of eye--those
with a simple retina and those with a compound retina--has led, in the most
natural manner, to the conception that the retina is developed, in the
higher animals, sometimes from the cells of the peripheral epidermis,
sometimes from the tissue of the brain--two modes of development termed by
Balfour 'peripheral' and 'cerebral.' An historical survey of the question
shows most conclusively that all investigators are agreed in ascribing the
origin of the simple retina to the peripheral method of development, the
retina being formed from the hypodermal cells by a process of invagination,
while the cerebral type of development has been described only in the
development of the compound retina. The natural conclusion from this fact
is that, in watching the development of the compound retina, it is more
difficult to differentiate the layers formed from the epidermal retinal
cells and those formed from the epidermal optic ganglion-cells, than in the
case of the simple retina, where the latter cells withdraw entirely from
the surface. This is the conclusion to which Patten has come, and, indeed,
judging from the text-book of Korschelt and Heider, it is the generally
received opinion of the day that, as far as the Appendiculata are
concerned, the retina, in the true sense--the retinal end-cells, with their
cuticular rods,--is formed, in all cases, from the peripheral cells of the
hypodermal layer, the cuticular rods being modifications of the general
cuticular surface of the body. The apparent cerebral development of the
crustacean {74}retina, as quoted from Bobretsky by Balfour, is therefore in
reality the development of the retinal ganglion, and not of the retina
proper.
There is, I imagine, a universal belief that the natural mode of origin of
a sense-organ, such as the eye, must always have been from the cells
forming the external surface of the animal, and that direct origin from the
central nervous system is _a priori_ most improbable. It is, therefore, a
matter of satisfaction to find that the evidence for the latter origin has
universally broken down, with the single exception of the eyes of
vertebrates and their degenerated allies; a fact which points strongly to
the probability that a reconsideration of the evidence upon which the
present teaching of the origin of the vertebrate eye is based will show
that here, too, a confusion has arisen between that part formed from the
epidermal surface and that from the optic ganglion.
THE MEDIAN OR PINEAL EYES.
Undoubtedly, in recent times, the most important clue to the ancestry of
vertebrates has been given by the discovery that the so-called pineal gland
in the vertebrate brain is all that remains of a pair of median or pineal
eyes, the existence of which is manifest in the earliest vertebrates; so
that the vertebrate, when it first arose, possessed a pair of median eyes
as well as a pair of lateral eyes. The ancestor of the vertebrate,
therefore, must also have possessed a pair of median eyes as well as a pair
of lateral eyes.
Very instructive, indeed, is the evidence with regard to these median eyes,
for one of the great characteristics of the ancient palæostracan forms is
the invariable presence of a pair of median eyes as well as a pair of
lateral eyes. In the living representative of such forms--Limulus--the pair
of median eyes (Fig. 5) is well shown, and it is significant that here,
according to Lankester and Bourne, these eyes are already in a condition of
degeneration; so also in many of the Palæostraca (Fig. 7) the lateral eyes
are the large, well-developed eyes, while the median eyes resemble those of
Limulus in their insignificance.
We see, then, that in the dominant arthropod race at the time when the
fishes first appeared, the type of eyes consisted of a pair of
well-developed lateral eyes and a pair of insignificant, partially
degenerated, median eyes. Further, according to all palæontologists, {75}in
the best-preserved head-shields of the most ancient fishes, especially well
seen in the Osteostraci, in Cephalaspis, Tremataspis, Auchenaspis,
Keraspis, a pair of large, prominent lateral eyes existed, between which,
in the mid-line, are seen a pair of small, insignificant median eyes.
The evidence of the rocks, therefore, proves that the pair of median eyes
which were originally the principal eyes (Hauptaugen), had already, in the
dominant arthropod group been supplanted by a pair of lateral eyes, and
had, in consequence, become small and insignificant, at the time when
vertebrates first appeared. This dwindling process thus initiated in the
arthropod itself has steadily continued ever since through the whole
development of the vertebrates, with the result that, in the highest
vertebrates, these median or pineal eyes have become converted into the
pineal gland with its 'brain-sand.'
In the earliest vertebrate these median eyes may have been functional; they
certainly were more conspicuous than in later forms. Alone among living
vertebrates the right median eye of Ammocoetes is so perfect and the skin
covering it so transparent that I have always felt doubtful whether it may
not be of use to the animal, especially when one takes into consideration
the undeveloped state of the lateral eyes in this animal, hidden as they
are under the skin. Thus the one living vertebrate which is comparable with
these extinct fishes is the one in which one of the pineal eyes is most
well defined, most nearly functional.
Before passing to the consideration of the structure of the median eyes of
Ammocoetes, it is advisable to see whether these median eyes in other
animals, such as arachnids and crustaceans, belong to any particular type
of eyes, for then assuredly the median eyes of Ammocoetes ought to belong
to the same type if they are derived from them.
In the specialized crustacean, as in the specialized vertebrate, the median
eyes have disappeared, at all events in the adult, but still exist in the
primitive forms, such as Branchipus, which resemble the trilobites in some
respects. On the other hand, the median eyes have persisted, and are well
developed in the arachnids, both scorpions and spiders possessing a
well-developed pair. The characteristics of the median eyes must then be
especially sought for in the arachnid group.
Both scorpions and spiders possess many eyes, of which two are {76}always
separate and median in position, while the others form lateral groups; all
these eyes possess a simple retina and a simple corneal lens. Grenacher was
the first to point out that in the spiders two very distinct types of eye
are found. In the one the retina is upright; in the other the retina is
inverted, and the eye possesses a tapetal layer. The distribution of these
two types is most suggestive, for the inverted retina is always found in
the lateral eyes, never in the two median eyes; these always possess a
simple upright retina.
In the crustaceans, the lateral eyes differ also from the median eyes, but
not in the same way as in the arachnids; for here both types of eye possess
an upright retina, but the retina of the lateral eyes is compound, while
that of the median eyes is simple. In other words, the median eyes are in
all cases eyes with a simple upright retina and a simple cuticular lens,
while the retina of the lateral eyes is compound or may be inverted,
according as the animal in question possesses crustacean or arachnid
affinities. The lateral eye of the vertebrate, possessing, as it does, an
inverted compound retina, indicates that the vertebrate arose from a stock
which was neither arachnid nor crustacean, but gave rise to both groups--in
fact, was a member of the great palæostracan group. What, then, is the
nature of the median eyes in the vertebrate?
THE MEDIAN EYES OF AMMOCOETES.
The evidence of Ammocoetes is so conclusive that I, for one, cannot
conceive how it is possible for any zoologist to doubt whether the parietal
organ, as they insist on calling it, had ever been an eye, or rather a pair
of eyes.
Anyone who examines the head of the larval lamprey will see on the dorsal
side, in the median line, first, a somewhat circular orifice--the unpaired
nasal opening; and then, tailwards to this, a well-marked circular spot,
where the skin is distinctly more transparent than elsewhere. This spot
coincides in position with the underlying dorsal pineal eye, which shines
out conspicuously owing to the glistening whiteness of its pigment. Upon
opening the brain-case the appearance as in Fig. 20 is seen, and the mass
of the right _ganglion habenulæ_ (_G.H.R._), as it has been called, stands
out conspicuously as well as the right or dorsal pineal eye (_Pn._). Both
eye and ganglion appear at first sight to be one-sided, but further
examination shows that a left _ganglion habenulæ_ is present, though much
smaller than on {77}the right side. In connection with this is another
eye-like organ--the left or ventral pineal eye,--much more aborted, much
less like an eye than the dorsal one; so also there are two bundles of
peculiar fibres called Meynert's bundles, which connect this region with
the infra-infundibular region of the brain; of these, the right Meynert's
bundle is much larger than the left.
[Illustration: FIG. 31.--ONE OF A SERIES OF HORIZONTAL SECTIONS THROUGH THE
HEAD OF AMMOCOETES.
_l.m._, upper lip muscles; _m.c._, muco-cartilage; _n._, nose; _na.c._,
nasal cartilage; _pn._, right pineal eye and nerve; _g.h.r._, right
_ganglion habenulæ_; _s.m._, somatic muscles; _cr._, membranous wall of
cranium; _ch._, choroid plexus; _gl._, glandular substance and pigment
filling up brain-case.]
{78}[Illustration: FIG. 32.--EYE OF ACILIUS LARVA, WITH ITS OPTIC GANGLION.
On the right side the nerve end-cells have been drawn free from pigment.]
[Illustration: FIG. 33.--PINEAL EYE OF AMMOCOETES, WITH ITS _Ganglion
Habenulæ._
On the left side the eye is drawn as it appeared in the section. On the
right side I have removed the pigment from the nerve end-cells, and drawn
the eye as, in my opinion, it would appear if it were functional.]
This difference between right and left indicates a greater degeneration on
the left side, and points distinctly to a close relationship between the
nerve-masses known as _ganglia habenulæ_ and the median eyes. In my opinion
this ganglion is, in part, at all events, the optic ganglion of the median
eye on each side. It is built up on the same type as the optic ganglia of
invertebrate simple eyes, with a cortex of small round cells and a medulla
of fine nerve-fibres. Into this ganglion, on the right side, there passes a
very well-defined nerve--the nerve of the dorsal eye. The eye itself with
its nerve, _pn._, and its optic ganglion, _g.h.r._, is beautifully shown by
means of a horizontal section through the head of Ammocoetes (Fig. 31).
Originally, as described by Scott, the eye stood vertically {79}above its
optic ganglion, and presented an appearance remarkably like Fig. 32, which
represents one of the simple eyes and optic ganglia of a larva of Acilius
as described by Patten; then, with the forward growth of the upper lip, the
right pineal eye was dragged forward and its nerve pulled horizontally over
the _ganglion habenulæ_. For this reason the eye, nerve, and ganglion are
better shown in a nearly horizontal than in a transverse section.
The optic nerve belonging to this eye is most evident and clearly shown in
Fig. 31, and in the series of consecutive sections which follow upon this
section; no doubt can arise as to the structure in question having been the
nerve of the eye, even though, as is possible, it does not contain any
functional nerve-fibres.
