afterwards disappear. The thin anterior layer remains throughout life as a simple layer of cubical cells, and forms the so-called lens-epithelium; but the cells of the posterior layer grow forwards into the cavity of the lens-vesicle as the lens-fibres : those in the middle being the longest and straight, while the rest are slightly curved with their concavity towards the equator, and become gradually shorter towards the circumference, where they pass through gradually shortening columnar cells (transitional zone) into continuity with the anterior epithelium. By the growth of these fibres the cavity of the lens-vesicle becomes obliterated.

In this manner the central part of the lens is formed, and it consists in the main of fibres which pass in an antero-posterior direction. The remainder of the lens is formed of fibres which are so disposed as to curve round its margin and over the ends of the first formed fibres; they are, moreover, deposited in successive layers and in three (or more) separate sections, so that their ends abut against one another in front and behind along tri-radiate (or multi-radiate) lines, such as may be seen in the macerated lens. These later deposited fibres are all formed at the equator (at the transitional zone), where chiefly cell-multiplication takes place, and they grow hence meridionally backwards over the ends of the already developed antero-posteriorly disposed fibres of the central part of the lens.

The capsule of the lens is early visible as a thin homogeneous membrane, the origin of which is still undetermined. According to some observers (Lieberkühn, Arnold, Löwe) it is derived from a thin layer of mesoblast, which passes in between the lens and the optic cup; according to others (Kölliker, Kessler, Balfour), it appears before any mesoblast has passed in, and they therefore regard it as a cuticular deposit from the lens cells. In the human embryo, His figures mesoblast as existing from the first between the lens invagination and the optic cup (v. fig. 97).

In connection with this question it must be remembered that the substantia propria of the cornea (see below), which is formed of connective tissue, and is therefore mesoblastic in nature, also at first makes its appearance as a homogeneous deposit before any mesoblast cells have passed in behind the corneal epithelium. Its chemical nature, and its continuity at the equator with the suspensory ligament and hyaloid membrane, certainly point to the lens capsule as being a connective tissue, i.e. a mesoblastic structure.

Although the fœtal lens like that of the adult is itself non-vascular, it is nevertheless externally freely supplied with blood-capillaries, which form a vascular tunic completely surrounding it outside the capsule. These capillaries are supplied by a branch of the arteria centralis retina which passes forwards through the centre of the vitreous humour; in front, at the margin of the pupil, they come into continuity with the vessels of the iris. The most anterior part of this vascular tunic forms a membrane which closes the aperture of the pupil in the middle periods of fœtal life. In the human eye the whole tunic, together with the artery which supplies its vessels, becomes atrophied and is lost sight of before birth, but in some animals the pupillary membrane remains apparent for a few days after birth.

The vitreous humour appears to be formed from the mesoblastic tissue which has passed in between the lens and the inner layer of the optic cup by a gradual formation of a large quantity of ground-substance, whilst the cells of the tissue almost entirely disappear. The development of the hyaloid membrane has not been fully traced out, and the same may be said with regard to the zonule of Zinn. They are probably both formed by part of the same mesoblast as forms the vitreous humour (Lieberkühn, Angelucci).

The corneo-sclerotic coat, the choroid coat, and the iris are all derived from the mesoblast surrounding the optic cup.

The corneal epithelium is a portion of the external epiblast, which originally rests against the front of the lens rudiment. The substantia propria corneæ first appears in the chick as a thin homogeneous layer lying immediately within

1 Kessler, however, looks upon this homogeneous deposit as being also a cuticular deposit formed by the epithelial cells.

this epithelium. Into this homogeneous layer mesoblast cells pass from the margin, greatly thickening it and producing eventually the regular layers of fibrous tissue, which are characteristic of the cornea. No cells pass into the most anterior or into the most posterior stratum, which remain homogeneous (anterior and posterior

Fig. 103. HORIZONTAL SECTION

THROUGH THE EYE OF AN EMBRYO

RABBIT OF 18 DAYS. 30. (Kölliker.)

o, optic nerve; p, hexagonal pigment layer; 7, retina; re, ciliary part of the retina; p', forepart of the optic cup (rudiment of the iris pigment); g, vitreous, shrunk away from the retina, except where the vessels from the arteria centralis retinæ enter it; i, iris; mp, membrana pupillaris ; c, cornea with epithelium e; pp, pa, palpebræ; 7, lens; ', lens epithelium; f, sclerotic; m, recti muscles.

homogeneous lamella of Bowman). The epithelium of the posterior homogeneous lamella, or membrane of Descemet, is derived from mesoblast cells which grow in like the corneal corpuscles from the margin and spread themselves over the posterior surface of the cornea, thus separating this from the iris and anterior surface of the lens. For a

long while, however, there is no anterior chamber; this eventually appears as a (left-like space between the cornea and the structures immediately behind it.

