ORGANS OF VISION.
In the lowest forms of animal life the whole surface is sensitive to
light, and organs of vision have no doubt arisen in the first instance
from limited areas becoming especially sensitive to light in
conjunction with a deposit of pigment. Lens-like structures, formed
either as a thickening of the cuticle, or as a mass of cells, were
subsequently formed; but their function was not, in the first
instance, to throw an image of external objects on the perceptive part
of the eye, but to concentrate the light on it. From such a simple
form of visual organ it is easy to pass by a series of steps to an eye
capable of true vision.
There are but few groups of the Metazoa which are not provided with
optic organs of greater or less complexity.
In a large number of instances these organs are placed on the anterior
part of the head, and are innervated from the anterior ganglia. It is
possible that many of the eyes so situated may be modifications of a
common prototype. In other instances organs of vision are situated in
different regions of the body, and it is clear that such eyes have
been independently evolved in each instance.
The percipient elements of the eye would invariably appear to be
cells, one end of each of which is continuous with a nerve, while the
other terminates in a cuticular structure, or indurated part of the
cell forming what is known as the rod or cone.
The presence of such percipient elements in various eyes is therefore
no proof of genetic relationship between these eyes, but merely of
similarity of function.
Embryological data as to the development of the eye do not exist
except in the case of the Arthropoda, Mollusca and Chordata. From such
data as there are, combined with study of the adult structure of the
eye, it can be shewn that two types of development are found. In one
of these the percipient elements are formed from the central nervous
system, in the other from the epidermis. The former may be called
cerebral eyes. It is probable however that this distinction is not, in
all cases at any rate, so fundamental as might be supposed; but that
in both instances the eye may have taken its origin from the
epidermis. In the eyes in which the retina is continuous with the
central nervous system, these two organs were probably evolved
simultaneously as differentiations of the epidermis, and continue to
develop together in the ontogenetic growth of the eye.
Some of the eyes in which the retina is formed from the epidermis have
also probably arisen simultaneously with part of the central nervous
system, while in other instances they have arisen as later formations
subsequently to the complete establishment of a central nervous
system.
Coelenterata. The actual evolution of the eye is best shewn in the
Hydrozoa. The simplest types are those found in Oceania and
Lizzia[182]. In Lizzia the eye is placed at the base of a tentacle and
consists of (fig. 276) a lens (l) and a percipient bulb (_oc_). The
lens is a simple thickening of the cuticle, while the percipient part
of the eye is formed of three kinds of elements:--(1) pigment cells;
(2) sense cells, forming the true retinal elements, and consisting of
a central swelling with the nucleus, a peripheral process representing
a hardly differentiated rod, and a central process continuous with (3)
ganglion cells at the base of the eye. In this eye there is present a
commencing differentiation of a ganglion as well as of a retina.
[182] O. and R. Hertwig. _Das Nervensystem u. Sinnesorgane d.
Medusen._ Leipzig, 1878.
[FIG. 276. EYE OF LIZZIA KOELLIKERI. (From Lankester; after
Hertwig.)
_l._ lens; _oc._ perceptive part of eye.]
The eye of Oceania is simpler than that of Lizzia in the absence of a
lens. Claus has shewn that in Charybdea amongst the Acraspeda a more
highly differentiated eye is present, with a lens formed of cells like
the vertebrate eye.
Mollusca. In a large number of the odontophorous Mollusca eyes,
innervated by the supraoesophageal ganglia, are present on the dorsal
side of the head. These eyes exhibit very various degrees of
complexity, but are shewn both by their structure and development to
be modifications of a common prototype.
The simplest type of eye is that found in the Nautilus, and although
the possibility of this eye being degenerated must be borne in mind,
it is at the same time very interesting to note (Hensen) that it
retains permanently the early embryonic structure of the eyes of the
other groups.
It has (fig. 277 A) the form of a vesicle, with a small opening in the
outer wall, placing the cavity of the vesicle in free communication
with the exterior. The cells lining the posterior face of the vesicle
form a retina (_R_); and are continuous with the fibres of the optic
nerve (_N.op_). We have no knowledge of the development of this eye.
[FIG. 277. THREE DIAGRAMMATIC SECTIONS OF THE EYES OF MOLLUSCA.
(After Grenacher.)
A. Nautilus. B. Gasteropod (Limax or Helix). C. Dibranchiate
Cephalopod.
_Pal._ eyelid; _Co._ cornea; _Co.ep._ epithelium of ciliary body;
_Ir._ iris; _Int, Int^1 ... Int^4._ different parts of the
integument; _l._ lens; _l^1._ outer segment of lens; _R._ retina;
_N.op._ optic nerve; _G.op._ optic ganglion; _x._ inner layer of
retina; _N.S._ nervous stratum of retina.]
In the Gasteropods the eye (fig. 277 B) has the form of a closed
vesicle: the cells lining the inner side form the retina, while the
outer wall of the vesicle constitutes the cornea. A cuticular lens is
placed in the cavity, on the side adjoining the cornea. This eye
originates from the ectoderm, within the velar area, and close to the
supraoesophageal ganglia, usually at the base of the tentacles.
According to Rabl (Vol. II. No. 268) it is formed as an invagination,
the opening of which soon closes; while according to Bobretzky (Vol.
II. No. 242) and Fol it arises as a thickening of the epiblast, which
becoming detached takes the form of a vesicle. It is quite possible
that both types of development may occur, the second being no doubt
abbreviated. The vesicle, however formed, soon acquires a covering of
pigment, except for a small area of its outer wall, where the lens
becomes formed as a small body projecting into the lumen of the
vesicle. The lens seems to commence as a cuticular deposit, and to
grow by the addition of concentric layers. The inner wall of the
vesicle gives rise to the retina.
The most highly differentiated molluscan eye is that of the
Dibranchiate Cephalopoda, which is in fact more highly organized than
any other invertebrate eye.
A brief description of its adult structure[183] will perhaps render
more clear my account of the development. The most important features
of the eye are shewn in fig. 277 C. The outermost layer of the optic
bulb forms a kind of capsule, which may be called the sclerotic.
Posteriorly the sclerotic abuts on the cartilaginous orbit, which
encloses the optic ganglion (_G. op_); and in front it becomes
transparent and forms the cornea _Co_, which may be either completely
closed, or (as represented in the diagram) perforated by a larger or
smaller opening. Behind the cornea is a chamber known as the anterior
optic chamber. This chamber is continued back on each side round a
great part of the circumference of the eye, and separates the
sclerotic from a layer internal to it.
[183] _Vide_ Hensen, _Zeit. f. wiss. Zool._ Bd. XV.
In the anterior optic chamber there are placed (1) the anterior part
of the lens (_l^1_) and (2) the folds of the iris (_Ir_). The whole
chamber, except the part formed by the lens, is lined by the epidermis
(_Int^1_ and _Int^2_). Bounding the inner side of the anterior optic
chamber is a layer which is called the choroid (_Int^1_) which is
continued anteriorly into the fold of the iris (_Ir_). The most
superficial layer of the choroid is the epithelium already mentioned,
next comes a layer of obliquely placed plates known as the argentea
externa, then a layer of muscles, and finally the argentea interna.
The argentea interna abuts on a cartilaginous capsule, which
completely invests the inner part of the eye.
The lens is a nearly spherical body composed of concentric lamellæ of
a structureless material. It is formed of a small outer (_l^1_) and
large inner (_l_) segment, the two being separated by a thin membrane.
It is supported by a peculiar projection of the wall of the optic cup,
known as the ciliary body (_Co.ep_), inserted at the base of the iris,
and mainly formed of a continuation of the retina. This body is
however muscular, and presents a series of folds on its outer and
inner surfaces, which are especially developed on the latter.
The membrane dividing the lens into two parts is continuous with the
ciliary body. Within the lens is the inner optic chamber, bounded in
front by the lens and the ciliary body, and behind by the retina.
The retina is formed of two main divisions, an anterior division
adjoining the inner optic chamber, and a posterior division (_N.S_)
adjoining the cartilage of the choroid. The two layers are separated
by a membrane. Passing from within outwards the following layers in
the retina may be distinguished:
(1) Homogeneous membrane. } Anterior division of
(2) Layer of rods. } retina.
(3) Layer of granules imbedded in pigment }
(4) Cellular layer. }
(5) Connective tissue layer. } Posterior layer of retina.
(6) Layer of nerve-fibres. }
At the side of the optic ganglion is a peculiar body, known as the
white body (not shewn in the figure), which has the histological
characters of glandular tissue.
[FIG. 278. TWO SECTIONS THROUGH THE DEVELOPING EYE OF A CEPHALOPOD
TO SHEW THE FORMATION OF THE OPTIC CUP. (After Lankester.)]
The first satisfactory account of the development of the eye is due to
Lankester (No. 365). The more important features in it were also
independently worked out by Grenacher (No. 363), and are beautifully
illustrated in Bobretzky's paper (No. 362). The eye first appears as
an oval pit of the epiblast, the edge of which is formed by a
projecting rim (fig. 278 A). The epiblast layer lining the floor of
the pit soon becomes considerably thickened. By the growth inwards of
the rim the mouth of the pit is gradually narrowed (fig. 278 B),
resembling at this stage the eye of Nautilus, and finally closed.
There is thus formed a flattened sack, lined by epiblast, which may be
called the primary optic vesicle. Its cavity eventually forms the
inner optic chamber. The anterior wall of the sack is lined by a much
less columnar layer than the posterior, the former giving rise to the
epithelium on the inner side of the ciliary processes, the latter to
the retina.
[FIG. 279. TRANSVERSE SECTION THROUGH THE HEAD OF AN ADVANCED EMBRYO
OF LOLIGO. (After Bobretzky.)
_gls._ salivary gland; _g.vs._ visceral ganglion; _gc._ cerebral
ganglion; _g.op._ optic ganglion; _adk._ optic cartilage; _ak._ and
_y._ lateral cartilage or (?) white body; _rt._ retina; _gm._
limiting membrane of retina; _vk._ ciliary region of eye; _cc._
iris; _ac._ auditory sack (the epithelium lining the auditory sacks
is not represented); _vc._ vena cava; _ff._ folds of funnel; _x._
epithelium of funnel.]
The cavity of the sack rapidly enlarges, and assumes a spherical form.
At the same time a layer of mesoblast grows in between the walls of
the sack and the external epiblast. Two new structures soon arise
nearly simultaneously (fig. 279),--which become in the adult eye the
iris (_cc_) and the posterior segment of the lens. The iris is formed
as a circular fold of the skin in front of the optic vesicle. It
consists both of epiblast and mesoblast, and gives rise to a pit lined
by epiblast. The posterior segment of the lens arises as a
structureless rod-like body, which is shewn in fig. 279 depending from
the inner side of the anterior wall of the optic vesicle. Its exact
mode of origin is somewhat obscure. The following is Lankester's
account of it[184]: "It is formed entirely within the primitive optic
chamber, and at first depends as a short cylindrical rod from the
middle point of the anterior wall of that chamber, that is to say,
from the point at which the chamber finally closed up. It grows
subsequently by the deposition of concentric layers of a horny
material round this cone. No cells appear to be immediately concerned
in effecting the deposition, and it must be looked upon as an organic
concretion, formed from the liquid contained in the primitive optic
chamber."