[Illustration: FIG. 34.--HORIZONTAL SECTION THROUGH BRAIN OF AMMOCOETES, TO
SHOW THE LEFT, OR VENTRAL PINEAL EYE.
_pn._2_, left or ventral pineal eye; _pn._1_, last remnant of right, or
dorsal pineal eye; _g.h.r._, right _ganglion habenulæ_; _g.h.l._1_,
_g.h.l._3_, parts of left _ganglion habenulæ_; _pi._, fold of _pia mater_
which separates the left _ganglion habenulæ_ from the left pineal eye;
_f._, strands of nerve-fibres connecting the left eye with its ganglion,
_g.h.l._3_; _V_3_, third ventricle; _V.aq._, ventricle of aquæduct.]
The second, ventral or left, eye, belonging to the left ganglion habenulæ
is very different in appearance, being much less evidently an eye. Fig. 34
is one of the same series of horizontal sections as Fig. 31, _pn._1_ being
the last remnant of the right, or dorsal, eye, while _pn._2_ shows the
left, or ventral, eye with its connection with the left _ganglion
habenulæ_.
{80}In a series of sections I have followed the nerve of the right pineal
eye to its destination, as described in my paper in the _Quarterly Journal
of Microscopical Science_, and have found that it enters into the _ganglion
habenulæ_ just as the nerve to any simple eye enters into its optic
ganglion. This nerve, as I have shown, is a very distinct, well-defined
nerve, with no admixture of ganglion-cells or of connective tissue, very
different indeed to the connection between the left pineal eye and its
optic ganglion. Here there is no defined nerve at all; but the cells of the
_ganglion habenulæ_ stretch right up to the remains of the eye itself.
Seeing, then, that both the eye and ganglion on this side have reached a
much further grade of degeneration than on the right side, it may be fairly
concluded that the original condition of these two median eyes is more
nearly represented by the right eye, with its well-defined nerve and optic
ganglion, than by the left eye, or by the eyes in lizards and other animals
which do not show so well-defined a nerve as is possessed by Ammocoetes.
Quite recently Dendy has examined the two median eyes in the New Zealand
lamprey _Geotria australis_. In this species the second eye is much better
defined than in the European lamprey, and its connection with the _ganglion
habenulæ_ is more nerve-like. In neither eye, however, is the nerve so
clean cut and isolated as is the nerve of the dorsal, or right, eye in the
Ammocoetes stage of _Petromyzon Planeri_; in both, cells resembling those
of the cortex of the _ganglion habenulæ_ and connective tissues are mixed
up with the nerve-fibres which pass from each eye to its respective optic
ganglion.
THE RIGHT PINEAL EYE OF AMMOCOETES.
The optic fibres of the right median eye of Ammocoetes are connected with a
well-defined retina, the limits of which are defined by the white pigment
so characteristic of this eye. This pigment is apparently calcium
phosphate, which still remains as the 'brain-sand' of the human pineal
gland. The cells, which are hidden by this pigment, were described by me in
1890 as the retinal end-cells with large nuclei. In 1893, Studniçka
examined them more closely, and concluded that the retinal cells are of two
kinds: the one, nerve end-cells, the sensory cells proper; the other,
pigmented epithelial cells, which surround the sense-cells. The sense-cells
may contain some of the white pigment, but not so much as the other cells.
Similarly, in the {81}median eyes of Limulus, Lankester and Bourne find it
difficult to determine how far the retinal end-cells contain pigment and
how far that pigment really is in the cells surrounding these nerve
end-cells.
The interior of the eye presents the appearance of a cavity in shape like a
cornucopia, the stalk of which terminates at the place where the nerve
enters. This cavity is not empty, but the posterior part of it is filled
with the termination of the nerve end-cells of the retina, as pointed out
by me and confirmed by Studniçka. These terminations are free from pigment,
and contain strikingly translucent bodies, which I have described in my
paper in the _Quarterly Journal_, and called rhabdites, for they present
the same appearance and are situated in the same position as are many of
the rhabdites on the terminations of the retinal end-cells of arthropod
eyes. Studniçka has also seen these appearances, and figures them in his
second paper on the nerve end-cells of the pineal eye of Ammocoetes.
Up to this point the following conclusions may be drawn:--
1. Ammocoetes possesses a pair of median eyes, just as was the case with
the most ancient fishes, and with the members of the contemporary
palæostracan group.
2. The retina of one of these eyes is well-defined and upright, not
inverted, and therefore in this respect agrees with that of all median
eyes.
3. The presence of nerve end-cells, with pigment either in them or in
cells around them, to the unpigmented ends of which translucent bodies
resembling rhabdites are attached, is another proof that this retina
agrees with that of the median eyes of arthropods.
4. The simple nature of the nerve with its termination in an optic
ganglion closely resembling in structure an arthropod optic ganglion,
together with Studniçka's statement that the nerve end-cells pass
directly into the nerve, points directly to the conclusion that this
retina is a simple, not a compound, retina, and that it therefore in this
respect also agrees with the retina of all median eyes.
With respect to this last conclusion, neither I myself nor Studniçka have
been able to see any definite groups of cells between the nerve end-cells
and the optic nerve such as a compound retina necessitates.
{82}On the other hand, Dendy describes in the New Zealand lamprey, _Geotria
australis_, a cavity where the nerve enters into the eye, which he calls
the atrium. This cavity is distinct from the main cavity of the eye, and is
separated from it by a mass of cells similar in appearance to those of the
cortex of the _ganglion habenulæ_. In these two eyes then, groups of cells,
resembling in appearance those belonging to an optic ganglion, exist in the
eyes themselves. This atrium is evidently that part of the central cavity
which I have called the handle of the cornucopia in the European lamprey,
and the very fact that it is separated from the rest of the central cavity
is evidence that we are dealing here with a later stage in the history of
the pineal eyes than in the case of the Ammocoetes of _Petromyzon Planeri_.
Taking also into consideration the continuity of the mass of small
ganglion-cells which surround this atrium with the cells of the _ganglion
habenulæ_ by means of the similar cells scattered along the course of the
nerve, and also bearing in mind the fact already stated that in the more
degenerate left eye of Ammocoetes the cells of the _ganglion habenulæ_
extend right up to the eye itself, it seems more likely than not that these
cells do not represent the original optic ganglion of a compound retina,
but rather the subsequent invasion, by way of the pineal nerve, of
ganglion-cells belonging to a portion of the brain. In the last chapter it
has been suggested that the presence of the trochlear or fourth cranial
nerve has given rise to the formation of the cerebellum by a similar
spreading.
There is certainly no appearance in the least resembling a compound retina
such as is seen in the vertebrate or crustacean lateral eye. In the median
eyes of scorpions and of Limulus, cells are found within the capsule of the
eye among the nerve-fibres and the nerve end-cells. These are especially
numerous in the median eyes of Limulus, as described by Lankester and
Bourne, and are called by them intrusive connective tissue cells. The
meaning of these cells is not, to my mind, yet settled. It is sufficient
for my purpose to point out that the presence of cells interneural in
position among the nerve end-cells of the retina of the median eyes of
Ammocoetes is more probable than not, on the assumption that the retina of
these eyes is built up on the same plan as that of the median eyes in
Limulus and the scorpions.
It is further to be borne in mind that these specimens of _Geotria_ worked
at by Dendy were in the 'Velasia' stage of the New Zealand {83}lamprey, and
correspond, therefore, more nearly to the Petromyzon than to the Ammocoetes
stage of the European lamprey.
THE DIOPTRIC APPARATUS.
Besides the retina, all eyes possess a dioptric apparatus. What is the
evidence as to its nature in these vertebrate median eyes? Lankester and
Bourne have divided the eyes of scorpions and Limulus into two kinds,
monostichous and diplostichous. In the first the retinal cells are supposed
to give rise to not only rhabdites but also the cuticular chitinous lens,
so that the eye is one-layered; in the second the lens is formed by a
well-marked hypodermal layer, in front of the retina, composed of elongated
cells, so that these eyes are two-layered or diplostichous. The lateral
eyes, according to them, are all monostichous, but the median eyes are
diplostichous.
[Illustration: FIG. 35.--EYE OF ACILIUS LARVÆ. (After PATTEN.)
_l._, chitinous lens; _c._, corneagen; _pr._, pre-retinal layer; _rh._,
rhabdites; _ret._, retinal end-cells.]
This distinction is not considered valid by other observers. Thus, {84}as
already indicated, Patten looks on all these eyes as three-layered, and
states that in all cases a corneagen or vitreogen layer exists, which gives
origin to the lens. For my own part I agree with Patten, but we are not
concerned here with the lateral eyes. It is sufficient to note that all
observers are agreed that the median eyes are characterized by this
well-marked cell-layer, the so-called vitreous or corneagen layer of cells.
[Illustration: FIG. 36.--EYE OF HYDROPHILUS LARVA, WITH THE PIGMENT OVER
THE RETINAL END-CELLS.
_l._, chitinous lens; _c._, corneagen; _pr._, pre-retinal layer; _rh._,
rhabdites; _ret._, retinal end-cells.]
This layer (_c._, Fig. 35) is composed of much-elongated cells of the
hypodermal layer, in each of which the large nucleus is always situated
towards the base of the cell. The space between it and the retina contains,
according to Patten the cells of the pre-retinal layer _(pr.)_. These may
be so few and insignificant as to give the impression that the vitreous
layer is immediately adjacent to the retina (_ret._).
Let us turn now to the right pineal eye of Ammocoetes (Fig. 37) and see
what its further structure is. The anterior part of the eye is free from
pigment, and is composed, as is seen in hæmatoxylin or carmine specimens,
of an inner layer of nuclei which are frequently arranged in a wavy line.
From this nucleated layer, strands of tissue, free from nuclei, pass to the
anterior edge of the eye.
