The Works of Francis Balfour 3-16

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Foster M. and Sedgwick A. The Works of Francis Balfour Vol. III. A Treatise on Comparative Embryology 2 (1885) MacMillan and Co., London.

Cephalochorda | Urochorda | Elasmobranchii | Teleostei | Cyclostomata | Ganoidei | Amphibia | Aves | Reptilia | Mammalia | Comparison of the Formation of Germinal Layers and Early Stages in Vertebrate Development | Ancestral form of the Chordata | General Conclusions | Epidermis and Derivatives | The Nervous System | Organs of Vision | Auditory, Olfactory, and Lateral Line Sense Organs | Notochord, Vertebral Column, Ribs, and Sternum | The Skull | Pectoral and Pelvic Girdles and Limb Skeleton | Body Cavity, Vascular System and Glands | The Muscular System | Excretory Organs | Generative Organs and Genital Ducts | The Alimentary Canal and Appendages in Chordata

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This historic 1885 book edited by Foster and Sedgwick is the third of Francis Balfour's collected works published in four editions. Francis (Frank) Maitland Balfour, known as F. M. Balfour, (November 10, 1851 - July 19, 1882) was a British biologist who co-authored embryology textbooks. Foster M. and Sedgwick A. The Works of Francis Balfour Vol. I. Separate Memoirs (1885) MacMillan and Co., London. Foster M. and Sedgwick A. The Works of Francis Balfour Vol. II. A Treatise on Comparative Embryology 1. (1885) MacMillan and Co., London. Foster M. and Sedgwick A. The Works of Francis Balfour Vol. III. A Treatise on Comparative Embryology 2 (1885) MacMillan and Co., London. Foster M. and Sedgwick A. The Works of Francis Balfour Vol. IV. Plates (1885) MacMillan and Co., London. Modern Notes:

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Vol. III. A Treatise on Comparative Embryology 2 (1885)

ChapterXVI. 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 1 . In "Lizzia. the eye is placed at the base of a tentacle and consists of (fig. 276) a lens (/) 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: (i) 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.

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.

(From Lankester; after Hertwig.)

/. lens; oc. perceptive part of eye.

1 O. and R. Hertwig. Das Nei~uen system #. Sinnesorgane d. Medtisen. 1878.

Leipzig,

Mollusca. In a large number of the odontophorous Mollusca eyes, innervated by the supracesophageal 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 (7?); and are continuous with the fibres of the optic nerve (N.op). We have no knowledge of the development of this eye.

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 supra-cesophageal 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.

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*. different parts of the integument; /. lens; I 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.

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 l 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.

In the anterior optic chamber there are placed (i) the anterior part of the lens (7 1 ) and (2) the folds of the iris (Ir). The whole chamber, except the part formed by the lens, is lined by the epidermis (InP and Infi}. 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 lamellae of a structureless material. It is formed of a small outer (7 1 ) and large inner

1 Vide Hensen, Zeit. f. wiss. Zool. Bd. XV.

(/) 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. J

(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.

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. 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.

FlG. 278. TWO SECTIONS THROUGH THE DEVELOPING EYE OF A CEI'HALUl'OD

TO SHEW THE FORMATION OF THE OPTIC CUP. (After Lankester.)

FIG. 279. TRANSVERSE SECTION THROUGH THE HEAD OF AN ADVANCED EMBRYO OF LoLlGO. (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.

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 1 : "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."

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, /if).

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 pocketlike 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."

1 "Devel. of Cephalopoda." Q. J. Micro. Scien. 1875, p. 44.

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.

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 rf}. The former is composed of two strata, (i) 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.

FIG. 280. SECTIONS THROUGH THE DEVELOPING EYE OF LOLIGO

AT TWO STAGES. (After Bobretzky.)

///. 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; of. and if. fold of iris; rt. retina; rt". inner layer of retina; st. rods ; aq. cartilage of the choroid.