[184] "Devel. of Cephalopoda." _Q. J. Micro. Scien._ 1875, p. 44.
The lens would thus appear to be a cuticular structure. It gradually
assumes a nearly spherical form; and is then composed of
concentrically arranged layers (fig. 280, _hl_).
While the lens is being formed, the ciliary epithelium of the optic
vesicle becomes divided into two layers, an outer layer of large cells
and an inner of small cells. Both layers are at first continuous
across the anterior wall of the optic chamber in front of the lens,
but soon become confined to the sides (fig. 280 A, _cc_ and _gz_). The
inner layer is stated by Lankester to give rise to the muscles present
in the adult. The mesoblast cells also disappear from the region in
front of the lens, and the outer epithelium is converted into a kind
of cuticular membrane. By these changes the original layers of cells
in front of the lens become reduced to mere membranes,--a change which
appears to be preparatory to the appearance of the anterior segment of
the lens. The formation of the latter has not been fully followed out
by any investigator except Bobretzky. His figures would seem to
indicate that it is formed as a cuticular deposit in front of the
membrane already spoken of (fig. 280 B, _vl_). The two segments of the
lens appear at any rate to be separated by a membrane continuous with
the ciliary region of the optic vesicle.
Grenacher believes that the front part of the lens is formed in a
pocket-like depression of the epiblastic layer covering the outer side
of the optic cup; and Lankester thinks that the lens "pushes its way
through the median anterior area of the primitive optic chamber, and
projects into the second or anterior optic chamber where the iridian
folds lie closely upon it."
While the lens is attaining its complete development there appears a
fresh fold round the circumference of the eye, which gradually grows
inwards so as to form a chamber outside the parts already present.
This chamber is the anterior optic chamber of the adult. In most
Cephalopods (fig. 277 C) the edges of the fold do not quite meet, but
leave a larger or smaller aperture leading into the chamber containing
the iris, outer segment of the lens, etc. In some forms however they
meet and coalesce, and so shut off this chamber from communication
with the exterior. The edge of the fold constitutes the cornea while
the remainder of it gives rise to the sclerotic.
[FIG. 280. SECTIONS THROUGH THE DEVELOPING EYE OF LOLIGO AT TWO
STAGES. (After Bobretzky.)
_hl._ inner segment of lens; _vl._ outer segment of lens; _a_ and
_a´_. epithelium lining the anterior optic chamber; _gz._ large
epiblast cells of ciliary body; _cc._ small epiblast cells of
ciliary body; _ms._ layer of mesoblast between the two epiblastic
layers of the ciliary body; _af._ and _if._ fold of iris; _rt._
retina; _rt´´._ inner layer of retina; _st._ rods; _aq._ cartilage of
the choroid.]
The retina is at first a thick layer of numerous rows of oval cells
(fig. 279). When the inner segment of the lens is far advanced towards
its complete formation pigment becomes deposited in the anterior part
of the retina, and a layer of rods grows out from the surface turned
towards the cavity of the optic vesicle (fig. 280 A, _st_). At a
slightly later stage the retina becomes divided into two layers
(Bobretzky), a thicker anterior layer, and a thinner posterior layer
(fig. 280, _rt_ and _rt´´_). The former is composed of two strata, (1)
the rods and (2) a stratum with numerous rows of nuclei which becomes
in the adult the granular layer with its pigment. The posterior layer
gives rise to the cellular part of the posterior division of the
retina, while layers of connective tissue around it give rise to the
connective tissue of this portion of the retina (layer 6 in the scheme
on p. 474). The nervous layer is derived from the optic ganglion which
attaches itself to the inner side of the connective tissue layer.
The greater part of the choroid is formed from the mesoblast adjoining
the retina, but the epithelium covering its outer wall is of
epiblastic origin.
It is difficult to decide from development whether the Molluscan eyes,
so far dealt with, originated in the first instance _pari passu_ with
the supraoesophageal ganglia or independently at a later period. On
purely _à priori_ ground I should be inclined to adopt the former
alternative.
In addition to the above eyes there occur amongst Mollusca highly
complicated eyes, of a very different kind, in two widely separated
groups, viz. certain species of a genus of slug (Onchidium), and
certain Lamellibranchiata. These eyes, though they have no doubt been
evolved independently of each other, present certain remarkable points
of agreement. In both of them the rods of the retina are turned _away
from the surface_, and the nerve-fibres are placed, as in the
Vertebrate eye, _on the side of the retina which faces outwards_.
The peculiar eyes of Onchidium, investigated by Semper[185], are
scattered on the dorsal surface, there being normal eyes in the usual
situation on the head. The eyes on the dorsal surface are formed of a
cornea, a lens composed of 1-7 cells, and a retina surrounded by
pigment; which is perforated in the centre by an optic nerve, the
retinal elements being in the inverted position above mentioned.
[185] _Ueber Sehorgane von Typus d. Wirbelthieraugen, etc._,
Wiesbaden, 1877, and _Archiv f. mikr. Anat._ Vol. XIV. pp.
118-122.
The development of these eyes has been somewhat imperfectly studied in
the adult, in which they continue to be formed anew. They arise by a
differentiation of the epidermis at the end of a papilla. At first a
few glandular cells appear in the epidermis in the situation where an
eye is about to be formed. Then, by a further process of growth, an
irregular mass of epidermic cells becomes developed, which pushes the
glandular cells to one side, and constitutes the rudiment of the eye.
This mass, becoming surrounded by pigment, unites with the optic
nerve, and its cells then differentiate themselves, _in situ_, into
the various elements of the eye. No explanation is offered by Semper
of the inverted position of the rods, nor is any suggested by his
account of the development. As pointed out by Semper these eyes are no
doubt modifications of the sensory epithelium of the papillæ.
The eyes of Pecten and Spondylus[186] are placed on short stalks at
the edge of the mantle, and are probably modifications of the
tentacular processes of the mantle edge. They are provided with a
cornea, a cellular lens, a vitreous chamber, and a retina. The retinal
elements are inverted, and the optic nerve passes in at the side, but
occupies, in reference to its ramifications, the same relative
situation as the optic nerve in the Vertebrate eye. The development
has unfortunately not yet been studied.
[186] _Vide_ Hensen (No. 364) and S. J. Hickson, "The Eye of
Pecten," _Quart. J. of Micr. Science_, Vol. XX. 1880.
Our knowledge of the structure or still more of the development of the
organ of vision of the Platyelminthes, Rotifera, and Echinodermata is
too scanty to be of any general interest.
Chætopoda. Amongst the Chætopoda the cephalic eyes of Alciope (fig.
281) have been adequately investigated as to their anatomy by Greeff.
These are provided with a large cuticular lens (_l_), separated from
the retina by a wide cavity containing the vitreous humour. The retina
is formed of a single row of cells, with rods at their free
extremities, continuous at their opposite ends with nerve-fibres. The
development of this eye has not been worked out. Eyes _not_ situated
on the head are found in Polyophthalmus, and have probably been
evolved from the more indifferent type of sense-organ found by Eisig
in the allied Capitellidæ.
Chætognatha[187]. The paired cephalic eyes of Sagitta are spherical
bodies imbedded in the epidermis. They are formed of a central mass of
pigment with three lenses partially imbedded in it. The outer covering
of the eye is the retina, which is mainly composed of rod-bearing
cells; the rods being placed in contact with the outer surface of each
of the lenses. In the presence of three lenses the eye of Sagitta
approaches in some respects the eye of the Arthropoda.
[187] O. Hertwig. "Die Chætognathen." _Jenaische Zeitschrift_,
Vol. XIV. 1880.
Arthropodan eye. A satisfactory elucidation of the phylogeny of
Arthropodan eyes has not yet been given.
All the types of eyes found in the group (with exception of that of
Peripatus)[188] present marked features of similarity, but I am
inclined to view this similarity as due rather to the character of the
exoskeleton modifying in a more or less similar way all the forms of
visual organs, than to the descent of all these eyes from a common
prototype. In none of these eyes is there present a chamber filled
with fluid between the lens and the retina, but the space in question
is filled with cells. This character sharply distinguishes them from
such eyes as those of Alciope (fig. 281). The types of eyes which are
found in the Arthropoda are briefly the following:
[188] The eye of Peripatus is similar neither to the eye of the
Arthropoda, nor to that of the Chætopoda, but resembles much more
closely the Molluscan eye. The hypodermis and cuticle form
together a highly convex cornea, within which is a large optic
chamber, the posterior wall of which is formed by the retina. The
optic chamber would appear to contain a structureless lens, but
it is possible that what I regard as a lens may, on fuller
investigation, turn out to be only a coagulum.
[FIG. 281. EYE OF AN ALCIOPID (NEOPHANTA CELOX). (From Gegenbaur;
after Greef.)
_i._ cuticle; _c._ continuation of cuticle in front of eye; _l._
lens; _h_. vitreous humour; _o._ optic nerve; _o´._ expansion of the
optic nerve; _b._ layer of rods; _p._ pigment layer.]
(1) _Simple eyes._ In all simple eyes the corneal lens is formed by a
thickening of the cuticle. Such eyes are confined to the Tracheata.
There are three types of simple eyes. (_a_) A type in which the
retinal cells are placed immediately behind the lens, found (Lowne) in
the larvæ of some Diptera (Eristalis), and also in some Chilognatha.
(_b_) A type of simple eye found in some Chilopoda, and in some Insect
larvæ (Dytiscus, etc.) (fig. 282), the parts of which are entirely
derived from the epidermis. There is present a lens (_l_) formed as a
thickening of the cuticle, a so-called vitreous humour (_gl_) formed
of modified hypodermis cells, and a retina (_r_) derived from the same
source. The outer ends of the retinal cells terminate in rods, and
their inner ends are continuous with nerve-fibres.
(_c_) A type of simple eye found in the Arachnida, and apparently some
Chilopoda, and forming the simple eyes of most Insects, which differs
from type (_a_) in the cells of the retina forming _a distinct layer
beneath the hypodermis_; the latter only obviously giving rise to the
vitreous humour.
[FIG. 282. SECTION THROUGH THE SIMPLE EYE OF A YOUNG DYTISCUS LARVA.
(From Gegenbaur; after Grenacher.)
_l._ corneal lens; _g._ vitreous humour; _r._ retina; _o._ optic
nerve; _h._ hypodermis.]
The development of the simple eyes has not yet been studied.
The simple eyes so far described are always placed on the head, and
are usually rather numerous.