In the horizontal longitudinal sections it is seen that these strands are
confined to the middle of the eye. On each side of them the nuclear layer
reaches the periphery, so that if we consider these strands to represent
long cylindrical cells, as described by Beard, then the anterior wall may
be described as consisting of long cylindrical cells, which are flanked on
either side by shorter cells of a similar kind. The nuclei at the base of
these cylindrical cells are not all alike. We see, in the first place,
large nuclei resembling the large nuclei belonging to the nerve end-cells;
these are the nuclei of {85}the long cylindrical cells. We see also smaller
nuclei in among these larger ones, which look like nuclei of intrusive
connective tissue, or may perhaps form a distinct layer of cells, situated
between the cells of the anterior wall and the terminations of the nerve
end-cells already referred to.
These elongated cells are in exactly the same position and present the same
appearance as the cells of the corneagen layer of any median eye. Like the
latter they are free from pigment and never show with osmic staining any
sign of the presence of translucent rhabdite-like bodies, such as are seen
in the termination of the retinal cells, and like the latter their nuclei
are at the base. The resemblance between this layer and the corneagen cells
of any median eye is absolute. Between it and the terminations of the
retinal cells there exists some ill-defined material certainly containing
cells which may well correspond to Patten's pre-retinal layer of cells.
Retina, corneagen, nerve, optic ganglion, all are there, all in their right
position, all of the right structure, what more is needed to complete the
picture?
[Illustration: FIG. 37.--PINEAL EYE OF AMMOCOETES, WITH ITS _Ganglion
Habenulæ_.]
In order to complete the dioptric apparatus a lens is necessary. Where,
then, is the lens in these pineal eyes? In all the arachnid eyes, whether
median or lateral, the lens is a single corneal lens composed of the
external cuticle, which is thickened over the corneagen cells. This
thickened cuticle is composed of chitin, and is not cellular, but is dead
material formed out of the living underlying corneagen cells. Such a lens
is in marked contrast to the lens of the lateral vertebrate eye, which is
formed by living cells themselves. This {86}thickening of the cuticular
layer to form a lens could only exist as long as that layer is absolutely
external, so that the light strikes immediately upon it; for, if from any
cause the eye became situated internally, the place of such a lens must be
filled by the structures situated between it and the surface, and the
thickened cuticle would no longer be formed.
In all vertebrates these pineal eyes are separated from the external
surface by a greater or less thickness of tissues; in the case of
Ammocoetes, as is seen in Fig. 31, the eye lies within the membranous
cranial wall, and is attached closely to it. The position, then, of the
cuticular, or corneal lens, as it is often called, on the supposition that
this is a median eye of the arachnid type, is taken by the membranous
cranium, and, as I have described in my paper in the _Quarterly Journal_,
on carefully lifting the eye in the fresh condition from the cranial wall,
it can be seen under a dissecting microscope that the cranial wall often
forms at this spot a lens-like bulging, which fits the shallow concavity of
the surface of the eye, and requires some little force to separate it from
the eye.
As will appear in a subsequent chapter, this cranial wall has been formed
by the growth, laterally and dorsally, of a skeletal structure known by the
name of the _plastron_. The last part of it to be completed would be that
part in the mid-dorsal line, where apparently, in consequence of the
insinking of the degenerating eyes, a dermal and subdermal layer already
intervened between the source of light and the eyes themselves.
When the membranous cranium was completed in the mid-dorsal region, it was
situated here as elsewhere just internally to the subdermal layer, and
therefore enclosed the pineal eyes. This, to my mind, is the reason why the
pineal eyes, which, in all other respects, conform to the type of the
median eyes of an arachnid-like animal, do not possess a cuticular lens.
Other observers have attempted to make a lens out of the elongated cells of
the anterior wall of the eye (my corneagen layer), but without success.
Studniçka, who calls this layer the _pellucida_, does not look upon it as
the lens, but considers, strangely enough, that the translucent appearances
at the ends of each nerve end-cell represent a lens for that cell, so that
every nerve end-cell has its own lens. Still more strange is it that,
holding this view, he should yet consider these knobs {87}to be joined by
filaments to the cells in the anterior wall of the eye, a conception fatal
to the action of such knobs as lenses.
The discovery that the vertebrate possesses, in addition to the lateral
eyes, a pair of median eyes, which are most conspicuous in the lowest
living vertebrate, together with the fact that such eyes are built up on
the same plan as the median eyes of living crustaceans or arachnids, not
only with respect to the eye itself but also to its nerve and optic
ganglion, constitutes a fact of the very greatest importance for any theory
of the origin of vertebrates; especially in view of the further fact, that
similar eyes in the same position are found not only in all the members of
the Palæostraca, but also in all those ancient forms (classed as fishes)
which lived at that time. At one and the same moment it proves the utter
impossibility of reversing dorsal and ventral surfaces, points in the very
strongest manner to the origin of the vertebrate from some member or other
of the palæostracan group, and insists that the advocates of the origin of
vertebrates from the Hemichordata, etc., should give an explanation of the
presence of these two median eyes of a more convincing character than that
given here.
THE LATERAL EYES.
Turning now to the consideration of the lateral eyes, we see that these
eyes in the arachnids often possess an inverted retina, in the crustaceans
always an upright retina. In the arachnids they possess a simple retina,
while in the crustaceans their retina is compound; so that in the latter
case the so-called optic nerve is in reality a tract of fibres connecting
together the brain-region with a variable number of optic ganglia, which
have been left at the periphery in close contact with the retinal cells,
when the brain sunk away from the superficial epithelial covering.
There is, then, this difference between the lateral eyes of crustaceans and
arachnids, that the retina of the former is compound, but never inverted,
while that of the latter may be inverted, but is always simple.
The retina of the lateral eyes of the vertebrate resembles both of these,
for it is compound, as in the crustacean, and inverted as in the arachnid.
It must always be borne in mind that in the palæostracan epoch {88}the
dominant race was neither crustacean nor arachnid, but partook of the
characters of both; also, as is characteristic of dominance, there was very
great variety of form, so that it seems more probable than not that some of
these forms may have combined the arachnid and crustacean characteristics
to the extent of possessing lateral eyes with an inverted yet compound
retina. A certain amount of evidence points in this direction. As already
stated, the compound retina which characterizes the vertebrate lateral eyes
is characteristic of all facetted eyes, and in the trilobites facetted
lateral eyes are commonly found. From this it may be concluded that many of
the trilobites possessed eyes with a compound retina. There have, however,
been found in certain species, e.g. _Harpes vittatus_ and _Harpes ungula_,
lateral eyes which were not facetted, and are believed by Korschelt and
Heider to be of an arachnid nature. They say, "Palæontologists have
appropriately described them as ocelli, although, from a zoological point
of view, they do not deserve this name, having most probably arisen in a
way similar to that conjectured in connection with the lateral eyes of
scorpions." If this conjecture is right, then in these forms the retina may
have been inverted, but because they belonged to the trilobite group, the
retina was most probably compound, so that here we may have had the
combination of the arachnid and crustacean characteristics. On the other
hand, in some forms of Branchipus, and many of the Gammaridæ, a single
corneal lens is found in conjunction with an eye of the crustacean type, so
that a non-facetted lateral eye, found in a fossil form, would not
necessarily imply the arachnid type of eye with the possibility of an
inverted retina. Whatever may be the ultimate decision upon these
particular forms, the striking fact remains, that both in the vertebrate
and in the arachnid the median eyes possess a simple upright retina, while
the lateral eyes possess an inverted retina, and that both in the
vertebrate and the crustacean the median eyes possess a simple upright
retina, while the lateral eyes possess a compound retina.
The resemblance of the retina of the lateral eyes of vertebrates to that of
the lateral eyes of many arthropods, especially crustaceans, has been
pointed out by nearly every one who has worked at these invertebrate
lateral eyes. The foundation of our knowledge of the compound retina is
Berger's well-known paper, the results of which are summed up by him in the
following two main conclusions.
{89}1. The optic ganglion of the Arthropoda consists of two parts, of
which the one stands in direct inseparable connection with the facetted
eye, and together with the layer of retinal rods forms the retina of the
facetted eye, while the other part is connected rather with the brain, and
is to be considered as an integral part of the brain in the narrower sense
of the word.
[Illustration: FIG. 38.--THE RETINA OF MUSCA. (After BERGER.)
_Br._, brain; _O.n._, optic nerve; _n.l.o.g._, nuclear layer of ganglion of
optic nerve; _m.l._, molecular layer (Punktsubstanz); _n.l.r.g.i._ and
_n.l.r.g.o._, inner and outer nuclear layers of the ganglion of the retina;
_f.br.r._, terminal fibre-layer of retina; _r._, layer of retinal end-cells
(indicated only).]
2. In all arthropods examined by him, the retina consists of five layers,
as follows:--
(1) The layer of rods and their nuclei.
(2) The layer of nerve-bundles.
(3) The nuclear layer.
(4) The molecular layer.
(5) The ganglion cell layer.
Berger passes under review the structure and arrangement of the optic
ganglion in a large number of different groups of arthropods, and concludes
that in all cases one part of the optic ganglion is always closely attached
to the visual end-cells, and this combination he calls the retina. On the
other hand, the nerve-fibres which connect the peripheral part of the optic
ganglion with the brain, the so-called optic nerve, are by no means
homologous in the different groups; for in some cases, as in many of the
stalk-eyed crustaceans, the whole optic ganglion is at the periphery, while
in others, as in the Diptera, only the retinal ganglion is at the
periphery, and the nerve-stalk connects this with the rest of the optic
ganglion, the latter being fused with the main brain-mass. In the Diptera,
in fact, according to Berger, the optic nerve {90}and retina are most
nearly comparable to those of the vertebrate. For this reason I give
Berger's picture of the retina of Musca (Fig. 38), in order to show the
arrangement there of the retinal layers.
[Illustration: FIG. 39.--THE BRAIN OF _Sphæroma serratum_. (After
BELLONCI.)
_Ant. I._ and _Ant. II._, nerves to 1st and 2nd antennæ. _f.br.r._,
terminal fibre-layer of retina; _Op. g. I._, first optic ganglion; _Op. g.