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 part passu with the supra-cesophageal ganglia or independently at a later period. On purely a 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 1 , 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 i 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.

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 papillce.

1 Ueber Sehorgane von Typus d. Wirbdthieraugen, etc., Wiesbaden, 1877, anf l Archiv f. mikr. Anat. Vol. xiv. pp. 118 122.

The eyes of Pecten and Spondylus 1 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.

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.

Chaetopoda. Amongst the Chaetopoda 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 (/), 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 senseorgan found by Eisig in the allied Capitellidas.

Chaetognatha 2 . 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.

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) 1 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 :

1 Vide Hensen (No. 364) and S. J. Hickson, "The Eye of Pecten," Quart. J. of Micr. Science, Vol. xx. 1 880.

2 O. Hertwig. " Die Chaetognathen." Jenaischc Zcitschrift, Vol. XTV. 1880.

FIG. 281. EYE OF AN ALCIOPID (NEOPHANTA CELOX). (From Gegenbaur; after Greef.)

i. cuticle; c. continuation of cuticle in front of eye; /. lens; h. vitreous humour; o. optic nerve; o. expansion of the optic nerve; b. layer of rods; /. pigment layer.

(i) 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 larvae of some Diptera (Eristalis), and also in some Chilognatha.

1 The eye of Peripatus is similar neither to the eye of the Arthropoda, nor to that of the Choetopoda, 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.

(b] A type of simple eye found in some Chilopoda, and in some Insect larvae (Dytiscus, etc.) (fig. 282), the parts of which are entirely derived from the epidermis. There is present a lens (/) 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 nervefibres.

(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.

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 retinulae (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.

FIG. 282. SECTION THROUGH THE SIMPLE EYE OF A YOUNG DYTISCUS LARVA. (From Gegenbaur ; after Grenadier.)

/. corneal lens ; g. vitreous hu mour ; r. retina ; o. optic nerve ; h. hypodermis.

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 aconouseyes, while eyes with incompletely formed crystalline cones are called by him pseudoconouseyes.

The rhabdom of the retinulae 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 retinulas is permanently retained in the eyes of the Tipulidae.

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 Palaemon 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, (i) the external epidermis of the procephalic lobes which will be spoken of as the epidermic layer of the eye, (2) a portion of the supracesophageal 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 retinulae with their rhabdoms, and to the optic ganglion.

After the separation of the supra-cesophageal 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 supra- oesophageal 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.

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. retinulae with rhabdoms ; n. optic nerve ; g. ganglionic swelling of optic nerve.

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 1 . These cells give rise in a later stage (not worked out in Astacus) to the cuticular corneal lenses.

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 retinuke 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 retinulas.

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.

1 There would appear to be some confusion as to the nomenclature of these parts in Bobretzky's account,

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, ff).

After the establishment of the optic nerves, there take place (i) 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 (), which remains thin, pigment is eventually deposited, and it ultimately becomes the tesselated pigmentlayer of the choroid.

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.

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, /). 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 / represents the cavity of the stalk leading away from the almost obliterated cavity of the primary vesicle.

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 /, now completely detached from the superficial epiblast xh.

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.

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. 1 18, 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).

FIG. 286. DIAGRAMMATIC SECTION OF THE EYE AND THE OPTIC NERVE AT AN EARLY STAGE. (From Lieberkiihn.)

To shew the lens / 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.

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.

With reference to the above description, taken with very slight alterations from the Elements of Embryology, Pt. I., 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.

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.

/. the lens ; /'. 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.

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 eye-ball 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 linejjy, 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 /, 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/, 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 zs, 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.

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.C/i). At about the 8oth 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.

FIG. 289. SECTION OF THE EYE OF CHICK AT THE FOURTH DAY.

e.p. superficial epiblast of the side of the head ; /?. 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. /. the lens. The hind wall, the nuclei of whose elongated cells are shewn at /, 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 d with the tissue a, which appears to be the rudiment of the capsule of the lens and suspensory ligament.