(2) _Compound eyes._ Compound eyes are almost always present in the
Crustacea, and are usually found in adult Insects. In both groups they
are paired, though in the Crustacea a median much simplified compound
eye may either take the place of the paired eyes in the Nauplius larva
and lower forms, or be present together with them during a period in
the development of higher forms.
The typical compound eye is formed (fig. 283) of a series of corneal
lenses (_c_) developed from the cuticle; below which are placed bodies
known as the crystalline cones, one to each corneal lens; and below
the crystalline cones are placed bodies known as the retinulæ (_r_)
constituting the percipient elements of the eye, each of them being
formed of an axial rod, the rhabdom, and a number of cells surrounding
it.
The crystalline cones are formed from the coalescence of cuticular
deposits in several cells, the nuclei of which usually remain as
Semper's nuclei. These cells are probably simple hypodermis cells, but
in some forms, _e.g._ Phronima, there may be a continuous layer of
hypodermis cells between them and the cuticle. In various Insect eyes
the cells which usually give rise to a crystalline cone may remain
distinct, and such eyes have been called by Grenacher aconous eyes,
while eyes with incompletely formed crystalline cones are called by
him pseudoconous eyes.
The rhabdom of the retinulæ is, like the crystalline cone, developed
by the coalescence of a series of parts, which are primitively
separate rods placed each in its own cell: this condition of the
retinulæ is permanently retained in the eyes of the Tipulidæ.
The development of the compound eye has so far only been
satisfactorily studied in some Crustacea by Bobretzky (No. 367); by
whom it has been worked out in Palæmon and Astacus, but more fully in
the latter, to which the following account refers:
The eye of Astacus takes its origin from two distinct parts, (1) the
external epidermis of the procephalic lobes which will be spoken of as
the epidermic layer of the eye, (2) a portion of the supraoesophageal
ganglia, which will be spoken of as the neural layer of the eye. The
mesoblast is moreover the source of some of the pigment between the
two above layers. The epidermic layer gives rise to the corneal
lenses, the crystalline cones, and the pigment around the latter. The
neural layer on the other hand seems to give rise to the retinulæ with
their rhabdoms, and to the optic ganglion.
[FIG. 283. DIAGRAMMATIC REPRESENTATIONS OF PARTS OF A COMPOUND
ARTHROPOD EYE. (From Gegenbaur.)
A. Section through the eye. B. Corneal facets. C. Two segments of
the eye.
_c._ corneal (cuticular) lenses; _r._ retinulæ with rhabdoms; _n._
optic nerve; _g._ ganglionic swelling of optic nerve.]
After the separation of the supraoesophageal ganglia from the
superficial epiblast, the cells of the epidermis in the region of the
future eye become columnar, and so form the above-mentioned epidermic
layer of the eye. This layer soon becomes two or three cells deep. At
the same time the most superficial part of the adjoining
supraoesophageal ganglion becomes partially constricted off from the
remainder as the neural layer of the eye, but is separated by a small
space from the thickened patch of epidermis. Into this space some
mesoblast cells penetrate at a slightly later period. Both the
epidermic and neural layers next become divided into two strata. The
outer stratum of the epidermic layer gives rise to the crystalline
cones and Semper's nuclei; each crystalline cone being formed from
four coalesced rods, developed as cuticular differentiations of four
cells, the nuclei of which may be seen in the embryo on its outer
side. The lower ends of the cones pass through the inner stratum of
the epidermic disc, the cells of which become pigmented, and
constitute the pigment cells surrounding the lower part of the
crystalline cones in the adult. The outer end of each of the
crystalline cones is surrounded by four cells, believed by Bobretzky
to be identical with Semper's nuclei[189]. These cells give rise in a
later stage (not worked out in Astacus) to the cuticular corneal
lenses.
[189] There would appear to be some confusion as to the
nomenclature of these parts in Bobretzky's account.
Of the two strata of the neural layer the outer is several cells deep,
while the inner is formed of elongated rod-like cells. Unfortunately
however the fate of the two neural layers has not been worked out,
though there can be but little doubt that the retinulæ originate from
the outer layer.
The mesoblast which grows in between the neural and epidermic layers
becomes a pigment layer, and probably also forms the perforated
membrane between the crystalline cones and the retinulæ.
The above observations of Bobretzky would appear to indicate that the
paired compound eyes of Crustacea belong to the type of cerebral eyes.
How far this is also the case with the compound eyes of Insects is
uncertain, in that it is quite possible that the latter eyes may have
had an independent origin.
The relation between the paired and median eye of the Crustacea is
also uncertain.
In the genus Euphausia amongst the Schizopods there is present a
series of eyes placed on the sides of some of the thoracic legs and on
the sides of the abdomen. The structure of these eyes, though not as
yet satisfactorily made out, would appear to be very different from
that of other Arthropodan visual organs.
The Eye of the Vertebrata. In view of the various structures which
unite to form it, the eye is undoubtedly the most complicated organ of
the Vertebrata; and though its mode of development is fairly constant
throughout the group, it will be convenient shortly to describe what
may be regarded as its typical development, and then to proceed to a
comparative view of the origin of its various parts, and to enter into
greater detail with reference to some of them. At the end of the
section there is an account of the accessory structures connected with
the eye.
The formation of the eye commences with the appearance of a pair of
hollow outgrowths from the anterior cerebral vesicle or
thalamencephalon, which arise in many instances, even before the
closure of the medullary canal. 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 (fig. 284, _a_), united to the base of
the brain by comparatively narrow hollow stalks, the rudiments of the
optic nerves. The constriction to which the stalk or optic nerve is
due takes place obliquely downwards and backwards, so that the optic
nerves open into the base of the front part of the thalamencephalon
(fig. 284, _b_).
[FIG. 284. SECTION THROUGH THE HEAD OF AN EMBRYO TELEOSTEAN, TO SHEW
THE FORMATION OF THE OPTIC VESICLES, ETC. (From Gegenbaur; after
Schenk.)
_c._ fore-brain; _a._ optic vesicle; _b._ stalk of optic vesicle;
_d._ epidermis.]
After the establishment of the optic nerves, there take place (1) the
formation of the lens, and (2) the formation of the optic cup from the
walls of the primary optic vesicle.
The external or superficial epiblast which covers, and is in most
forms in immediate contact with, the most projecting portion of the
optic vesicle, becomes thickened. This thickened portion is then
driven inwards in the form of a shallow open pit with thick walls
(fig. 285 A, _o_), carrying before it the front wall (_r_) of the
optic vesicle. To such an extent does this involution of the
superficial epiblast take place, that the front wall of the optic
vesicle is pushed close up to the hind wall, and the cavity of the
vesicle becomes almost obliterated (fig. 285 B).
The bulb of the optic vesicle is thus converted into a cup with double
walls, containing in its cavity the portion of involuted epiblast.
This cup, in order to distinguish its cavity from that of the original
optic vesicle, is generally called the _secondary optic vesicle_. We
may, for the sake of brevity, speak of it as the _optic cup_; in
reality it never is a vesicle, since it always remains widely open in
front. Of its double walls the inner or anterior (fig. 285 B, _r_) is
formed from the front portion, the outer or posterior (fig. 285 B,
_u_) from the hind portion of the wall of the primary optic vesicle.
The inner or anterior (_r_), which very speedily becomes thicker than
the other, is converted into the retina: in the outer or posterior
(_u_), which remains thin, pigment is eventually deposited, and it
ultimately becomes the tesselated pigment-layer of the choroid.
[FIG. 285. DIAGRAMMATIC SECTIONS ILLUSTRATING THE FORMATION OF THE
EYE. (After Remak.)
In A the thin superficial epiblast _h_ is seen to be thickened at
_x_, in front of the optic vesicle, and involuted so as to form a
pit _o_, the mouth of which has already begun to close in.
Accompanying this involution, which forms the rudiment of the lens,
the optic vesicle is doubled in, its front portion _r_ being pushed
against the back portion _u_, and the original cavity of the vesicle
thus reduced in size. The stalk of the vesicle is shewn as still
broad.
In B the optic vesicle is still further doubled in so as to form a
cup with a posterior wall _u_ and an anterior wall _r_. In the
hollow of this cup lies the lens _l_, now completely detached from
the superficial epiblast _xh_.]
By the closure of its mouth the pit of the involuted epiblast becomes
a completely closed sac with thick walls and a small central cavity
(fig. 285 B, _l_). At the same time it breaks away from the external
epiblast, which forms a continuous layer in front of it, all traces of
the original opening being lost. There is thus left lying in the cup
of the secondary optic vesicle, an isolated elliptical mass of
epiblast. This is the rudiment of the lens. The small cavity within it
speedily becomes still less by the thickening of the walls, especially
of the hinder one.
At its first appearance the lens is in immediate contact with the
anterior wall of the secondary optic vesicle (fig. 285 B). In a short
time however, the lens is seen to lie in the mouth of the cup (fig.
288 D), a space (_vh_) (which is occupied by the vitreous humour)
making its appearance between the lens and anterior wall of the
vesicle.
In order to understand how this space is developed, the position of
the optic vesicle and the relations of its stalk must be borne in
mind.
The vesicle lies at the side of the head, and its stalk is directed
downwards, inwards and backwards. The stalk in fact slants away from
the vesicle. Hence, when the involution of the lens takes place, the
direction in which the front wall of the vesicle is pushed in is not
in a line with the axis of the stalk, as for simplicity's sake has
been represented in the diagram (fig. 285), but forms an obtuse angle
with that axis, after the manner of fig. 286, where _s´_ represents
the cavity of the stalk leading away from the almost obliterated
cavity of the primary vesicle.
Fig. 286 represents the early stage at which the lens fills the whole
cup of the secondary vesicle. The subsequent condition is brought
about through the rapid growth of the walls of the cup. This growth
however does not take place equally in all parts of the cup. The walls
of the cup rise up all round except that point of the circumference of
the cup which adjoins the stalk. While elsewhere the walls increase
rapidly in height, carrying so to speak the lens with them, at this
spot, which in the natural position of the eye is on its under
surface, there is no growth: the wall is here imperfect, and a gap is
left. Through this gap, which afterwards receives the name of the
choroidal fissure, a way is open from the mesoblastic tissue
surrounding the optic vesicle and stalk into the interior of the
cavity of the cup.
[FIG. 286. DIAGRAMMATIC SECTION OF THE EYE AND THE OPTIC NERVE AT AN
EARLY STAGE. (From Lieberkühn.)
To shew the lens _l_ occupying the whole hollow of the optic cup,
the inclination of the stalk _s_ to the optic cup, and the
continuity of the cavity of the stalk _s´_ with that of the primary
vesicle _c_; _r_. anterior, _u_. posterior wall of the optic cup.]
From the manner of its formation the gap or fissure is evidently in a
line with the axis of the optic stalk, and in order to be seen must be
looked for on the under surface of the optic vesicle. In this position
it is readily recognised in the embryo seen as a transparent object
(fig. 118, _chs_).
Bearing in mind these relations of the gap to the optic stalk, the
reader will understand how sections of the optic vesicle at this stage
present very different appearances according to the plane in which the
sections are taken.