In mammals, all the above stages of formation have not been described. A complete layer of mesoblast is early visible lying between the corneal epiblast and the lens epiblast, and continuous around the margin of the lens with the mesoblast of the vitreous chamber. In this mesoblast a cleft makes its appearance, separating it into two parts, one of which adheres to the corneal epiblast, where it forms the substance of the cornea, the other to the lens capsule forming the pupillary membrane. This cleft is the rudiment of the anterior chamber. It does not become actually distended with fluid until a short time before birth (Kölliker).

The sclerotic is formed entirely from mesoblast around the optic cup, probably continuous with that which forms the cornea, although it is only later that the cornea and sclerotic come to be completely amalgamated.

The choroid coat is formed from the mesoblast which is immediately in contact with the outer layer of the optic cup, and the forward growth of the middle tunic closely follows that of the margin of the cup. The latter ceases at first at the margin of the lens, but subsequently grows forwards over the front of the lens as a thin double layer, which is closely covered externally with a continuation of the choroidal mesoblast. This is the iris, over the back of which both the layers of the cup-margin eventually acquire pigment and remain permanently as the uvea. The ciliary body is formed by a kind of hypertrophy of the optic cup, which developes radial folds, enclosing thin portions of mesoblastic choroidal tissue, in which, as in the rest of the choroid, numerous blood-vessels and branched pigment-cells become formed.

Accessory structures.-The eyelids make their appearance gradually as folds of integument, subsequently to the formation of the eyeball (fig. 103). About the third month of fœtal life the two folds, one forming the upper and the other the lower lid, meet and unite by a growth together of the epithelium at the margins of the folds, so as to cut off the conjunctival sac from the exterior. A short time before birth they again become disunited.

A third fold (of the conjunctiva) appears at the inner canthus, and in many vertebrates developes into a well-marked third eyelid, the membrana nictitans. In man it remains rudimentary, forming the plica semilunaris.

The glands, hairs, and other structures belonging to the eyelids, are developed in the same way as the corresponding structures in the rest of the integument.

The lachrymal gland is developed in the third month as a number of outgrowths from the deeper layer of the epithelium, at the upper and outer part of the conjunctival sac. The outgrowths are at first solid, and branch into the surrounding connective tissue as with other racemose glands, subsequently becoming hollowed out and differentiated into ducts and acini.

The lachrymal canals and ducts are usually described as being directly developed by the enclosure of the fissure which separates the lateral nasal process from the maxillary process (see Development of Nose, p. 95, and figs. 111, 112), and which passes in the early embryo from the eye to the upper part of the nasobuccal cavity (lachrymal fissure). But it has been shown, chiefly by the researches of Born, that in most animals the canal is at first formed as a thickening of the rete mucosum of the epidermis, which sinks into the corium along the line of that fissure. The thickening subsequently becomes separated from the rest of the epidermis, and hollowed out to form an epithelial tube, which leads from the conjunctiva into the nasal cavity.

The bifurcation of the duct where it opens on the conjunctiva is produced, according to Ewetsky, by a broadening out of the epithelial cord at the inner canthus, and its subsequent separation into two parts by an ingrowth of connective tissue in its middle, the two parts developing into the upper and lower lachrymal canals.

The essential part of the ear, viz., the epithelial lining of the labyrinth, is developed in much the same way as the crystalline lens, as an invagination of the external epiblast, which at first appears as a pit of thickened epithelium (auditory pit, fig. 104, A.), but is gradually converted by a growing together of the margins of the pit into a hollow island of epiblast, the auditory or otic vesicle (fig. 104, B). This process occurs somewhat after the formation of the eye is laid, and at quite a different part of the head, viz., on either side of the hind-brain just over the upper end of the first post-oral visceral cleft. The vesicle comes at first into close contact with the hind-brain, except where the ganglionic rudiment of the auditory nerve projects between them, but it subsequently becomes entirely surrounded by mesoblast, which separates it from both the neural and external epiblast.

The hind-brain does not send out a hollow process towards the otic vesicle corresponding to the optic processes of the fore-brain, but the auditory nerve developes from a solid outgrowth of the neural crest in the same way as the posterior roots of the spinal nerves and parts of many other of the cranial nerves (see p. 78).