II._, second optic ganglion; _O.n._, optic nerve-fibres forming an optic
chiasma.]
In Branchipus and other primitive Crustacea, Berger also finds the same
retinal layers, but is unable to distinguish in the brain the rest of the
optic ganglion. Judging from Berger's description of Branchipus, and
Bellonci's of Sphæroma, it would almost appear as though the cerebral part
of the retina in the higher forms originated from two ganglionic
enlargements, an external and internal enlargement, as Bellonci calls them.
The external ganglion (_Op. g. I._, Fig. 39) may be called the ganglion of
the retina, the cells of which form the nuclear layer of the higher forms,
and the internal ganglion (_Op. g. II._, Fig. 39), from which the optic
nerve-fibres to the brain arise, may therefore be called the ganglion of
the optic nerve. Bellonci describes how in this latter ganglion cells are
found very different to the small ones of the external ganglion or ganglion
of the retina. So also in Branchipus, judging from the pictures of Berger,
Claus, and from my own observations (_cf._ Fig. 46, in which the double
nature of the retinal ganglion is indicated), the peripheral part of the
optic ganglion--_i.e._ the retinal ganglion--may be spoken {91}of as
composed of two ganglia. The external of these is clearly the ganglion of
the retina; its cells form the nuclear layer, the striking character of
which, and close resemblance to the corresponding layer in vertebrates, is
shown by Claus' picture, which I reproduce (Fig. 40). The internal ganglion
with which the optic nerve is in connection contains large ganglion cells,
which, together with smaller ones, form the ganglionic layer of Berger.
The most recent observations of the structure of the compound retina of the
crustacean eye are those of Parker, who, by the use of the methylene blue
method, and Golgi's method of staining, has been able to follow out the
structure of the compound retina in the arthropod on the same lines as had
already been done for the vertebrate. These two methods have led to the
conclusion that the arthropod central nervous system and the vertebrate
central nervous system are built up in the same manner--viz. by means of a
series of ganglia connected together, each ganglion being composed of
nerve-cells, nerve-fibres, and a fine reticulated substance called by
Leydig in arthropods 'Punktsubstanz,' and known in vertebrates and in
invertebrates at the present time as 'neuropil.' A further analysis
resolves the whole system into a combination of groups of neurones, the
cells and fibres of which form the cells and fibres of the ganglia, while
their dendritic connections with the terminations of other neurones,
together with the neuroglia-cells form the 'neuropil.' As is natural to
expect, that part of the central nervous system which helps to form the
compound retina is built up in the same manner as the rest of the central
nervous system.
[Illustration: FIG. 40.--BIPOLAR CELLS OF NUCLEAR LAYER IN RETINA OF
BRANCHIPUS. (After CLAUS.)
_f.br.r._, terminal fibre-layer of retina; _n.l.r.g._, bipolar cells of the
ganglion of the retina = inner nuclear layer; _m.l._, Punktsubstanz = inner
molecular layer; _b.m._, basement membrane formed by neurilemma round
central nervous system.]
Thus, according to Parker, the mass of nervous tissue which occupies the
central part of the optic stalk in Astacus is composed {92}of four distinct
ganglia; the retina is connected with the first of these by means of the
retinal fibres, and the optic nerve extends proximally from the fourth
ganglion to the brain. Each ganglion consists of ganglion-cells,
nerve-fibres, and 'neuropil,' and, in addition, supporting cells of a
neuroglial type. By means of the methylene blue method and the Golgi
method, it is seen that the retinal end-cells, with their visual rods, are
connected with the fibres of the optic nerve by means of a system of
neurones, the synapses of which take place in and help to form the
'neuropil' of the various ganglia. Thus, an impulse in passing from the
retina to the brain would ordinarily travel over five neurones, beginning
with one of the first order and ending with one of the fifth. He makes five
neurones although there are only four ganglia, because he reckons the
retinal cell with its elongated fibre as a neurone of the first order, such
fibre terminating in dendritic processes which form synapses in the
'neuropil' of the first ganglion with the neurones of the second order.
Similarly the neurones of the second order terminate in the 'neuropil' of
the second ganglion, and so on, until we reach the neurones of the fifth
order, which terminate on the one hand in the 'neuropil' of the fourth
ganglion, and on the other pass to the optic lobes of the brain by their
long neuraxons--the fibres of the optic nerve.
He compares this arrangement with that of Branchipus, Apus, Estheria,
Daphnia, etc., and shows that in the more primitive crustaceans the
peripheral optic apparatus was composed, not of four but of two optic
ganglia, not, therefore, of five but of three neurones, viz.--
1. The neurone of the first order--_i.e._ the retinal cell with its fibre
terminating in the 'neuropil' of the first optic ganglion (ganglion of the
retina).
2. The neurone of the second order, which terminates in the 'neuropil' of
the second ganglion (ganglion of the optic nerve).
3. The neurone of the third order, which terminates in the optic lobes of
the brain by means of its neuraxons (the optic nerve).
We see, then, that the most recent researches agree with the older ones of
Berger, Claus, and Bellonci, in picturing the retina of the primitive
crustacean forms as formed of two ganglia only, and not of four, as in the
specialized crustacean group the Malacostraca.
{93}The comparison of the arthropod compound retina with that of the
vertebrate shows, as one would expect upon the theory of the origin of
vertebrates put forward in this book, that the latter retina is built up of
two ganglia, as in the more primitive less specialized crustacean forms.
The modern description of the vertebrate retina, based upon the Golgi
method of staining, is exactly Parker's description of the simpler form of
crustacean retina in which the 'neuropil' of the first ganglion is
represented by the external molecular layer, and that of the second
ganglion by the internal molecular layer; the three sets of neurones being,
according to Parker's terminology:--
1. The neurones of the first order--viz. the visual cells--the nuclei of
which form the external nuclear layer, and their long attenuated processes
form synapses in the external molecular layer with
2. The neurones of the second order, the cells of which form the internal
nuclear layer, and their processes form synapses in the internal molecular
layer with
3. The neurones of the third order, the cells of which form the ganglionic
layer and their neuraxons constitute the fibres of the optic nerve which
end in the optic lobes of the brain.
Strictly speaking, of course, the visual cells with their elongated
processes have no right to be called neurones: I only use Parker's
phraseology in order to show how closely the two retinas agree even to the
formation of synapses between the fine drawn-out processes of the visual
cells and the neurones of the ganglion of the retina.
THE RETINA OF THE LATERAL EYE OF AMMOCOETES.
As in the case of all other organs, it follows that if we are dealing here
with a true genetic relationship, then the lower we go in the vertebrate
kingdom the more nearly ought the structure of the retina to approach the
arthropod type. It is therefore a matter of intense interest to determine
the nature of the retina in Ammocoetes in order to see whether it differs
from that of the higher vertebrates, and if so, whether such differences
are explicable by reference to the structure of the arthropod eye.
Before describing the structure of this retina it is necessary to clear
away a remarkable misconception, shared among others by {94}Balfour, that
this eye is an aborted eye, and that it cannot be considered as a primitive
type. Thus Balfour says: "Considering the degraded character of the
Ammocoete eye, evidence derived from its structure must be received with
caution," and later on, "the most interesting cases of partial degeneration
are those of Myxine and the Ammocoete. The development of such aborted eyes
has as yet been studied only in the Ammocoete, in which it resembles in
most important features that of other Vertebrata."
Again and again the aborted character of the eye is stated to be evidence
of degeneration in the case of the lamprey. What such a statement means,
why the eye is in any way to be considered as aborted, is to me a matter of
absolute wonderment: it is true that in the larval form it lies under the
skin, but it is equally true that at transformation it comes to the
surface, and is most evidently as perfect an eye as could be desired. There
is not the slightest sign of any degeneration or abortion, but simply of
normal development, which takes a longer time than usual, lasting as it
does throughout the life-time of the larval form.
Kohl, who has especially studied degenerated vertebrate eyes, discusses
with considerable fulness the question of the Ammocoetes eye, and concludes
that in aborted eyes a retarded development occurs, and this applies on the
whole to Ammocoetes, "but with the important difference that in this case
the period of retarded development is not followed by a stoppage, but on
the contrary by a period of very highly intensified progressive development
during the metamorphosis," with the result that "the adult eye of
_Petromyzon Planeri_ does not diverge from the ordinary type."
Referring in his summing up to this retarded development, he says: "Such
reminiscences of embryonic conditions are after all present here and there
in normally developed organs, and by no means entitle us to speak of
abnormal development."
The evidence, then, is quite clear that the eye of Petromyzon, or, indeed,
of the full-grown Ammocoetes, is in no sense an abnormal eye, but simply
that its development is slow during the ammocoete stage. The retina of
Petromyzon was figured and described by Langerhans in 1873. He describes it
as composed of the following layers:--
(1) _Membrana limitans interna._
(2) Thick inner molecular layer.
(3) Optic fibre layer.
(4) Thick inner nuclear layer.
(5) Peculiar double-layered ganglionic layer.
(6) External molecular layer.
(7) External nuclear layer.
(8) _Membrana limitans externa._
(9) Layer of rods.
(10) Pigment-epithelium.
{95}[Illustration: FIG. 41.--RETINA AND OPTIC NERVE OF PETROMYZON. (AFTER
MÜLLER AND LANGERHANS.)
On the left side the Müllerian fibres and pigment-epithelium are
represented alone. The retina is divided into an epithelial part, _C_ (the
layer of visual rod-cells), and a neurodermal or cerebral part which is
formed of, _A_, the ganglion of the optic nerve and, _B_, the ganglion of
the retina. 1, int. limiting membrane; 2, int. molecular layer with its two
layers of cells; 3, layer of optic nerve fibres; 4, int. nuclear layer; 5,
double row of tangential fulcrum cells; 6, layer of terminal retinal
fibres; 7, ext. nuclear layer; 8, ext. limiting membrane; 9, layer of rods;
10, layer of pigment-epithelium. _D_, axial cell layer (Axenstrang) in
optic nerve. The layer 6 is drawn rather too thick.]