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 (i) an outer part, corresponding morphologically to the epithelial lining of the cerebro-spinal 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 Lowe, 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 (Kolliker, 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 (Kolliker, Babuchin), or cuticularizations (Schultze, W. Miiller) 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.

Lowe'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 Lowe'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. Lieberkiihn, Arnold, Lowe, and other authors regard it however as a mesoblastic product ; and Kolliker 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 Kolliker) 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 / 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 Kolliker, appears to be formed as a cuticular membrane deposited by the epithelial cells of the lens.

The views of Lieberkiihn, Arnold, Lowe and others, according to which the lens-capsule is a mesoblastic structure, do not appear to be well founded. The contrary view, held by Kolliker, 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. Miiller has actually found cellular elements in what he believes to be the lens-capsule of the Ammoccete lens. Considering the degraded character of the Ammoccete 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 capsulopupillaris. In the Ammoccete 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 Lieberkiihn (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. Kolliker confirms Lieberkiihn'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 Lowe 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.

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 ringshaped 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 1 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.

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.

1 It appears to me possible that Lieberkiihn 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 Kolliker is right in regarding it as derived from the mesoblast. The grounds for this view are, (i) 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 Lieberkiihn. 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 Lieberkiihn (No. 374) and Arnold (No. 370), and are not entirely accepted by Kolliker. 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 Ammoccete. The development of such aborted eyes has as yet been studied only in the

THE AMMOCCETE EYE.

Ammocoete 1 , in which it resembles in most important features that of other Vertebrata.

Eye of Ammoccetes. 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, /). 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. Muller, 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 Ammoccete is fully developed, the secondary optic cup forms a deep pit (fig. 292, r) ; in the mouth of which is placed the lens (/). 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 (i) 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.

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 ; /. lens of eye ; h.c. head cavity.

The most detailed account is that of W. Muller (No. 377).

In a still later stage a distinct mesoblastic element for the iris is formed. When the Ammoccete 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.

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, /). 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. Gotte describes the lens as formed by a solid thickening of the nervous layer in Bombinator. This is probably a mistake.

FIG. 292. EYE OF AN AMMOCCETES LYING BENEATH THE SKIN.

ep. epidermis; d.c. dermal connective tissue continuous with the sub-dermal 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 ; /.lens; v.h. vitreous humour ; r. retina; rp. retinal pigment.

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 laminae of the cornea. The cornea continues to increase in thickness by the addition of laminae 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 pec ten. 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 (Lieberkiihn) 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.

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

FIG. 293. SECTION THROUGH THE FRONT PART OF THE HEAD OF A LEPIDOS TEUS EMBRYO ON THE SEVENTH DAY AFTER IMPREGNATION. al. alimentary tract ; fb. thalamencephalon ; /. lens of eye ; op.v. optic vesicle. The mesoblast is not represented.

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 laminae 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.

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 (Lieberkiihn) an artery, which breaks up into a capillary system on the retinal border of the vitreous humour.

In the Ammoccete 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 fcetal 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 Lieberkiihn, Arnold, Kolliker, 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 retinas. It is homologous with the arterial limb of the vascular loop projecting into the vitreous humour in Birds, Lizards, Teleostei, etc.

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.

Before becoming enveloped in the optic nerve this artery is continued through the vitreous humour (fig. 295), and when it comes in close proximity

FIG. 295. SECTION THROUGH THE EYE OF A RABBIT EMBRYO OF ABOUT TWELVE DAYS.

c. epithelium of cornea ; /. lens ; mec. mesoblast growing in from the side to form the cornea: rt. retina ; a.c.r. arteria centralis retinae; of.n. optic nerve.

The figure shews (i) 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 retinae forming the vascular capsule of the lens and continuous with vascular structures round the edges of the optic cup.