When the head is viewed from underneath as a transparent object the
eye presents very much the appearance represented in the diagram (fig.
287).
A section of such an eye taken along the line _y_, perpendicular to
the plane of the paper, would give a figure corresponding to that of
fig. 288 D. The lens, the cavity and double walls of the secondary
vesicle, the remains of the primary cavity, would all be represented
(the superficial epiblast of the head would also be shewn); but there
would be nothing seen of either the stalk or the fissure. If on the
other hand the section were taken in a plane parallel to the plane of
the paper, at some distance above the level of the stalk, some such
figure would be obtained as that shewn in fig. 288 E. Here the fissure
_f_ is obvious, and the communication of the cavity _vh_ of the
secondary vesicle with the outside of the eye evident; the section of
course would not go through the superficial epiblast. Lastly, a
section, taken perpendicular to the plane of the paper along the line
_z_, _i.e._ through the fissure itself, would present the appearances
of fig. 288 F, where the wall of the vesicle is entirely wanting in
the region of the fissure marked by the position of the letter _f_.
The external epiblast has been omitted in this figure.
[FIG. 287. DIAGRAMMATIC REPRESENTATION OF THE EYE OF THE CHICK OF
ABOUT THE THIRD DAY AS SEEN WHEN THE HEAD IS VIEWED FROM UNDERNEATH
AS A TRANSPARENT OBJECT.
_l._ the lens; _l´._ the cavity of the lens, lying in the hollow of
the optic cup; _r_. the anterior, _u._ the posterior wall of the
optic cup; _c._ the cavity of the primary optic vesicle, now nearly
obliterated. By inadvertence _u_ has been drawn in some places
thicker than _r_, it should have been thinner throughout. _s._ the
stalk of the optic cup with _s´_ its cavity, at a lower level than
the cup itself and therefore out of focus; the dotted line indicates
the continuity of the cavity of the stalk with that of the primary
vesicle.
The line _z z_, through which the section shewn in fig. 288 F is
supposed to be taken, passes through the choroidal fissure.]
With reference to the above description, taken with very slight
alterations from the _Elements of Embryology_, Pt. 1., two points
require to be noticed. Firstly it is extremely doubtful whether the
invagination of the secondary optic vesicle is to be viewed as an
actual mechanical result of the ingrowth of the lens. Secondly it
seems probable that the choroid fissure is not simply due to an
inequality in the growth of the walls of the secondary optic cup, but
is partly due to a doubling up of the primary vesicle from the side
along the line of the fissure, at the same time that the lens is being
thrust in in front. In Mammalia, the doubling up involves the optic
stalk, which becomes flattened (whereby its original cavity is
obliterated) and then folded in on itself, so as to embrace a new
central cavity continuous with the cavity of the vitreous humour. And
in other forms a partial phenomenon of the same kind is usually
observable, as is more particularly described in the sequel.
Before describing the development of the cornea, aqueous humour, etc.
we may consider the further growth of the parts, whose first
development has just been described, commencing with the optic cup.
During the above changes the mesoblast surrounding the optic cup
assumes the character of a distinct investment, whereby the outline of
the eyeball is definitely formed. The internal portions of this
investment, nearest to the retina, become the _choroid_ (_i.e._ the
_chorio-capillaris_, and the _lamina fusca_; the pigment epithelium,
as we have seen, being derived from the epiblastic optic cup), and
pigment is subsequently deposited in it. The remaining external
portion of the investment forms the _sclerotic_.
The complete differentiation of these two coats of the eye does not
however take place till a late period.
The cavity of the original optic vesicle was left as a nearly
obliterated space between the two walls of the optic cup. By the end
of the third day the obliteration is complete, and the two walls are
in immediate contact.
The inner or anterior wall is, from the first, thicker than the outer
or posterior; and over the greater part of the cup this contrast
increases with the growth of the eye, the anterior wall becoming
markedly thicker and undergoing changes of which we shall have to
speak directly (fig. 289).
In the front portion however, along, so to speak, the lip of the cup,
anterior to a line which afterwards becomes the ora serrata, both
layers cease to take part in the increased thickening, accompanied by
peculiar histological changes, which the rest of the cup is
undergoing. Thus a hind portion or true retina is marked off from a
front portion.
The front portion, accompanied by the mesoblast which immediately
overlies it, is behind the lens thrown into folds, the ciliary ridges;
while further forward it bends in between the lens and the cornea to
form the iris. The original wide opening of the optic cup is thus
narrowed to a smaller orifice, the pupil; and the lens, which before
lay in the open mouth of the cup, is now inclosed in its cavity. While
in the hind portion of the cup or retina proper no deposit of black
pigment takes place in the layer formed out of the inner or anterior
wall of the vesicle; in the front portion forming the region of the
iris, pigment is largely deposited throughout both layers, though
first of all in the outer one, so that eventually this portion seems
to become nothing more than a forward prolongation of the pigment
epithelium of the choroid.
[FIG. 288.
D. Diagrammatic section taken perpendicular to the plane of the
paper, along the line _yy_, fig. 287. The stalk is not seen, the
section falling quite out of its region. _vh._ hollow of optic cup
filled with vitreous humour; other letters as in fig. 285 B. (After
Remak.)
E. Section taken parallel to the plane of the paper through fig.
287, so far behind the front surface of the eye as to shave off a
small portion of the posterior surface of the lens _l_, but not so
far behind as to be carried at all through the stalk. Letters as
before; _f._ the choroidal fissure.
F. Section along the line _zz_, perpendicular to the plane of the
paper, to shew the choroidal fissure _f_, and the continuity of the
cavity of the optic stalk with that of the primary optic vesicle.
Had this section been taken a little to one side of the line _zz_,
the wall of the optic cup would have extended up to the lens below
as well as above. Letters as before. The external epiblast is
omitted in this section.]
Thus, while the hind moiety of the optic cup becomes the retina
proper, including the choroid-pigment in which the rods and cones are
imbedded, the front moiety is converted into the ciliary portion of
the retina, covering the ciliary processes, and into the uvea of the
iris; the bodies of the ciliary processes and the substance of the
iris, their vessels, muscles, connective tissue and ramified pigment,
being derived from the mesoblastic choroid. The margin of the pupil
marks the extreme lip of the optic vesicle, where the outer or
posterior wall turns round to join the inner or anterior.
The ciliary muscle and the ligamentum pectinatum are both derived from
the mesoblast between the cornea and the iris.
[FIG. 289. SECTION OF THE EYE OF CHICK AT THE FOURTH DAY.
_e.p._ superficial epiblast of the side of the head; _R._ true
retina: anterior wall of the optic cup; _p.Ch._ pigment-epithelium
of the choroid: posterior wall of the optic cup. _b_ is placed at
the extreme lip of the optic cup at what will become the margin of
the iris. _l._ the lens. The hind wall, the nuclei of whose
elongated cells are shewn at _nl_, now forms nearly the whole mass
of the lens, the front wall being reduced to a layer of flattened
cells _el_. _m._ the mesoblast surrounding the optic cup and about
to form the choroid and sclerotic. It is seen to pass forward
between the lip of the optic cup and the superficial epiblast.
Filling up a large part of the hollow of the optic cup is seen a
hyaline mass, the rudiment of the hyaloid membrane, and of the
coagulum of the vitreous humour, _y_. In the neighbourhood of the
lens it seems to be continuous as at _cl_ with the tissue _a_, which
appears to be the rudiment of the capsule of the lens and suspensory
ligament.]
The Retina. At first the two walls of the optic cup do not greatly
differ in thickness. On the third day the outer or posterior becomes
much thinner than the inner or anterior, and by the middle of the
fourth day is reduced to a single layer of flattened cells (fig. 289,
_p.Ch_). At about the 80th hour its cells commence to receive a
deposit of pigment, and eventually form the so-called pigmentary
epithelium of the choroid; from them no part of the true retina (or no
other part of the retina, if the pigment-layer in question be supposed
to belong more truly to the retina than to the choroid) is derived.
On the fourth day, the inner (anterior) wall of the optic cup (fig.
289, _R_) has a perfectly uniform structure, being composed of
elongated somewhat spindle-shaped cells, with distinct nuclei. On its
external (posterior) surface a distinct cuticular membrane, the
membrana limitans externa, early appears.
As the wall increases in thickness, its cells multiply rapidly, so
that it soon becomes several cells thick: each cell being however
probably continued through the whole thickness of the layer. The wall
at this stage corresponds closely in its structure with the brain, of
which it may properly be looked upon as part. According to the usual
view, which is not however fully supported by the development, the
retina becomes divided in the subsequent growth into (1) an outer
part, corresponding morphologically to the epithelial lining of the
cerebrospinal canal, composed of what may be called the visual cells
of the eye, _i.e._ the cells forming the outer granular (nuclear)
layer and the rods and cones attached to them; and (2) an inner
portion consisting of the inner granular (nuclear) layer, the inner
molecular layer, the ganglionic layer and the layer of nerve-fibres
corresponding morphologically to the walls of the brain. According to
Löwe, however, only the outer limbs of the rods and cones, which he
holds to be metamorphosed cells, correspond to the epithelial layer of
the brain.
The actual development of the retina is not thoroughly understood.
According to the usual statements (Kölliker, No. 298, p. 693) the
layer of ganglion cells and the inner molecular layer are first
differentiated, while the remaining cells give rise to the rest of the
retina proper, and are bounded externally by the membrana limitans
externa. On the inner side of the ganglionic layer the stratum of
nerve-fibres is also very early established. The rods and cones are
formed as prolongations (Kölliker, Babuchin), or cuticularizations
(Schultze, W. Müller) of the cells which eventually form the outer
granular layer. The layer of cells external to the molecular layer is
not divided till comparatively late into the inner and outer granular
(nuclear) layers, and the interposed outer molecular layer.
Löwe's account of the development of the retina in the Rabbit is in
many points different from the above. He finds that three stages in
the differentiation of the layers of the retina may be distinguished.
In the first stage, in an embryo of four or five millimetres, the
following layers are present, commencing at the outer side, adjoining
the external wall of the secondary optic cup.
(1) A membrane, which does not however, as usually believed, become
the membrana limitans externa.
(2) A layer of clear elements, derived from metamorphosed cells,
constituting the outer limbs of the rods and cones.
(3) A layer of dark rounded elements.
(4) An indistinctly striated layer, the future layer of nerve-fibres.
The third of these layers gives rise to all the eventual strata of the
retina proper, except the outer limbs of the rods and cones.
In the next stage, when the embryo has reached a length of 2 cm., this
layer becomes divided into three strata: viz. an outer and inner layer
of dark elements and a middle one of clearer elements. The two inner
of these layers become respectively the inner molecular layer and the
layer of ganglion cells, while the outer layer gives rise to the parts
of the retina external to the inner molecular layer.