He points out especially the peculiarity of layer (2) (2, Fig. 41), the
inner molecular, in which two rows of nuclei are arranged with great
regularity, the one row closely touching the _membrana limitans interna_,
the other at the inner boundary of the middle third of the {96}molecular
layer. Of these two rows of nuclei, he describes the innermost as composed
almost entirely of large nuclei belonging to ganglion cells, while the
outermost is composed mainly of distinctly smaller nuclei, which in
staining and appearance appear to belong not to nerve-cells but to the true
reticular tissue of the molecular layer.
He also draws special attention to the remarkable layer (5) (5, Fig. 41),
which is not found in the retina of the higher vertebrates, the cells of
which, in his opinion, are of the nature of ganglion-cells.
W. Müller, in 1874, gave a most careful description of the eye of
Ammocoetes and Petromyzon, and traced the development of the retina; the
subsequent paper of Kohl does not add anything new, and his drawings are
manifestly diagrams, and do not represent the appearances so accurately as
Müller's illustrations. In the accompanying figure (Fig. 41) I reproduce on
the right-hand side Müller's picture of the retina of Petromyzon, but have
drawn it, as in Langerhans' picture, at the place of entry of the optic
nerve.
From his comparison of this retina with a large number of other vertebrate
retinas, he comes to the conclusion that the retina of all vertebrates is
divisible into
_A._ An ectodermal (epithelial) part consisting of the layer of the
visual cells, and
_B._ A neurodermal (cerebral) part which forms the rest of the retina.
Further, Müller points out that the neuroderm gives origin throughout the
central nervous system to two totally different structures, on the one hand
to the true nervous elements, on the other to a system of supporting cells
and fibres which cannot be classed as connective tissue, for they do not
arise from mesoblast, and are therefore called by him 'fulcrum-cells.' In
the retina he recognizes two distinct groups of such supporting
structures--(1) a system of radial fibres with well-marked elongated
nuclei, which extend between the two limiting layers, and form at their
outer ends a membrane-like expansion which was originally the outer limit
of the retina, but becomes afterwards co-terminous with the _membrana
limitans externa_, owing to the piercing through it of the external limbs
of the rods. This system, which is known by the name of the radial
Müllerian fibres (shown on the left-hand side of Fig. 41), has no
connection with (2) the spongioblasts and neurospongium, which form a
framework of neuroglia, in which the terminations of the {97}optic ganglion
and of the retinal ganglion ramify to form the molecular layers.
It is evident from Fig. 41 that the retina of Ammocoetes and Petromyzon
differs in a striking manner from the typical vertebrate retina. The
epithelial part (C) remains the same--viz. the visual rods, the external
limiting membrane, and the external nuclear layer; but the cerebral part,
the retinal ganglion (A and B), is remarkably different. It is true, it
consists in the main of the small-celled mass known as the inner nuclear
layer, and of the reticulated tissue or 'neuropil' known as the inner
molecular layer, just as in all other compound retinal eyes; but neither
the ganglion cell-layer nor the optic fibre-layer is clearly defined as
separate from this molecular layer; on the contrary, it is matter of
dispute as to what cells represent the ganglionic layer of higher
vertebrates, and the optic fibres do not form a distinct innermost layer,
but pass into the inner molecular layer at its junction with the inner
nuclear layer. A comparison of this innermost part of the retina (A, Fig.
41), with the corresponding part in Berger's picture of Musca (_n.l.o.g._,
Fig. 38), shows a most striking similarity between the two. In both cases
the fibres of the optic nerve (_O.n._, Fig. 38) which cross at their
entrance pass into the 'neuropil' of this part of the retinal ganglion, and
are connected probably (though that is not proved in either case) with the
cells of the ganglionic layer. In both cases we find two well-marked
parallel rows of cells in this part of the retina, of which one, the
innermost, is composed in Ammocoetes of large ganglion-cells, and the other
mainly of smaller, deeper staining cells apparently supporting in function.
Similarly, also, in Branchipus, as I conclude from my own observations as
well as from those of Berger and Claus, the ganglionic layer is composed
partly of true ganglion-cells and partly of supporting cells arranged in a
distinct layer. This part, then, of the retina of Ammocoetes is remarkably
like that of a typical arthropod retina, and forms that part of the retinal
ganglion which may be called the ganglion of the optic nerve.
Next comes the ganglion of the retina (B, Fig. 41) (Parker's first optic
ganglion), the cells of which form the small bipolar granule-cells of the
inner nuclear layer; granule-cells arranged in rows just as they are shown
in Claus' picture of the same layer in the retina of Branchipus (Fig. 40),
just as they are found in the cortical layers of the optic ganglion of the
pineal eye (_ganglion habenulæ_), in the {98}optic lobes and other parts of
the Ammocoetes brain, or in the cortical layers of the optic ganglia of all
arthropods.
Between this small-celled nuclear layer (4, Fig. 41) and the layer of
nuclei of the visual rod cells (7, Fig. 41) (the external nuclear layer),
we find in the eye of Ammocoetes and Petromyzon two well-marked rows of
cells of a most striking character--viz. the two remarkably regular rows of
large epithelial-like cells with large conspicuous nuclei, which give the
appearance of two opposing rows of limiting epithelium (5, Fig. 41),
already mentioned in connection with the researches of Langerhans and W.
Müller. Here, then, is a striking peculiarity of the retina of the lamprey,
and according to Müller the obliteration of these two layers can be traced
as we pass upwards in the vertebrate kingdom. Among fishes, they are
especially well seen in the perch; in the higher vertebrates the whole
layer is only a rudiment represented, he thinks, by the simple layer of
round cells which lies close against the inner surface of the layer of
terminal fibres (Nervenansätze), and is especially evident in birds and
reptiles. In man and the higher mammals they are probably represented by
the horizontal cells of the outer part of the inner nuclear layer.
Seeing, then, that they are most evident in Ammocoetes, and become less and
less marked in the higher vertebrates, it is clear that their origin cannot
be sought among the animals higher in the scale than Ammocoetes, but must,
therefore, be searched for in the opposite direction.
Müller describes them as forming a very conspicuous landmark in the
embryology of the retina, dividing it distinctly into two parts, an outer
thinner, and an inner somewhat thicker part, the zone formed by them
standing out conspicuously on account of the size and regularity of the
cells and their lighter appearance when stained. Thus in his description of
the retina of an Ammocoetes 95 mm. in length, he says, "The layer of pale
tangentially elongated cells formed a double layer and produced the
appearance of a pale, very characteristic zone between the outer and inner
parts of the retina."
Let us now turn to the retina of the crustacean and see whether there is
any evidence there that the retina is divisible into an outer and inner
part, separated by a zone of characteristically pale staining cells with
conspicuous nuclei. The most elaborate description of the development of
the retina of Astacus is given by Reichenbach, {99}according to whom the
earliest sign of the formation of the retina is an ectodermic involution
(Augen-einstülpung), which soon closes, so that the retinal area appears as
a thickening. In close contiguity to this thickening, the thickening of the
optic ganglion arises, so that that part of the optic ganglion which will
form the retinal ganglion fuses with the thickened optic plate and forms a
single mass of tissue. Later on a fold (Augen-falte) appears in this mass
of tissue, in consequence of which it becomes divided into two parts. The
lining walls of this fold form a double row of cells, the nuclei of which
are most conspicuous because they are larger and lighter in colour than the
surrounding nuclei, so that by this fold the retina is divided into an
outer and an inner wall, the line of demarcation being conspicuous by
reason of these two rows of large, lightly-staining nuclei.
Reichenbach is unable to say that this secondary fold is coincident with
the primary involution, and that therefore the junction between the two
rows of large pale nuclei is the line of junction between the retinal
ganglion and the retina proper, because all sign of the primary involution
is lost before the secondary fold appears.
Parker compares the appearances in the lobster with Reichenbach's
description in the crayfish, and says that he finds only a thickening, no
primary involution; at the same time he expressly states that in the very
early stages his material was deficient, and that he had not grounds
sufficient to warrant the statement that no involution occurs. He also
finds that in the lobster the ganglionic tissue which arises by
proliferation is divided into an outer and inner part; the separation is
effected by a band of large, lightly-staining nuclei, which, in position
and structure, resemble the band figured by Reichenbach. According to
Parker, then, the line of separation indicated in the development by
Reichenbach's outer and inner walls is not the line of junction between the
retina and the retinal ganglion, as Reichenbach was inclined to think, but
rather a separation of two rows of large ganglion-cells belonging to the
retinal ganglion.
The similarity between these conspicuous layers of lightly-staining cells
in Ammocoetes and in crustaceans is remarkably close, and in both cases
observers have found the same difficulty in interpreting their meaning. In
each case one group of observers looks upon them as ganglion-cells, the
other as supporting structures. Thus in the lamprey, Müller considers them
to belong to the supporting elements, while Langerhans and Kohl describe
them as a double {100}layer of ganglion-cells. In the crustacean, Berger in
Squilla, Grenacher in Mysis, and Parker in Astacus, look upon them as
supporting elements, while Viallanes in Palinurus considers them to be true
ganglionic cells.
Whatever the final interpretation of these cells may prove to be, we may,
it seems to me, represent an ideal compound retina of the crustacean type
by combining the investigations of Berger, Claus, Reichenbach, and Parker
in the following figure.
[Illustration: FIG. 42.--IDEAL DIAGRAM OF THE LAYERS IN A CRUSTACEAN EYE.
The retina is divided into an epithelial part, _C_ (the layer of retinular
cells and rhabdomes), and a neurodermal or cerebral part, which is formed
of, _A_, the ganglion of the optic nerve, and, _B_, the ganglion of the
retina. 1, optic nerve fibres which cross at their entrance into the
retina; 2, int. molecular layer with its two rows of cells; 3, int. nuclear
layer; 4, Reichenbach's double row of large lightly-staining cells; 5,
layer of terminal retinal fibres; 6, ext. nuclear layer; 7, ext. limiting
membrane; 8, layer of crystalline cones; 9, cornea.]