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 Kolliker believes it to be derived from a mesoblastic ingrowth between the front wall of the lens and the epiblast. According to Kolliker this ingrowth subsequently becomes split into two laminae, 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 retinas, 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 retinas 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 laminas. 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, Kdlliker) 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 Kolliker 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 papillae. In view of Born's discoveries Kolliker's statements must be received with some caution.

The Eye of tJte 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 im

FIG. 296. LARVA OF ASCIDIA MENTULA. (From Gegenbaur ; after Kupffer. ) Only the anterior part of the tail is represented.

IV'. 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; 0. eye; a. otolith ; o. mouth ; s. papilla for attachment.

bedded 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.

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 1 "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."

1 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 1 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.

1 F. Leydig. "Ueber Nebenaugen d. Chauliodus Sloani." Archiv f. Anal, und Phys., 1879. M. Ussow. " Ueb. d. Bau d. augenahnlichen Flicken einiger Knochenfische." Bui. d. la Soc. d. Naturalistes de Moscon, Vol. i.iv. 1879. Vide for general description and further literature, Giinther, The Study of Fish>-s t Edinburgh, 1880.

Eye of the Mollusca.

(362) N. Bobretzky. " Observations on the development of the Cephalopoda " (Russian). Nachrichten d. kaiserlichen Gesell.d. Freundcder Natunviss. Anthropolog. Ethnogr. bei d. Universitiit 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, y. of Micr. Science, Vol. xv. 1875.

(366) C. Semper. Ueber Sehorgane von Typus d. Wirbelthieratigen. Wiesbaden, 1877 Eye of the Arthropoda.

(367) N. Bobretzky. Development of Astacus and Palaemon. Kiew, 1873.

(368) A. Dohrn. " Untersuchungen lib. Bau u. Entwicklung d. Arthropoden. Palinurus nnd Scyllarus. " Zeit. f. wiss. Zool., Bd. xx. 1870, p. 264 et seq.

(369) E. Claparede. " Morphologic d. zusammengesetzten Auges bei den Arthropoden." Zeit. f. wiss. Zool., Bd. x. 1860.

(370) H. Grenacher. Untersuchungen iib. d. Sehorgane d. Arthropoden. Gottingen, 1879.

Vertebrate Eye.

(371) J.Arnold. Beitrage zur Entwicklungsgeschichte des Auges. Heidelberg, 1874.

(372) Babuchin. "Beitrage zur Entwicklungsgeschichte des Auges." Wilrzburger natiinuissenschaftliche Zeitschrift, Bd. 8.

(373) L. Kessler. Zur Entwicklung d. Attges d. Wirbelthiere. Leipzig, 1877.

(374) N. Lieberkiihn. Ueber das Auge des Wirbelthierembryo. Cassel, 1872.

(375) N. Lieberkiihn. "Beitrage z. Anat. d. embryonalen Auges." Archiv f. Anat. imd Phys., 1879.

(376) L. Lowe. "Beitrage zur Anatomic des Auges" and "Die Histogenese der Retina." Archiv f. mikr. Anat., Vol. xv. 1878.

(377) V. Mihalkowics. " Untersuchungen iiber den Kamm des Vogelauges." Archiv f. mikr. Anat., Vol. ix. 1873.

(378) W. Miiller. " Ueber die Stammesentwickelung des Sehorgans der Wirbelthiere." Festgabe Carl Ltidwig. 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 Nasenhohlen u. d. Thranennasengang d. Amphibien. Morphologisches Jahrbuch, Bd. II. 1876.

(381) G. Born. " Die Nasenhohlen u. d. Thranennasengang d. amnioten Wirbelthiere. I. Lacertilia. II. Aves." Morphologisches Jahrbuch, Bd. v. 1879.

Eye of the Tunicata.

(382) A. Kowalevsky. "Weitere Studien lib. d. Entwicklung d. einfachen Ascidien." Archiv f. mikr. Anat., Vol. vil. 1871.

(383) C. Kupffer. "Zur Entwicklung d. einfachen Ascidien." Archiv f. mikr. Anat., Vol. vii. 1872.