In the newly born animal the outer darker layer of the previous stage
has become considerably subdivided. Its outermost part forms a stratum
of darkly coloured elements, which develop into the inner limbs of the
rods and cones. It is bounded internally by a membrane--the true
membrana elastica externa. The part of the layer within this is soon
divided into the outer and inner granular layers, separated from each
other by the delicate outer molecular layer. Thus, shortly after
birth, all the layers of the retina are established in the Rabbit. It
is important to notice that, according to Löwe's views, the outer and
inner limbs of the rods and cones are metamorphosed cells. The outer
limbs at first form a continuous layer, in which separate elements
cannot be recognised.
At a very early period there appears a membrane on the side of the
retina adjoining the vitreous humour. This membrane is the hyaloid
membrane. The investigations of Kessler and myself lead to the
conclusion that it may be formed at a time when there is no trace of
mesoblastic structures in the cavity of the vitreous humour, and that
it is therefore necessarily developed as a cuticular deposit of the
cells of the optic cup. Lieberkühn, Arnold, Löwe, and other authors
regard it however as a mesoblastic product; and Kölliker believes that
a primitive membrane is developed from the cells of the optic cup, and
that a true hyaloid membrane is developed much later as a product of
the mesoblast.
For fuller information on this subject the reader is referred to the
authors quoted above.
The optic nerve. The optic nerves are derived, as we have said, from
the at first hollow stalks of the optic vesicles. Their cavities
gradually become obliterated by a thickening of the walls, the
obliteration proceeding from the retinal end inwards towards the
brain. While the proximal ends of the optic stalks are still hollow
the rudiments of the optic chiasma are formed from fibres at the roots
of the stalks, the fibres of the one stalk growing over into the
attachment of the other. The decussation of the fibres would appear to
be complete. The fibres arise in the remainder of the nerves somewhat
later. 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.
There does not appear to me any ground for doubting (as has been done
by His and Kölliker) that the fibres of the optic nerve are derived
from a differentiation of the epithelial cells of which the nerve is
at first formed.
Choroid Fissure. With reference to the choroid fissure we may state
that its behaviour varies somewhat in the different types. It becomes
for the greater part of its extent closed, though its proximal end is
always perforated by the optic nerve, and in many forms by a
mesoblastic process also.
The lens when first formed is an oval vesicle with a small central
cavity, the front and hind walls being of nearly equal thickness, and
each consisting of a single layer of elongated columnar cells. In the
subsequent stages the mode of growth of the hind wall is of precisely
an opposite character to that of the front wall. The hind wall becomes
much thicker, and tends to obliterate the central cavity by becoming
convex on its front surface. At the same time its cells, still
remaining as a single layer, become elongated and fibre-like. The
front wall on the contrary becomes thinner and thinner and its cells
flattened.
These modes of growth continue until, as shewn in fig. 289, the hind
wall _l_ is in absolute contact with the front wall _el_, and the
cavity thus becomes entirely obliterated. The cells of the hind wall
have by this time become veritable fibres, which, when seen in
section, appear to be arranged nearly parallel to the optic axis,
their nuclei _nl_ being seen in a row along their middle. The front
wall, somewhat thickened at either side where it becomes continuous
with the hind wall, is now a single layer of flattened cells
separating the hind wall of the lens, or as we may now say the lens
itself, from the front limb of the lens-capsule; of the latter it
becomes the epithelium.
The subsequent changes undergone consist chiefly in the continued
elongation and multiplication of the lens-fibres, with the partial
disappearance of their nuclei.
During their multiplication they become arranged in the manner
characteristic of the adult lens of the various forms. The
lens-capsule, as was originally stated by Kölliker, appears to be
formed as a cuticular membrane deposited by the epithelial cells of
the lens.
The views of Lieberkühn, Arnold, Löwe and others, according to which
the lens-capsule is a mesoblastic structure, do not appear to be well
founded. The contrary view, held by Kölliker, Kessler, etc., is
supported mainly by the fact that at the time when the lens-capsule
first appears there are no mesoblast cells to give rise to it. It
should however be stated that W. Müller has actually found cellular
elements in what he believes to be the lens-capsule of the Ammocoete
lens. Considering the degraded character of the Ammocoete eye,
evidence derived from its structure must be accepted with caution.
The vitreous humour. The vitreous humour is derived (except in
Cyclostomata) from a vascular ingrowth, which differs considerably in
different types, through the choroid slit. Its real nature is very
much disputed. According to Kessler's view, it is of the nature of a
fluid transudation, but the occasional presence in it of ordinary
embryonic mesoblast cells, in addition to more numerous
blood-corpuscles, gives it a claim to be regarded as intercellular
substance. The number of cells in it is however at best extremely
small and in many cases there is no trace of them. In Mammals there
appear to be some mesoblast cells invaginated with the lens, which are
not improbably employed in the formation of the vessels of the
so-called membrana capsulo-pupillaris. In the Ammocoete the vitreous
humour originates from a distinct mesoblastic ingrowth, though the
cells which give rise to it subsequently disappear.
The development of the zonula of Zinn in Mammalia, which ought to
throw some light on the nature of the vitreous humour, has not been
fully investigated. According to Lieberkühn (No. 373, p. 43), this
structure appears in half-grown embryos of the sheep and calf.
He says "At the point where the ciliary processes and the ciliary part
of the retina are entirely removed, one sees in the meridian bundles
of fine fibres, which correspond to the valleys between the ciliary
processes and fill them; also between these bundles there extend, as a
thin layer, similar finely striated masses, and these would have been
on the top of the ciliary processes." He further states that these
fibres may be traced to the anterior and posterior limb of the
lens-capsule, and that amongst them are numerous cells. Kölliker
confirms Lieberkühn's statements. There can be little doubt that the
fibres of the zonula are of the nature of connective tissue: they are
stated to be elastic. By Löwe they are believed to be developed out of
the substance of the vitreous humour, but this does not appear to me
to follow from the observations hitherto made. It seems quite possible
that they arise from mesoblast cells which have grown into the cavity
of the vitreous humour, solely in connection with their production.
The integral parts of the eye in front of the lens are the cornea, the
aqueous humour, and the iris. The development of the latter has
already been described, and there remain to be dealt with the cornea,
and the cavity containing the aqueous humour.
The cornea. The cornea is formed by the coalescence of two structures,
viz. the epithelium of the cornea and the cornea proper. The former is
directly derived from the external epiblast, which covers the eye
after the invagination of the lens. The latter is formed in a somewhat
remarkable manner, first clearly made out by Kessler.
[FIG. 290. SECTION THROUGH THE EYE OF A FOWL ON THE EIGHTH DAY OF
DEVELOPMENT, TO SHEW THE IRIS AND CORNEA IN THE PROCESS OF
FORMATION. (After Kessler.)
_ep._ epiblastic epithelium of cornea; _cc._ corneal corpuscles
growing into the structureless matrix of the cornea; _dm._
Descemet's membrane; _ir._ iris; _cb._ mesoblast of the iris (this
reference letter points a little too high).
The space between the layers _dm._ and _ep._ is filled with the
structureless matrix of the cornea.]
When the lens is completely separated from the epidermis its outer
wall is directly in contact with the external epiblast (future corneal
epithelium). At its edge there is a small ring-shaped space bounded by
the outer skin, the lens and the edge of the optic cup. In the chick,
which we may take as typical, there appears at about the time when the
cavity of the lens is completely obliterated a structureless layer
external to the above ring-like space and immediately adjoining the
inner face of the epiblast. This layer, which forms the commencement
of the cornea proper, at first only forms a ring at the border of the
lens, thickest at its outer edge, and gradually thinning off to
nothing towards the centre. It soon however becomes broader, and
finally forms a continuous stratum of considerable thickness,
interposed between the external skin and the lens. As soon as this
stratum has reached a certain thickness, a layer of flattened cells
grows in along its inner side from the mesoblast surrounding the optic
cup (fig. 290, _dm_). This layer is the epithelioid layer of the
membrane of Descemet. After it[190] has become completely established,
the mesoblast around the edge of the cornea becomes divided into two
strata; an inner one (fig. 290, _cb_) destined to form the mesoblastic
tissue of the iris already described, and an outer one (fig. 290,
_cc_) adjoining the epidermis. The outer stratum gives rise to the
corneal corpuscles, which are the only constituents of the cornea not
yet developed. The corneal corpuscles make their way through the
structureless corneal layer, and divide it into two strata, one
adjoining the epiblast, and the other adjoining the inner epithelium.
The two strata become gradually thinner as the corpuscles invade a
larger and larger portion of their substance, and finally the
outermost portion of them alone remains as the membrana elastica
anterior and posterior (Descemet's membrane) of the cornea. The
corneal corpuscles, which have grown in from the sides, thus form a
layer which becomes continually thicker, and gives rise to the main
substance of the cornea. Whether the increase in the thickness of the
layer is due to the immigration of fresh corpuscles, or to the
division of those already there, is not clear. After the cellular
elements have made their way into the cornea, the latter becomes
continuous at its edge with the mesoblast which forms the sclerotic.
[190] It appears to me possible that Lieberkühn may be right in
stating that the epithelium of Descemet's membrane grows in
between the lens and the epiblast before the formation of the
cornea proper, and that Kessler's account, given above, may on
this point require correction. From the structure of the eye in
the Ammocoete it seems probable that Descemet's membrane is
continuous with the choroid.
The derivation of the original structureless layer of the cornea is
still uncertain. Kessler derives it from the epiblast, but it appears
to me more probable that Kölliker is right in regarding it as derived
from the mesoblast. The grounds for this view are, (1) the fact of its
growth inwards from the border of the mesoblast round the edge of the
eye, (2) the peculiar relations between it and the corneal corpuscles
at a later period. This view would receive still further support if a
layer of mesoblast between the lens and the epiblast were really
present as believed by Lieberkühn. It must however be admitted that
the objections to Kessler's view of its epiblastic nature are rather
_a priori_ than founded on definite observation.
The observations of Kessler, which have been mainly followed in the
above account, are strongly opposed by Lieberkühn (No. 374) and Arnold
(No. 370), and are not entirely accepted by Kölliker. It is especially
on the development of these parts in Mammalia (to be spoken of in the
sequel) that the above authors found their objections. I have had
through Kessler's kindness an opportunity of looking through some of
his beautiful preparations, and have no hesitation in generally
accepting his conclusions, though as mentioned above I cannot agree
with all his interpretations.
The aqueous humour. The cavity for the aqueous humour has its origin
in the ring-shaped space round the front of the lens, which, as
already mentioned, is bounded by the external skin, the edge of the
optic cup, and the lens. By the formation of the cornea this space is
shut off from the external skin, and on the appearance of the
epithelioid layer of Descemet's membrane a continuous cavity is
developed between the cornea and the lens. This cavity enlarges and
receives its final form on the full development of the iris.
_Comparative view of the development of the Vertebrate Eye._
The organ of vision, when not secondarily aborted, contains in all
Vertebrata the essential parts above described. 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[191], in which it resembles in most important features
that of other Vertebrata.