The comparison of this figure (Fig. 42) with that of the Petromyzon retina
(Fig. 41) shows how great is the similarity of the latter with the
arthropod type, and how the very points in which it deviates from the
recognized vertebrate type are explainable by comparison with that of the
arthropod. The most striking difference between the retinas in the two
figures is that the layer of terminal nerve fibres (5, Fig. 42), which,
after all, are only the elongated terminations of the retinal cells
belonging to Parker's neurones of the first order, is very much longer than
in Petromyzon or in any vertebrate, for the external molecular layer (6,
Fig. 41) (Müller's layer of Nervenansätze) is very short and inconspicuous
(in Fig. 41 it is drawn too thick).
Turning from the retina to the fibres of the optic nerve we again find a
remarkable resemblance, for in Ammocoetes, as pointed out by
{101}Langerhans and carefully figured by Kohl, a crossing of the fibres of
the optic nerve occurs as the nerve leaves the retina, just as is so
universally the case in all compound retinas. To this crossing Kohl has
given the name _chiasma nervi optici_, in distinction to the cerebral
chiasma, which he calls _chiasma nervorum opticorum_. Further, we find that
even this latter chiasma is well represented in the arthropod brain; thus
Bellonci in Sphæroma, Berger, Dietl, and Krieger in Astacus, all describe a
true optic chiasma, the only difference in opinion being, whether the
crossing of the optic nerves is complete or not. Especially instructive are
Bellonci's figures and description. He describes the brain of Sphæroma as
composed of three segments--a superior segment, the cerebrum proper, a
middle segment, and an inferior segment; the optic fibres, as is seen in
Fig. 39, after crossing, pass direct into the middle segment, in the
ganglia of which they terminate. From this segment also arises the nerve to
the first antenna of that side--_i.e._ the olfactory nerve. The optic part,
then, of this middle segment is clearly the brain portion of the optic
ganglionic apparatus, and may be called the optic lobes, in
contradistinction to the peripheral part, which is usually called the optic
ganglion, and is composed of two ganglia, Op. g. I. and Op. g. II., as
already mentioned. These optic lobes are therefore homologous with the
optic lobes of the vertebrate brain.
The resemblance throughout is so striking as to force one to the conclusion
that the retina of the vertebrate eye is a compound retina, composed of a
retina and retinal ganglion of the type found in arthropods. From this it
follows that the development of the vertebrate retina ought to show the
formation of (1) an optic plate formed from the peripheral epidermis and
not from the brain; (2) a part of the brain closely attached to this optic
plate forming the retinal ganglion, which remains at the surface when the
rest of the optic ganglion withdraws; (3) an optic nerve formed in
consequence of this withdrawal, as the connection between the retinal and
cerebral parts of the optic ganglion.
This appears to me exactly what the developmental process does show
according to Götte's investigations. He asserts that the retina arises from
an optic plate, being the optical portion of his 'Sinnes-platte.' At an
early stage this is separated by a furrow (Furche) from the general mass of
epidermal cells which ultimately form the brain. This separation then
vanishes, and the retina and brain-mass {102}become inextricably united
into a mass of cells, which are still situated at the surface. By the
closure of the cephalic plate and the withdrawal of the brain away from the
surface, a retinal mass of cells is left at the surface connected with the
tubular central nervous system by the hollow optic diverticulum or primary
optic vesicle. If we regard only the retinal and nervous elements, and for
the moment pay no attention to the existence of the tube, Götte's
observation that the true retina has been formed from the optic plate
(Sinnes-platte) to which the retinal portion of the brain (retinal
ganglion) has become firmly fixed, and that then the optic nerve has been
formed by the withdrawal of the rest of the brain (optic lobes), is word
for word applicable to the description of the development of the compound
retina of the arthropod eye, as has been already stated.
THE SIGNIFICANCE OF THE OPTIC DIVERTICULA.
The origin of the retina from an optic epidermal plate in vertebrates, as
in all other animals, brings the cephalic eyes of all animals into the same
category, and leaves the vertebrate eye no longer in an isolated and
unnatural position. In one point the retina of the vertebrate eye differs
from that of a compound retina of an invertebrate; in the former, a
striking supporting tissue exists, known as Müller's fibres, which is
absent in the latter. This difference of structure is closely associated
with another of the same character as in the central nervous system, viz.
the apparent development of the nervous part from a tube. We see, in fact,
that the retinal and nervous arrangements of the vertebrate eye are
comparable with those of the arthropod eye, in precisely the same way and
to the same extent as the nervous matter of the brain of the vertebrate is
comparable with the brain of the arthropod. In both cases the nervous
matter is, in structure, position, and function, absolutely homologous; in
both cases there is found in the vertebrate something extra which is not
found in the invertebrate--viz. a hollow tube, the walls of which, in the
case of the brain, are utilized as supporting tissues for the nerve
structures. The explanation of this difference in the case of the brain is
the fundamental idea of my whole theory, namely, that the hollow tube is in
reality the cephalic stomach of the invertebrate, around which the nervous
brain-matter was originally grouped in precisely the same manner as in the
invertebrate. What, then, are the optic diverticula?
{103}"The formation of the eye," as taught by Balfour, "commences with the
appearance of a pair of hollow outgrowths from the anterior cerebral
vesicle. These outgrowths, known as the optic vesicles, at first open
freely into the cavity of the anterior cerebral vesicle. From this they
soon, however, become partially constricted, and form vesicles united to
the base of the brain by comparatively narrow, hollow stalks, the rudiments
of the optic nerves."
"After the establishment of the optic nerves, there takes place (1) the
formation of the lens, and (2) the formation of the optic cup from the
walls of the primary optic vesicle."
He then goes on to explain how the formation of the lens forms the optic
cup with its double walls from the primary optic vesicle, and says--
"Of its double walls, the inner, or anterior, is formed from the front
portion, the outer, or posterior, from the hind portion of the wall of the
primary optic vesicle. The inner, or anterior, which very speedily becomes
thicker than the other, is converted into the retina; in the outer, or
posterior, which remains thin, pigment is eventually deposited, and it
ultimately becomes the tesselated pigment-layer of the choroid."
The difficulties in connection with this view of the origin of the eye are
exceedingly great, so great as to have caused Balfour to discuss seriously
Lankester's suggestion that the eye must have been at one time within the
brain, and that the ancestor of the vertebrate was therefore a transparent
animal, so that light might get to the eye through the outer covering and
the brain-mass; a suggestion, the unsatisfactory nature of which Balfour
himself confessed. Is there really evidence of any part of either retina or
optic nerve being formed from the epithelial lining of the tube?
This tube is formed as a direct continuation of the tube of the central
nervous system, and we can therefore apply to it the same arguments as have
been used in the discussion of the meaning of the latter tube. Now, the
striking point in the latter case is the fact that the lining membrane of
the central canal is in so many parts absolutely free from nervous matter,
and so shows, as in the so-called choroid plexuses, its simple, non-nervous
epithelial structure. This also we find in the optic diverticulum. Where
there is no evidence of any invasion of the tube by nervous elements, there
it retains its simple non-nervous character of a tube composed of a single
layer of {104}epithelial cells--viz. in that part of the tube which, as
Balfour says, remains thin, in which pigment is eventually deposited, and
which ultimately becomes the tesselated pigment-layer of the choroid.
Nobody has ever suggested that this pigment-layer is nervous matter, or
ever was, or ever will be, nervous matter; it is in precisely the same
category as the membranous roof of the brain in Ammocoetes, which never
was, and never will be, nervous matter. Yet, according to the old
embryology both in the case of the eye and the brain, the pigment-layer and
the so-called choroid plexuses are a part of the tubular nervous system.
Turning now to the optic nerve, Balfour describes it as derived from the
hollow stalk of the optic vesicle. He says--
"At first the optic nerve is equally continuous with both walls of the
optic cup, as must of necessity be the case, since the interval which
primarily exists between the two walls is continuous with the cavity of the
stalk. When the cavity within the optic nerve vanishes, and the fibres of
the optic nerve appear, all connection is ruptured between the outer wall
of the optic cup and the optic nerve, and the optic nerve simply perforates
the outer wall, and becomes continuous with the inner one."
In this description Balfour, because he derived the optic nerve fibres from
the epithelial wall of the optic stalk, of necessity supposed that such
fibres originally supplied both the outer and inner walls of the optic cup
and, therefore, seeing that when the fibres of the optic nerve appear they
do not supply the outer wall, he supposes that their original connection
with the outer wall is ruptured, because a discontinuity of the epithelial
lining takes place coincidently with the appearance of the optic
nerve-fibres, and, according to him, the optic nerve simply perforates the
outer wall and becomes continuous with the inner one. This last statement
is very difficult to understand. I presume he meant that some of the fibres
of the optic nerve supplied from the beginning the inner wall of the optic
cup, but that others which originally supplied the outer wall were first
ruptured, then perforated the outer wall, and finally completed the supply
to the inner wall or retina.
This statement of Balfour's is the necessary consequence of his belief,
that the epithelial cells of the optic stalk gave rise to the fibres of the
optic nerve. If, instead of this, we follow Kölliker and His, who state
that the optic nerve-fibres are formed outside the {105}epithelial walls of
the optic stalk, and that the cells of the latter form supporting
structures for the nerve-fibres, then the position of the optic nerve
becomes perfectly simple and satisfactory without any rupturing of its
connection with the outer wall and subsequent perforation, for the optic
nerve-fibres from their very first appearance pass directly to supply the
retina--_i.e._ the inner wall of the optic cup and nothing else.
They pass, as is well known, without any perforation by way of the
choroidal slit to the inner surface of the inner wall (retina) of the optic
cup; then, when the choroidal slit becomes closed by the expansion of the
optic cup, the optic nerve naturally becomes situated in the centre of the
base of the cup and spreads over its inner surface as that surface expands.
A section across the optic cup at an early stage at the junction of the
optic stalk and optic cup would be represented by the upper diagram in Fig.
43; at a later stage, when the choroidal slit is closed, by the lower
diagram.
[Illustration: FIG. 43.--DIAGRAM OF THE RELATION OF THE OPTIC NERVE TO THE
OPTIC CUP.