The otic vesicle is at first flask-shaped, with the somewhat elongated mouth of the flask directed externally towards the original point of connection with the

exterior. In elasmobranch fishes this connection is never closed, but remains throughout life in the form of a small duct-like tube which passes up through the cranial wall and opens on the epidermis. In other vertebrates the connection with the exterior becomes closed-in the chick during the third day—and what remains

Fig. 104.-SECTIONS THROUGH REGION OF THE HIND-BRAIN OF HUMAN EMBRYOS, SHOWING THREE

STAGES IN THE DEVELOPMENT OF THE OTIC VESICLE.

A, auditory pits; B, simple auditory vesicles; C, auditory vesicles beginning to be fashioned into parts of the membranous labyrinth.

r.l.

a

Fig. 105.-OUTLINE OF THE RIGHT LABYRINTH OF A 3

WEEKS HUMAN EMBRYO, TO SHOW ITS RELATIONS TO

THE PARTS OF THE AUDITORY NERVE. (W. His, jun.) a.t, section of hind brain; g.r, ganglion vestibuli in contact with the upper part of the labyrinth; g.c, ganglion cochleæ in contact with the lower part. The fibres of the corresponding parts of the auditory nerve which have grown from these ganglia into the hind brain, are seen to cross one another; f, facial nerve.

of the original mouth, or canal of connection with the exterior, is visible as a distinct but small process from the upper and inner angle of the vesicle, and is known as the recess of the labyrinth (fig. 104 c, r.l). Eventually it developes into a long epithelial tube, which passes through the petrous bone, with an expanded end lying within the skull underneath the dura mater. This tube and its expanded termination form respectively the endolymphatic canal and saccule (fig. 106).

In the meantime the auditory vesicle becomes elongated and begins to be irregular. Its ventral end projects as a distinct hollow process, at first straight, but soon becoming curved; this is the rudiment of the epithelial canal of the cochlea.

Other hollow projections appear near the dorsal end of the vesicle; these form the two superior semicircular canals; the horizontal canal appears a little later.

The mode of formation of the canals is somewhat peculiar. They first appear as flattened semicircular hollow protrusions of the wall of the vesicle. Their sides then come together and coalesce, except near the circumference of the semicircle, which now forms a tube connected at both ends with the vesicle. Subsequently a separation or breach of continuity occurs over the area of coalescence, so that the rest of the tube is free. One of the ends becomes dilated into an ampulla and connected with a branch of the auditory nerve.

Whilst these processes are occurring at the dorsal and ventral ends of the now elongated vesicle, a fold, or constriction, of the wall is beginning to make its appearance about the middle, and thus the posterior part which is connected with the semicircular canals becomes gradually separated (as the utricle) from the anterior part, which forms the saccule, and is connected with the cochlea. This fold extends into the beginning of the recess of the labyrinth, and separates it longitudinally for a

Fig. 106.-STAGES IN THE DEVELOPMENT OF THE MEMBRANOUS LABYRINTH. (W. His, jun.) A. Left labyrinth of a human embryo of about four weeks, viewed from the outer side. lar part; c, cochlear part; r., recessus labyrinthi (aquæductus vestibuli).

2, vestibu

B. Left labyrinth with parts of the facial and auditory nerves of a human embryo of about 4 weeks. b.b, surface of the hind brain; u, utricular; 8, saccular part of labyrinth; a.s.c., p.s.c., e.s c., rudimentary folds representing the two vertical and the horizontal semicircular canals; .7, upper part of recessus labyrinthi becoming enlarged into the endolymphatic saccule; c.c, rudiment of cochlea; n.v, vestibular branch of auditory nerve; 9.v, vestibular ganglion (ganglion of Scarpa); g.c, cochlear ganglion; n.f, facial nerve, with geniculate ganglion, g.g.

C. Left labyrinth of a human embryo of about five weeks, viewed from without and below. Lettering as before. The horizontal canal is still only a fold. The ampullæ are beginning to be visible on the

two vertical canals.

short distance into two tubes, one of which opens into the utricle, and the other into the saccule, forming the only permanent means of communication between their contents. Another fold, or constriction, appears presently, somewhat lower down, and converts the connection between the saccule and the cochlea rudiment into the narrow duct of Hensen (canalis re-uniens).

In the meantime the cochlea-rudiment at the ventral end of the now labyrinthic vesicle, becomes elongated into a tube, which, as it grows, becomes coiled upon itself in such a manner as to produce the spiral structure of this part of the auditory