[191] The most detailed account is that of W. Müller (No. 377).
[FIG. 291. HORIZONTAL SECTION THROUGH THE HEAD OF A JUST HATCHED
LARVA OF PETROMYZON SHEWING THE DEVELOPMENT OF THE LENS OF THE EYE.
_th.c._ thalamencephalon; _op.v._ optic vesicle; _l._ lens of eye;
_h.c._ head cavity.]
Eye of Ammocoetes. The optic vesicle arises as an outgrowth of the
fore-brain, but the secondary optic cup is remarkable in the young
larva for its small size (fig. 291, _opv_). The thicker outer wall
gives rise to the retina, and the thinner inner wall to the choroid
pigment. The lens is formed as an invagination of the single-layered
epidermis (fig. 291, _l_). As development proceeds the parts of the
eye gradually enlarge, and the mesoblast around the hinder and dorsal
part of the optic cup becomes pigmented. There is at first no cavity
for the vitreous humour, but eventually the growth of the optic cup
gives rise to a space, into which a cellular process of mesoblast
grows at a slight notch in the ventral edge of the optic cup (W.
Müller, No. 377). This notch is the only rudiment of the choroid
fissure of other types. The mesoblastic process is probably the
homologue of the processus falciformis and pecten, and appears to give
rise to the vitreous humour; for a long time it retains its connection
with the surrounding mesoblast. Its cells eventually disappear, and it
never contains any vascular structures.
The lens for a long time remains as an oval vesicle with a central
cavity. In a later stage, when the Ammocoete is fully developed, the
secondary optic cup forms a deep pit (fig. 292, _r_); in the mouth of
which is placed the lens (_l_). The two walls of the retina have now
the normal vertebrate structure, though the pigment is as yet
imperfectly present in the choroid layer. The lens has the embryonic
forms of higher types (cf. fig. 289), consisting of an inner thicker
segment, the true lens, and an outer layer forming the epithelium of
the lens capsule. The edge of the optic cup, which forms the rudiment
of the epiblast of the iris, is imperfectly separated from the
remainder of the optic cup; and a mesoblastic element of the iris,
distinct from Descemet's membrane (_dm_), can hardly be spoken of.
There is no cavity for the aqueous humour in front of the lens; and
there is no cornea as distinct from the epidermis and subepidermic
tissues. The elements in front of the lens are (1) the epidermis
(_ep_); (2) the dermis (_dc_); (3) the subdermal connective tissue
(_sdc_) which passes without any sharp line of demarcation into the
dermis; (4) a thick membrane, continuous with the mesoblastic part of
the choroid, which appears to represent Descemet's membrane. The
subdermal connective tissue is continued as an investment round the
whole eye; and there is no differentiated sclerotic and only an
imperfect choroid.
In a still later stage a distinct mesoblastic element for the iris is
formed. When the Ammocoete is becoming a Lamprey, the eye approaches
the surface; an anterior chamber is established; and the eye differs
from that of the higher types mainly in the fact that the cornea is
hardly distinguished from the remainder of the skin, and that a
sclerotic is very imperfectly represented.
Optic vesicles. The development of the primitive optic vesicles, so
far as is known, is very constant throughout the Vertebrata. In
Teleostei and Lepidosteus alone is there an important deviation from
the ordinary type, dependent however upon the mode of formation of the
medullary keel, the optic vesicles arising while the medullary keel is
still solid, and being at first also solid. They subsequently acquire
a lumen and undergo the ordinary changes.
[FIG. 292. EYE OF AN AMMOCOETES LYING BENEATH THE SKIN.
_ep._ epidermis; _d.c._ dermal connective tissue continuous with the
subdermal connective tissue (_s.d.c_), which is also shaded. There
is no definite boundary to this tissue where it surrounds the eye.
_m._ muscles; _dm._ membrane of Descemet; _l._ lens; _v.h._ vitreous
humour; _r._ retina; _rp._ retinal pigment.]
The lens. In the majority of groups, viz. Elasmobranchii, Reptilia,
Aves, and Mammalia, the lens is formed by an open invagination of the
epiblast, but in Amphibia, Teleostei and Lepidosteus, where the
nervous layer of the skin is early established, this layer alone takes
part in the formation of the lens (fig. 293, _l_). The lens is however
formed even in these types as a hollow body by an invagination; but
its opening remains permanently shut off from communication with the
exterior by the epidermic layer of the epiblast. Götte describes the
lens as formed by a solid thickening of the nervous layer in
Bombinator. This is probably a mistake.
The cornea. The mode of formation of the cornea already described
appears to be characteristic of most Vertebrata except the Ammocoete.
It has been found by Kessler in Aves, Reptilia and Amphibia, and
probably also occurs in Pisces. In Mammals it is not however so easy
to establish. There are at first no mesoblast cells between the lens
and the epiblast (fig. 295) but in many Mammals (_vide_ Kessler, No.
372, pp. 91-94) a layer of rounded mesoblast cells, which forms
Descemet's membrane, grows in between the two, at a time when it is
not easy to recognise a corneal lamina, as distinct from a simple
coagulum.
After the formation of this layer the mesoblast cells grow into the
corneal lamina from the sides, and becoming flattened arrange
themselves in rows between the laminæ of the cornea. The cornea
continues to increase in thickness by the addition of laminæ on the
side adjoining the epiblast.
We have already seen that in the Lamprey the cornea is nothing else
but the slightly modified and more transparent epidermis and dermis.
The optic nerve and the choroid fissure. It will be convenient to
consider together the above structures, and with them the vascular and
other processes which pass into the cavity of the optic cup through
the choroid fissure. These parts present on the whole a greater amount
of variation than any other parts of the eye.
I commence with the Fowl which is both a very convenient general type
for comparison, and also that in which these structures have been most
fully worked out.
During the third day of incubation there passes in through the choroid
slit a vascular loop, which no doubt supplies the transuded material
for the growth of the vitreous humour. Up to the fifth day this
vascular loop is the only structure passing through the choroid slit.
On this day however a new structure appears, which remains permanently
through life, and is known as the pecten. It consists of a lamellar
process of the mesoblast cells round the eye, passing through the
choroid slit near the optic nerve, and enveloping part of the afferent
branch of the vascular loop above mentioned. The proximal part of the
free edge of the pecten is somewhat swollen, and sections through this
part have a club-shaped form. On the sixth day the choroid slit
becomes rapidly closed, so that at the end of the sixth day it is
reduced to a mere seam. There are however two parts of this seam where
the edges of the optic cup have not coalesced. The proximal of these
adjoins the optic nerve, and permits the passage of the pecten and at
a later period of the optic nerve; and the second or distal one is
placed near the ciliary edge of the slit, and is traversed by the
efferent branch of the above-mentioned vascular loop. This vessel soon
atrophies, and with it the distal opening in the choroid slit
completely vanishes. In some varieties of domestic Fowl (Lieberkühn)
the opening however persists. The seam which marks the original site
of the choroid slit is at first conspicuous by the absence of pigment,
and at a later period by the deep colour of its pigment. Finally, a
little after the ninth day, no trace of it is to be seen.
[FIG. 293. SECTION THROUGH THE FRONT PART OF THE HEAD OF A
LEPIDOSTEUS EMBRYO ON THE SEVENTH DAY AFTER IMPREGNATION.
_al._ alimentary tract; _fb._ thalamencephalon; _l._ lens of eye;
_op.v._ optic vesicle. The mesoblast is not represented.]
Up to the eighth day the pecten remains as a simple lamina; by the
tenth or twelfth day it begins to be folded or rather puckered, and by
the seventeenth or eighteenth day it is richly pigmented and the
puckerings have become nearly as numerous as in the adult, there being
in all seventeen or eighteen. The pecten is almost entirely composed
of vascular coils, which are supported by a sparse pigmented
connective tissue; and in the adult the pecten is still extremely
vascular. The original artery which became enveloped at the formation
of the pecten continues, when the latter becomes vascular, to supply
it with blood. The vein is practically a fresh development after the
atrophy of the distal portion of the primitive vascular loop of the
vitreous humour.
There are no true retinal blood-vessels.
In the formation of the optic cup the extreme peripheral part of the
optic nerve, which is in immediate proximity with the artery of the
pecten, becomes folded. The permanent opening in the choroid fissure
for the pecten is intimately related to the entrance of the optic
nerve into the eyeball; the fibres of the optic nerve passing in at
the inner border of the pecten, coursing along its sides to its outer
border, and radiating from it as from a centre to all parts of the
retina.
In the Lizard the choroid slit closes considerably earlier than in the
Fowl. The vascular loop in the vitreous humour is however more
developed. The pecten long remains without vessels, and does not in
fact become at all vascular till after the very late disappearance of
the distal part of the vascular loop of the vitreous humour.
The arrangement of the ingrowth through the choroid slit in
Elasmobranchii (Scyllium) has been partially worked out, and so far as
is at present known the agreement between the Avian and Elasmobranch
type is fairly close.
At the time when the cavity between the lens and the secondary optic
cup is just commencing to be formed, a process of mesoblast
accompanied by a vascular loop passes into the vitreous humour,
through the choroid slit, close to the optic nerve. The vessel in this
process is no doubt equivalent to the vascular loop in the Avian eye,
but I have not made out that it projects beyond the mesoblastic
process accompanying it. As the cavity of the vitreous humour enlarges
and the choroid slit elongates, the process through it takes the form
of a lamina with a somewhat swollen border, and projects for some
distance into the cavity of the vitreous humour.
At a later stage, after the outer layer of the optic cup has become
pigmented, the distal part of the choroid slit adjoining the border of
the lens closes up; but along the line where it was present the walls
of the optic cup remain very thin and are thrown into three folds, two
lateral and one median, projecting into the cavity of the vitreous
humour. The median fold is in contact with the lens, and the vascular
mesoblast surrounding the eye projects into the space between the two
laminæ of which it is formed. In passing from the region of the lens
to that of the optic nerve the lateral folds of the optic cup
disappear, and the median fold forms a considerable projection into
the cavity of the vitreous humour. It consists of a core of mesoblast
covered by a delicate layer derived from both strata of the optic cup.
Still nearer the optic nerve the choroid slit is no longer closed, and
the mesoblast, which in the neighbourhood of the lens only extended
into the folds of the wall of the optic cup, now projects freely into
the cavity of the vitreous humour, and forms the lamina already
described. It is not very vascular, but close to the optic nerve there
passes into it a considerable artery.
In the young animal the choroid slit is no longer perforated by a
mesoblastic lamina. At its inner end it remains open to allow of the
passage of the optic nerve. The line of the slit can easily be traced
along the lower side of the retina; and close to the lens the retinal
wall continues, as in the embryo, to be raised into a projecting fold.
Traces of these structures are visible even in the fully grown
examples of Scyllium.