The upper diagram represents a stage before the formation of the choroidal
slit, the lower one the stage of closure of the choroidal slit. _R._,
retina; _O.n._, optic nerve; _p._, pigment epithelium.]
The evident truth of this manner of looking at the origin of the optic
nerve is demonstrated by the appearance of the optic nerve in Ammocoetes
and Petromyzon. In the latter, although the development is complete, and
the eye, and consequently also the optic nerve-fibres, are fully
functional, there is still present in the axial core of the nerve a row of
epithelial cells (Axenstrang) which are altered so as to form supporting
structures, in the same way as a row of epithelial cells in the retina is
altered to form the system of supporting cells known by the name of the
Müllerian fibres.
The origin of this axial core of cells is perfectly clear, as has been
pointed out by W. Müller. He says--
"The development of the optic nerve shows peculiarities in {106}Petromyzon
of such a character as to make this animal one of the most valuable objects
for deciding the various controversial questions connected with the genesis
of its elements. The lumen of the stalk of the primary optic vesicle is
obliterated quite early by a proliferation of its lining epithelium. Also
the original continuity of this epithelium with that of the pigment-layer
is at an early period interrupted at the point of attachment of the optic
stalk. This interruption occurs at the time when the fibres of the optic
nerve first become visible."
Further on he says--
"The epithelium of the optic stalk develops entirely into supporting cells,
which in Petromyzon fill up the original lumen and so form an axial core
(Axenstrang) to the nerve-fibres which are formed entirely outside them;
the projections of these supporting cells are directed towards the
periphery, and so separate the bundles of the optic nerve-fibres. The
mesodermal coat of the optic stalk takes no part in this separation; it
simply forms the connective tissue sheath of the optic nerve. The
development of the optic nerve in the higher vertebrates also obeys the
same law, as I am bound to conclude from my own observations."
The evidence, then, of Ammocoetes is very conclusive. Originally a tube
composed of a single layer of epithelial cells became expanded at the
anterior end to form a bulb. On the outside of this tube or stalk the
fibres of the optic nerve make their appearance, arising from the
ganglion-cell layer of the retina, and, passing over the surface of the
epithelial tube at the choroidal fissure, proceed to the brain by way of
the optic chiasma. Owing to the large number of fibres, their crossing at
the junction of the stalk with the bulb, and the narrowness at this neck,
the obliteration of the lumen of the tube which takes place in the stalk is
carried out to a still greater extent at this narrow part. The result of
this is that all continuity of the cell-layers of the original tube of the
optic stalk with those of both the inner and outer walls of the bulb is
interrupted, and all that remains in this spot of the original continuous
line of cells which connected the tube of the stalk with that of the bulb
are possibly some of the groups of cells which are found scattered among
the fibres of the optic nerve at their entrance into the retina. Such
separation of the originally continuous elements of the epithelial wall of
the optic stalk, which is apparent only at this neck of the nerve in
Petromyzon, takes place {107}along the whole of the optic nerve in the
higher vertebrates, so that no continuous axial core of cells exist, but
only scattered supporting cells.
If further proof in support of this view be wanted, it is given by the
evidence of physiology, which shows that the fibres of the optic nerve are
not different from other nerve-fibres of the central nervous system, but
that they degenerate when separated from their nerve-cell, and that the
nerve-cell of which the optic nerve-fibre is a process is the large
ganglion-cell of the ganglionic layer of the retina. The origin of the
ganglionic layer of the retina cannot therefore be separated from that of
the optic nerve-fibres. If the one is outside the epithelial tube, so is
the other, and what holds true of the ganglionic layer must hold good of
the rest of the retinal ganglion and, from all that has been said, of the
retina itself. We therefore come to the conclusion that the evidence is
distinctly in favour of the view, that the retina and optic nerve in the
true sense are structures which originally were outside a non-nervous tube,
but, just like the central nervous system as a whole, have amalgamated so
closely with the elements of this tube as to utilize them for supporting
structures. One part of this non-nervous tube, its dorsal wall, like the
corresponding part of the brain-tube, still retains its original character,
and by the deposition of pigment has been pressed into the service of the
eye to form the pigmented epithelial layer.
We can, however, go further than this, for we know definitely in the case
of the retina what the fate of the epithelial cells lining this tube has
been. They have become the system of supporting structures known as
Müllerian fibres.
The epithelial layer of the primary optic vesicle can be traced into direct
continuity with the lining epithelium of the brain cavity, as a single
layer of epithelial cells in the core of the optic nerve, forming the optic
stalk, which, in consequence of close contact, becomes the well-known axial
layer of supporting cells. This epithelial layer of the optic stalk then
expands to form the optic bulb, the outer or dorsal wall of which still
remains as a single layer of epithelium and becomes the layer of pigment
epithelium. This layer of epithelium becomes doubled on itself by the
approximation of the inner or ventral wall of the optic cup to the outer or
dorsal wall in consequence of the presence of the lens, and still remaining
a single layer, forms the _pars ciliaris retinæ_; then suddenly, at the
_ora {108}serrata_, the single epithelial layer vanishes, and the layers of
the retina take its place. It has long been known, however, that even
throughout the retina this single epithelial layer still continues, being
known as the fibres of Müller. This is how the fact is described in the
last edition of Foster's "Text-book of Physiology," p. 1308--
"Stretching radially from the inner to the outer limiting membrane in all
regions of the retina are certain peculiar-shaped bodies known as the
radial fibres of Müller. Each fibre is the outcome of the changes undergone
by what was at first a simple columnar epithelial cell. The changes are, in
the main, that the columnar form is elongated into that of a more or less
prismatic fibre, the edges of which become variously branched, and that
while the nucleus is retained the cell substance becomes converted into
neuro-keratin. And, indeed, at the _ora serrata_ the fibres of Müller may
be seen suddenly to lose their peculiar features and to pass into the
ordinary columnar cells which form the _pars ciliaris retinæ_."
[Illustration: FIG. 44.--DIAGRAM REPRESENTING THE SINGLE-LAYERED EPITHELIAL
TUBE OF THE VERTEBRATE EYE AFTER REMOVAL OF THE NERVOUS AND RETINAL
ELEMENTS.
_O.n._, axial core of cells in optic nerve; _p._, pigment epithelium;
_p.c.r., pars ciliaris retinæ_; _m.f._, Müllerian fibres; _l._, lens.]
It is then absolutely clear that the essential parts of the eye may be
considered as composed of two parts--
1. A tube or diverticulum from the tube of the central nervous system,
composed throughout of a single layer of epithelium, which forms the
supporting axial cells in the optic nerve, the pigment epithelium and the
Müllerian fibres of the retina. Such a tube would be represented by the
accompanying Fig. 44, and the left side of Fig. 41.
2. The retina proper with the retinal ganglion and the optic nerve-fibres
as already described. In this part supporting elements are found, just as
in any other compound retina, of the nature of neuroglia, which are
independent of the Müllerian fibres.
{109}Of these two parts we have already seen that the second is to all
intents and purposes a compound retina of a crustacean eye, and seeing that
the single-layered epithelial tube is continuous with the single-layered
epithelial tube of the central nervous system--_i.e._ with the cephalic
part of the gut of the arthropod ancestor--it follows with certainty that
the ancestor of the vertebrates must have possessed two anterior
diverticula of the gut, with the wall of which, near the anterior
extremity, the compound retina has amalgamated on either side, just as the
infra-oesophageal ganglia have amalgamated with the ventral wall of the
main gut-tube. In this way, and in this way alone, does the interpretation
of the structure of the vertebrate lateral eye harmonize in the most
perfect manner with the rest of the conclusions already arrived at.
The question therefore arises:--Have we any grounds for believing that the
ancient forms of primitive crustaceans and primitive arachnids, which were
so abundant in the time when the Cephalaspids appeared, possessed two
anterior diverticula of the stomach, such as the consideration of the
vertebrate eye strongly indicates must have been the case?
The beautiful pictures of Blanchard, and his description, show how, on the
arachnid side, paired diverticula of the stomach are nearly universal in
the group. Thus, although they are not present in the scorpions, still, in
the Thelyphonidæ, Phrynidæ, Solpugidæ, Mygalidæ, the most marked
characteristic of the stomach-region is the presence of four pairs of
coecal diverticula, which spread laterally over the prosomatic region. In
the spiders the number of such diverticula increases, and the whole
prosomatic region becomes filled up with these tubes. Blanchard considers
that they form nutrient tubes for the direct nutrition of the organs in the
prosoma, especially the important brain-region of the central nervous
system. He points out that these animals are blood-suckers, and that,
therefore, their food is already in a suitable form for purposes of
nutrition when it is taken in by them, so that, as it were, the anterior
part of the gut is transformed into a series of vessels or diverticula
conveying blood directly to the important organs in the prosoma, by means
of which they obtain nourishment in addition to their own blood-supply.
The universality of such diverticula among the arachnids makes it highly
probable that their progenitors did possess an alimentary canal with one or
more pairs of anterior diverticula. In the {110}vertebrate, however, the
paired diverticula are associated with a compound retina, a combination
which does not occur among living arachnids; we must, therefore, examine
the crustacean group for the desired combination, and naturally the most
likely group to examine is the Phyllopoda, especially such primitive forms
as Branchipus and Artemia, for it is universally acknowledged that these
forms are the nearest living representatives of the trilobites. If,
therefore, it be found that the retina and optic nerve in Artemia is in
specially close connection with an anterior diverticulum of the gut on each
side, then it is almost certain that such a combination existed also in the
trilobites.
[Illustration: FIG. 45.--SECTION THROUGH ONE OF THE TWO ANTERIOR
DIVERTICULA OF THE GUT IN ARTEMIA AND THE RETINAL GANGLION.
The section is through the extreme anterior end of the diverticulum, thus
cutting through many of the columnar cells at right angles to their axis.
_Al._, gut diverticulum; _rt. gl._, retinal ganglion.]