As has been pointed out by Bergmeister the mesoblastic lamina
projecting into the vitreous humour resembles the pecten at an early
stage of development, and is without doubt homologous with it. The
artery which supplies it is certainly equivalent to the artery of the
pecten.
There can be no doubt that the mesoblastic lamina projecting into the
vitreous humour is equivalent to the processus falciformis of
Teleostei, and it seems probable that the whole of it, including the
free part as well as that covered by epiblast, ought to be spoken of
under this title. The optic nerve in Elasmobranchii is not included in
the folding to which the secondary optic vesicle owes its origin, and
would seem to perforate the walls of the optic cup only at the distal
end of the processus falciformis.
[FIG. 294. HORIZONTAL SECTION THROUGH THE EYE OF A TELEOSTEAN
EMBRYO. (From Gegenbaur; after Schenk.)
_s._ choroid fissure, with two folds forming part of the processus
falciformis; _a._ choroid layer of optic cup; _b._ retinal layer of
optic cup; _c._ cavity of vitreous humour; _d._ lens.]
In Teleostei there is at first a vascular loop like that in Birds,
passing through the choroid fissure. This has been noticed by Kessler
in the Pike, and by Schenk in the Trout. At a later period a
mesoblastic ingrowth with a blood-vessel makes its way in many forms
into the cavity of the vitreous humour, accompanied by two folds in
the walls of the free edges of the choroid fissure (fig. 294). These
structures, which constitute the processus falciformis, clearly
resemble very closely the mesoblastic process and folds of the optic
cup in Elasmobranchii. The processus falciformis comes in contact
with, and perhaps becomes attached to the wall of the lens; and
persists through life.
In Triton there is no vascular ingrowth through the choroid fissure,
but a few mesoblastic cells pass in which represent the vascular
ingrowth of other types. The optic nerve perforates the proximal
extremity of the original choroid slit.
The absence of an embryonic blood-vessel does not however hold good
for all Amphibia, as there is present in the embryo Alytes
(Lieberkühn) an artery, which breaks up into a capillary system on the
retinal border of the vitreous humour.
In the Ammocoete the choroid slit is merely represented by a slight
notch on the ventral edge of the optic cup, and the mesoblastic
process which passes through the choroid slit in most types is
represented by a large cellular process, from which the vitreous
humour would appear to be derived.
Mammalia differ from all the types already described in the immense
foetal development of the blood-vessels of the vitreous humour. There
are however some points in connection with the development of these
vessels which are still uncertain. The most important of these points
concerns the presence of a prolongation of the mesoblast around the
eye into the cavity of the vitreous humour. It is maintained by
Lieberkühn, Arnold, Kölliker, etc., that in the invagination of the
lens a thin layer of mesoblast is carried before it; and is thus
transported into the cavity of the vitreous humour. This is denied by
Kessler, but the layer is so clearly figured by the above
embryologists, that the existence of it in some Mammalia (the Rabbit,
etc.) must I think be accepted.
In the folding in of the optic vesicle, which accompanies the
formation of the lens, the optic nerve becomes included, and on the
development of the cavity of the vitreous humour an artery, running in
the fold of the optic nerve, passes through the choroid slit into the
cavity of the vitreous humour (fig. 295, _acr_). The sides of the
optic nerve subsequently bend over, and completely envelope this
artery, which at a later period gives off branches to the retina, and
becomes known as the arteria centralis retinæ. It is homologous with
the arterial limb of the vascular loop projecting into the vitreous
humour in Birds, Lizards, Teleostei, etc.
[FIG. 295. SECTION THROUGH THE EYE OF A RABBIT EMBRYO OF ABOUT
TWELVE DAYS..
_c._ epithelium of cornea; _l._ lens; _mec._ mesoblast growing in
from the side to form the cornea; _rt._ retina; _a.c.r._ arteria
centralis retinæ; _of.n._ optic nerve.
The figure shews (1) the absence at this stage of mesoblast between
the lens and the epiblast: the interval between the two has however
been made too great; (2) the arteria centralis retinæ forming the
vascular capsule of the lens and continuous with vascular structures
round the edges of the optic cup.]
Before becoming enveloped in the optic nerve this artery is continued
through the vitreous humour (fig. 295), and when it comes in close
proximity to the lens it divides into a number of radiating branches,
which pass round the edge of the lens, and form a vascular sheath
which is prolonged so as to cover the anterior wall of the lens. In
front of the lens they anastomose with vessels, coming from the iris,
many of which are venous (fig. 295)--and the whole of the blood from
the arteria centralis is carried away by these veins. The vascular
sheath surrounding the lens receives the name of the membrana
capsulo-pupillaris. The posterior part of it appears (Kessler, No.
372) to be formed of vessels without the addition of any other
structures and is either formed simply by branches of the arteria
centralis, or out of the mesoblast cells involuted with the lens. The
anterior part of the vascular sheath is however inclosed in a very
delicate membrane, the membrana pupillaris, continuous at the sides
with the epithelium of Descemet's membrane. On the formation of the
iris this membrane lies superficially to it, and forms a kind of
continuation of the mesoblast of the iris over the front of the lens.
The origin of this membrane is much disputed. By Kessler, whose
statements have been in the main followed, it is believed to appear
comparatively late as an ingrowth of the stroma of the iris; while
Kölliker believes it to be derived from a mesoblastic ingrowth between
the front wall of the lens and the epiblast. According to Kölliker
this ingrowth subsequently becomes split into two laminæ, one of which
forms the cornea, and the other the anterior part of the vascular
sheath of the lens with its membrana pupillaris. Between the two
appears the aqueous humour.
The membrana capsulo-pupillaris is simply a provisional embryonic
structure, subserving the nutrition of the lens. The time of its
disappearance varies somewhat for the different Mammalia in which this
point has been investigated. In the human embryo it lasts from the
second to the seventh month and sometimes longer. As a rule it is
completely absorbed at the time of birth. The absorption of the
anterior part commences in the centre and proceeds outwards.
In addition to the vessels of the vascular capsule round the lens,
there arise from the arteria centralis retinæ, just after its exit
from the optic nerve, in many forms (Dog, Cat, Calf, Sheep, Rabbit,
Man) provisional vascular branches which extend themselves in the
posterior part of the vitreous humour. Near the ciliary end of the
vitreous humour they anastomose with the vessels of the membrana
capsulo-pupillaris.
In Mammals the choroid slit closes very early, and is not perforated
by any structure homologous with the pecten. The only part of the slit
which remains open is that perforated by the optic nerve; and in the
centre of the latter is situated the arteria centralis retinæ as
explained above. From this artery there grow out the vessels to supply
the retina, which have however nothing to do with the provisional
vessels of the vitreous humour just described (Kessler). On the
atrophy of the provisional vessels the whole of the blood of the
arteria centralis passes into the retina.
It is interesting to notice (Kessler, No. 372, p. 78) that there seems
to be a blood-vessel supplying the vitreous humour in the embryos of
nearly all vertebrate types, which is homologous throughout the
Vertebrata. This vessel often exhibits a persisting and a provisional
part. The latter in Mammalia is the membrana capsulo-pupillaris and
other vessels of the vitreous humour; in Birds and Lizards it is the
part of the original vascular loop, not included in the pecten, and in
Osseous Fishes that part (?) not involved in the processus
falciformis. The permanent part is formed by the retinal vessels of
Mammalia, by the vessels of the pecten in Birds and Lizards, and by
those of the processus falciformis in Fishes.
The Iris and Ciliary processes. The walls of the edge of the optic cup
become very much thinner than those of the true retinal part. In many
Vertebrates (Mammalia, Aves, Reptilia, Elasmobranchii, etc.) the
thinner part, together with the mesoblast covering it, becomes divided
into two regions, viz. that of the iris, and that of the ciliary
processes. In the Newt and Lamprey this differentiation does not take
place, but the part in question simply becomes the iris.
_Accessory Organs connected with the Eye._
Eyelids. The most important accessory structures connected with the
eye are the eyelids. They are developed as simple folds of the
integument with a mesoblastic prolongation between their two laminæ.
They may be three in number, viz. an upper and lower, and a lateral
one--the nictitating membrane--springing from the inner or anterior
border of the eye. Their inner face is lined by a prolongation of
conjunctiva, which is the modified epiblast covering the cornea and
part of the sclerotic.
In Teleostei and Ganoidei eyelids are either not present or at most
very rudimentary. In Elasmobranchii they are better developed, and the
nictitating membrane is frequently present. The latter is also usually
found in Amphibia. In the Sauropsida all three eyelids are usually
present, but in Mammalia the nictitating membrane is rudimentary.
In many Mammalia the two eyelids meet together during a period of
embryonic life, and unite in front of the eye. A similar arrangement
is permanent through life in Ophidia and some Lacertilia; and there is
a chamber formed between the coalesced eyelids and the surface of the
cornea, into which the lacrymal ducts open.
Lacrymal glands. Lacrymal glands are found in the Sauropsida and
Mammalia. They arise (Remak, Kölliker) as solid ingrowths of the
conjunctival epithelium. They appear in the chick on the eighth day.
Lacrymal duct. The lacrymal duct first appears in Amphibia, and is
present in all the higher Vertebrates. Its mode of development in the
Amphibia, Lacertilia and Aves has recently been very thoroughly worked
out by Born (Nos. 380 and 381).
In Amphibia he finds that the lacrymal duct arises as a solid ridge of
the mucous layer of the epidermis, continued from the external opening
of the nasal cavity backwards towards the eye. It usually appears at
about the time when the nasal capsule is beginning to be chondrified.
As this ridge is gradually prolonged backwards towards the eye its
anterior end becomes separated from the epidermis, and grows inwards
in the mesoblast to become continuous with the posterior part of the
nasal sack. The posterior end which joins the eye becomes divided into
the two collecting branches of the adult. Finally the whole structure
becomes separated from the skin except at the external opening, and
develops a lumen.
In Lacertilia the lacrymal duct arises very much in the same manner as
in Amphibia, though its subsequent growth is somewhat different. It
appears as an internal ridge of the epithelium, at the junction of the
superior maxillary process and the fold which gives rise to the lower
eyelid. A solid process of this ridge makes its way through the
mesoblast on the upper border of the maxillary process till it meets
the wall of the nasal cavity, with the epithelium of which it becomes
continuous. At a subsequent stage a second solid growth from the upper
part of the epithelial ridge makes its way through the lower eyelid,
and unites with the inner epithelium of the eyelid; and at a still
later date a third growth from the lower part of the structure forms a
second junction with the epithelium of the eyelid. The two latter
outgrowths form the two upper branches of the duct. The ridge now
loses its connection with the external skin, and, becoming hollow,
forms the lacrymal duct. It opens at two points on the inner surface
of the eyelid, and terminates at its opposite extremity by opening
into the nasal cavity. It is remarkable, as pointed out by Born, that
the original epithelial ridge gives rise directly to a comparatively
small part of the whole duct.