{111}[Illustration: FIG. 46.--THE BRAIN, EYES, AND ANTERIOR TERMINATION OF
THE ALIMENTARY CANAL OF ARTEMIA, VIEWED FROM THE DORSAL ASPECT.
_Br._, brain; _l.e._, lateral eyes; _c.e._, median eyes; _Al._, alimentary
canal.]
[Illustration: FIG. 47.--A, THE FORMATION OF THE RETINA OF THE EYE OF
AMMOCOETES (after SCOTT); B, THE FORMATION OF THE RETINA OF THE EYE OF
AMMOCOETES, ON MY THEORY.
_R._, retina; _l._, lens; _O.n._, optic nerve fibres; _Al._, cephalic end
of invertebrate alimentary canal; _V._, cavity of ventricles of brain;
_Al.d._, anterior diverticulum of alimentary canal; _op.d._, optic
diverticulum.]
My friend Mr. W. B. Hardy has especially investigated the nervous system of
Artemia. In the course of his work he cut serial sections through the whole
animal, and, as mentioned in my paper in the _Journal of Anatomy and
Physiology_, he discovered that the retinal ganglion of each lateral eye is
so closely attached to the end of the corresponding diverticulum of the gut
that the lining cells of the ventral part of the diverticulum form a lining
to the retinal ganglion (Fig. 45). In this animal there are only two
gut-diverticula, which are situated most anteriorly. I have plotted out
this series of sections by means of a camera lucida, with the result that
the retina appears as a bulging attached ventro-laterally to the extremity
of each gut-diverticulum, as is shown in Fig. 46. It is instructive to
compare with this figure Scott's picture of the developing eye in
Ammocoetes, where he figures the retina as {112}a bulging attached
ventrally to the extremity of the narrow tube of the optic diverticulum. In
Fig. 47, A, I reproduce this figure of Scott, and by the side of it, Fig.
47, B, I have represented the origin of the vertebrate eye as I believe it
to have occurred.
We see, then, this very striking fact, that in the most primitive of the
Crustacea, not only are there two anterior diverticula of the gut, but also
the retinal ganglion of the lateral eye is in specially close connection
with the end of the diverticulum on each side. In fact, we find in the
nearest living representative of the trilobites a retina and retinal
ganglion and optic nerve, closely resembling that of the vertebrate, in
close connection with an epithelial tube which has nothing to do with the
organ of sight, but is one of a pair of anterior gut-diverticula. It is
impossible to obtain more decisive evidence that the trilobites possessed a
pair of gut-diverticula surrounded to a greater or less extent by the
retina and optic nerve of each lateral eye.
Such anterior diverticula are commonly found in the lower Crustacea; they
are usually known by the name of liver-diverticula, but as they take no
part in digestion, and, on the contrary, represent that part of the gut
which is most active in absorption, the term liver is not appropriate, and
it is therefore better to call them simply the pair of anterior
diverticula. Our knowledge of their function in Daphnia is given in a paper
by Hardy and M^cDougall, which does not appear to be widely known. Hardy
succeeded in feeding Daphnia with yolk of egg in which carmine grains were
mixed, and was able in the living animal to watch the whole process of
deglutition, digestion, and absorption. The food, which is made into a
bolus, is moved down to the middle region of the gut, and there digestion
takes place. Then by an antiperistaltic movement the more fluid products of
the digestion-process are sent right forward into the two anterior
diverticula, where the single layer of columnar cells lining these
diverticula absorbs these products, the cells becoming thickly studded with
fat-drops after a feed of yolk of egg. The carmine particles, which were
driven forward with the proteid- and fat-particles, are not absorbed, but
are at intervals driven back by contractions of the anterior diverticula to
the middle region of the gut.
These observations prove most clearly that the anterior diverticula have a
special nutrient function, being the main channels by which new nutrient
material is brought into the body, and, as {113}pointed out by the authors,
it is a remarkable exception in the animal kingdom that absorption should
occur in that portion of the gut which is anterior to the part in which
digestion occurs. In all these animals the two anterior diverticula extend
forwards over the brain, and, as we have seen in Artemia, the anterior
extremity of each one is so intimately related to a part of the brain--viz.
the retinal ganglion--as to form a lining membrane to that mass of
nerve-cells. It follows, therefore, that the nutrient fluid absorbed by the
cells of this part of the gut-diverticulum must be primarily for the
service of the retinal ganglion. In fact, the relations of this anterior
portion of the gut to the brain as a whole suggest strongly that the marked
absorptive function of this anterior portion of the gut exists in order to
supply nutrient material in the first place to the most vital, most
important organ in the animal--the brain and its sense-organs. This
conclusion is borne out by the fact that in these lower crustaceans the
circulation of blood is of a very inefficient character, so that the
tissues are mainly dependent for their nutrition on the fluid immediately
surrounding them. It stands to reason that the establishment of the
anterior portion of the gut as a nutrient tube to the brain would
necessitate a closer and closer application of the brain to that tube, so
that the process of amalgamation of the brain with the single layer of
columnar epithelial cells which constitutes the wall of the gut (which we
see in its initial stage in the retinal ganglion of Artemia), would tend
rapidly to increase as more and more demands were made upon the brain,
until at last both the supra- and infra-oesophageal ganglia, as well as the
retinal ganglia and optic nerves, were in such close intimate connection
with the ventral wall of the anterior portion of the gut and its
diverticula as to form a brain and retina closely resembling that of
Ammocoetes.
Such an origin for the lateral eyes of the vertebrate explains in a simple
and satisfactory manner why the vertebrate retina is a compound retina, and
why both retina and optic nerve have an apparent tubular development.
At the same time one discrepancy still exists which requires
consideration--viz. in no arthropod eye possessing a compound retina is the
retina inverted. All the known cases of inversion among arthropods occur in
eyes, the retina of which is simple, and are all natural consequences of
the process of invagination by which {114}the retina is formed. On the
other hand, eyes with an inverted compound retina are not entirely unknown
among invertebrates, for the eyes of Pecten and of Spondylus possess a
retina which is inverted after the vertebrate fashion and still may be
spoken of as compound rather than simple. It is clear that an invagination,
the effect of which is an inversion of the retinal layer, would lead to the
same result, whether the retinal optic nerves were short or long, whether,
in fact, a retinal ganglion existed or not. Undoubtedly the presence of the
retinal ganglion tends greatly to obscure any process of invagination, so
that, as already mentioned, many observers, with Parker, consider the
retina of the crustacean lateral eye to be formed by a thickening only,
without any invagination, while Reichenbach says an obscure invagination
does take place at a very early stage. So in the vertebrate eye most
observers speak only of a thickening to form the retina, but Götte's
observation points to an invagination of the optic plate at an early stage.
So also in the eye of Pecten, Korschelt and Heider consider that the
thickening, by which the retina is formed according to Patten, in reality
hides an invagination process by means of which, as Bütschli suggests, an
optic vesicle is formed in the usual manner. The retina is formed from the
anterior wall of this vesicle, and is therefore inverted.
The origin of the inverted retina of the vertebrate eye does not seem to me
to present any great difficulty, especially when one takes into
consideration the fact that the retina is inverted in the arachnid group,
only in the lateral eyes. The inversion is usually regarded as associated
with the tubular formation of the vertebrate retina, and it is possible to
suppose that the retina became inverted in consequence of the involvement
of the eye with the gut-diverticulum. I do not myself think any such
explanation is at all probable, because I cannot conceive such a process
taking place without a temporary derangement--to say the least of it--of
the power of vision, and as I do not believe that evolution was brought
about by sudden, startling changes, but by gradual, orderly adaptations,
and as I also believe in the paramount importance of the organs of vision
for the evolution of all the higher types of the animal kingdom, I must
believe that in the evolution from the Arthropod to the Cephalaspid, the
lateral eyes remained throughout functional. I therefore, for my own part,
would say that the inversion of the {115}retina took place before the
complete amalgamation with the gut-diverticulum, that, in fact, among the
proto-crustacean, proto-arachnid forms there were some sufficiently
arachnid to have an inverted retina, and at the same time sufficiently
crustacean to possess a compound retina, and therefore a compound inverted
retina after the vertebrate fashion existed in combination with the
anterior gut-diverticula. Thus, when the eye and optic nerve sank into and
amalgamated with the gut-diverticulum, neither the dioptric apparatus nor
the nervous arrangements would suffer any alteration, and the animal
throughout the whole process would possess organs of vision as good as
before or after the period of transition.
Further, not only the retina but also the dioptric apparatus of the
vertebrate eye point to its origin from a type that combined the
peculiarities of the arachnids and the crustaceans. In the former it is
difficult to speak of a true lens, the function of a lens being undertaken
by the cuticular surface of the cells of the corneagen (Mark's 'lentigen'),
while in the latter, in addition to the corneal covering, a true lens
exists in the shape of the crystalline cones. Further, these crustacean
lenses are true lenses in the vertebrate sense, in that they are formed by
modified hypodermal cells, and not bulgings of the cuticle, as in the
arachnid. We see, in fact, that in the compound crustacean eye an extra
layer of hypodermal cells has become inserted between the cornea and the
retina to form a lens. So also in the vertebrate eye the lens is formed by
an extra layer of the epidermal cells between the cornea and the retina.
The fact that the vertebrate eye possesses a single lens, though its retina
is composed of a number of ommatidia, while the crustacean eye possesses a
lens to each ommatidium, may well be a consequence of the inversion of the
vertebrate retina. It is most probable, as Korschelt and Heider have
pointed out, that the retina of the arachnid eyes is composed of a number
of ommatidia, just as in the crustacean eyes and in the inverted eyes it is
probable that the image is focussed on to the pigmented tapetal layer, and
thence reflected on to the percipient visual rods. In such a method of
vision a single lens is a necessity, and so it must also be if, as I
suppose, eyes existed with an inverted compound retina. Owing to the
crustacean affinities of such eyes, a lens would be formed and the retina
would be compound: owing to the arachnid affinities, the retina would be
inverted and the hypodermal cells which formed the lens would be massed
{116}together to form a single lens, instead of being collected in groups
of four to form a series of crystalline cones.