In the Fowl the lacrymal duct is formed as a solid ridge of the
epidermis, extending along the line of the so-called lacrymal groove
from the eye to the nasal pit (fig. 120). At the end of the sixth day
it begins to be separated from the epidermis, remaining however united
with it on the inner side of the lower eyelid. After its separation
from the epidermis it forms a solid cord, the lower end of which
unites with the wall of the nasal cavity. The cord so formed gives
rise to the whole of the duct proper and to the lower branch of the
collecting tube. The upper branch of the collecting tube is formed as
an outgrowth from this cord. A lumen begins to be formed on the
twelfth day of incubation, and first appears at the nasal end. It
arises by the formation of a space between the cells of the cord, and
not by an absorption of the central cells.
In Mammalia Kölliker states that he has been unable to observe
anything similar to that described by Born in the Sauropsida and
Amphibia, and holds to the old view, originally put forward by Coste,
that the duct is formed by the closure of a groove leading from the
eye to the nose between the outer nasal process and the superior
maxillary process. The upper extremity of the duct dilates to form a
sack, from which two branches pass off to open on the lacrymal
papillæ. In view of Born's discoveries Kölliker's statements must be
received with some caution.
_The Eye of the Tunicata._
The unpaired eye of the larva of simple Ascidians is situated somewhat
to the right side of the posterior part of the dorsal wall of the
anterior cephalic vesicle (fig. 296, _O_). It consists of a refractive
portion, turned towards the cavity of the vesicle of the brain, and a
retinal portion forming part of the wall of the brain. The refractive
parts consist of a convex-concave meniscus in front, and a spherical
lens behind, adjoining the concave side of the meniscus. The posterior
part of this lens is imbedded in a layer of pigment. The retina is
formed of columnar cells, with their inner ends imbedded in the
pigment which encloses the posterior part of the lens. The retinal
part of the eye arises in the first instance as a prominence of the
wall of the cerebral vesicle: its cells become very columnar and
pigmented at their inner extremities (fig. 8, _V_, _a_). The lens is
developed at a later period, after the larva has become hatched, but
the mode of its formation has not been made out.
[FIG. 296. LARVA OF ASCIDIA MENTULA. (From Gegenbaur; after
Kupffer.) Only the anterior part of the tail is represented.
_N´._ anterior swelling of neural tube; _N._ anterior swelling of
spinal portion of neural tube; _n._ hinder part of neural tube;
_ch._ notochord; _K._ branchial region of alimentary tract; _d._
oesophageal and gastric region of alimentary tract; _O._ eye; _a._
otolith; _o._ mouth; _s._ papilla for attachment.]
_General considerations on the Eye of the Chordata._
There can be but little doubt that the eye of the Tunicata belongs to
the same phylum as that of the true Vertebrata, different as the two
eyes are. The same may also be said with reference to the degenerate
and very rudimentary eye of Amphioxus.
The peculiarity of the eye of all the Chordata consists in the retina
being developed from part of the wall of the brain. How is this
remarkable feature of the eye of the Chordata to be explained?
Lankester, interpreting the eye in the light of the Tunicata, has made
the interesting suggestion[192] "that the original Vertebrate must
have been a transparent animal, and had an eye or pair of eyes inside
the brain, like that of the Ascidian Tadpole."
[192] _Degeneration_, London, 1880, p. 49.
This explanation may possibly be correct, but another explanation
appears to me possible, and I am inclined to think that the vertebrate
eyes have not been derived from eyes like those of Ascidians, but that
the latter is a degenerate form of vertebrate eye.
The fact of the retina being derived from the fore-brain may perhaps
be explained in the same way as has already been attempted in the case
of the retina of the Crustacea; _i.e._ by supposing that the eye was
evolved simultaneously with the fore part of the brain.
The peculiar processes which occur in the formation of the optic
vesicle are more difficult to elucidate; and I can only suggest that
the development of a primary optic vesicle, and its conversion into an
optic cup, is due to the retinal part of the eye having been involved
in the infolding which gave rise to the canal of the central nervous
system. The position of the rods and cones on the posterior side of
the retina is satisfactorily explained by this hypothesis, because, as
may be easily seen from figure 285, the posterior face of the retina
is the original external surface of the epidermis, which is infolded
in the formation of the brain; so that the rods and cones are, as
might be anticipated, situated on what is morphologically the external
surface of the epiblast of the retina.
The difficulty of this view arises in attempting to make out how the
eye can have continued to be employed during the gradual change of
position which the retina must have undergone in being infolded with
the brain in the manner suggested. If however the successive steps in
this infolding were sufficiently small, it seems to me not impossible
that the eye might have continued to be used throughout the whole
period of change, and a transparency of the tissues, such as Lankester
suggests, may have assisted in rendering this possible.
The difficulty of the eye continuing to be in use when undergoing
striking changes in form is also involved in Lankester's view, in that
if, as I suppose, he starts from the eye of the Ascidian Tadpole with
its lenses turned _towards_ the cavity of the brain; it is necessary
for him to admit that a fresh lens and other optical parts of the eye
became developed on the _opposite side_ of the eye to the original
lens; and it is difficult to understand such a change, unless we can
believe that the refractive media on the two sides were in operation
simultaneously. It may be noted that the same difficulty is involved
in supposing, as I have done, that the eye of the Ascidian Tadpole was
developed from that of a Vertebrate. I should however be inclined to
suggest that the eye had in this case ceased for a period to be
employed; and that it has been re-developed again in some of the
larval forms. Its characters in the Tunicata are by no means constant.
_Accessory eyes in the Vertebrata._
In addition to the paired eyes of the Vertebrata certain organs are
found in the skin of a few Teleostei living in very deep water, which,
though clearly not organs of true vision, yet present characters which
indicate that they may be used in the perception of light. The most
important of such organs are those found in Chauliodus, Stomias, etc.,
the significance of which was first pointed out by Leuckart, while the
details of their structure have been recently worked out by
Leydig[193] and Ussow. They are distributed not only in the skin, but
are also present in the mouth and respiratory cavity, a fact which
appears to indicate that their main function must be something else
than the perception of light. It has been suggested that they have the
function of producing phosphorescence.
Another organ, probably of the same nature, is found on the head of
Scopelus.
The organs in Chauliodus are spherical or nearly spherical bodies
invested in a special tunic. The larger of them, which alone can have
any relation to vision, are covered with pigment except on their outer
surface. The interior is filled with two masses, named by Leuckart the
lens and vitreous humour. According to Leydig each of them is cellular
and receives a nerve, the ultimate destination of which has not
however been made out. According to Ussow the anterior mass is
structureless, but serves to support a lens, placed in the centre of
the eye, and formed of a series of crystalline cones prolonged into
fibres, which in the posterior part of the eye diverge and terminate
by uniting with the processes of multipolar cells, placed near the
pigmented sheath. These cells, together with the fibres of the
crystalline cones which pass to them, are held by Ussow to constitute
a retina.
_Eye of the Mollusca._
(362) N. Bobretzky. "Observations on the development of the
Cephalopoda" (Russian). _Nachrichten d. kaiserlichen Gesell. d.
Freunde der Naturwiss. Anthropolog. Ethnogr. bei d. Universität
Moskau._
(363) H. Grenacher. "Zur Entwicklungsgeschichte d. Cephalopoden."
_Zeit. f. wiss. Zool._, Bd. XXIV. 1874.
(364) V. Hensen. "Ueber d. Auge einiger Cephalopoden." _Zeit. f. wiss.
Zool._, Vol. XV. 1865.
(365) E. R. Lankester. "Observations on the development of the
Cephalopoda." _Quart. J. of Micr. Science_, Vol. XV. 1875.
(366) C. Semper. _Ueber Sehorgane von Typus d. Wirbelthieraugen._
Wiesbaden, 1877.
_Eye of the Arthropoda._
(367) N. Bobretzky. _Development of Astacus and Palaemon._ Kiew, 1873.
(368) A. Dohrn. "Untersuchungen üb. Bau u. Entwicklung d. Arthropoden.
Palinurus und Scyllarus." _Zeit. f. wiss. Zool._, Bd. XX. 1870, p. 264
et seq.
(369) E. Claparède. "Morphologie d. zusammengesetzten Auges bei den
Arthropoden." _Zeit. f. wiss. Zool._, Bd. X. 1860.
(370) H. Grenacher. _Untersuchungen üb. d. Sehorgane d. Arthropoden._
Göttingen, 1879.
_Vertebrate Eye._
(371) J. Arnold. _Beiträge zur Entwicklungsgeschichte des Auges._
Heidelberg, 1874.
(372) Babuchin. "Beiträge zur Entwicklungsgeschichte des Auges."
_Würzburger naturwissenschaftliche Zeitschrift_, Bd. 8.
(373) L. Kessler. _Zur Entwicklung d. Auges d. Wirbelthiere._ Leipzig,
1877.
(374) N. Lieberkühn. _Ueber das Auge des Wirbelthierembryo._ Cassel,
1872.
(375) N. Lieberkühn. "Beiträge z. Anat. d. embryonalen Auges." _Archiv
f. Anat. und Phys._, 1879.
(376) L. Löwe. "Beiträge zur Anatomie des Auges" and "Die Histogenese
der Retina." _Archiv f. mikr. Anat._, Vol. XV. 1878.
(377) V. Mihalkowics. "Untersuchungen über den Kamm des Vogelauges."
_Archiv f. mikr. Anat._, Vol. IX. 1873.
(378) W. Müller. "Ueber die Stammesentwickelung des Schorgans der
Wirbelthiere." _Festgabe Carl Ludwig._ Leipzig, 1874.
(379) S. L. Schenk. "Zur Entwickelungsgeschichte des Auges der
Fische." _Wiener Sitzungsberichte_, Bd. LV. 1867.
_Accessory organs of the Vertebrate Eye._
(380) G. Born. "Die Nasenhöhlen u. d. Thränennasengang d. Amphibien."
_Morphologisches Jahrbuch_, Bd. II. 1876.
(381) G. Born. "Die Nasenhöhlen u. d. Thränennasengang d. amnioten
Wirbelthiere. I. Lacertilia. II. Aves." _Morphologisches Jahrbuch_,
Bd. V. 1879.
_Eye of the Tunicata._
(382) A. Kowalevsky. "Weitere Studien üb. d. Entwicklung d. einfachen
Ascidien." _Archiv f. mikr. Anat._, Vol. VII. 1871.
(383) C. Kupffer. "Zur Entwicklung d. einfachen Ascidien." _Archiv f.
mikr. Anat._, Vol. VII. 1872.
[193] F. Leydig. "Ueber Nebenaugen d. Chauliodus Sloani."
_Archiv f. Anat. und Phys._, 1879. M. Ussow. "Ueb. d. Bau d.
augenähnlichen Flicken einiger Knochenfische." _Bul. d. la Soc.
d. Naturalistes de Moscou_, Vol. LIV. 1879.
_Vide_ for general description and further literature,
Günther, _The Study of Fishes_, Edinburgh, 1880.