The optic vesicles arise from the anterior cerebral vesicle at a very early period, and their cavities are continuous with that of the fore-brain. With the development of the rudimentary cere-61 bral hemispheres the optic vesicles become displaced downward, and their cavities open into the junction of the cavities of the cerebral hemispheres, and that of the thalamencephalon, which becomes the third ventricle. Later, the optic vesicles open directly into the third ventricle, and finally are displaced backward, and come into connection with the mid-brain.

The optic vesicles are at first hollow prolongations, which consist of an anterior dilated portion, forming the primary optic vesicle, and a posterior tubular portion or stalk joining the vesicle to the fore-brain. This stalk forms the optic nerve.

As each vesicle grows forward toward the epiblast covering the head of the embryo, the epiblastic cells at the spot overlying the vesicle become thickened, an involution of the epiblast takes place toward the optic vesicle, and indents the latter, approximating its anterior to its posterior wall.

By this means the anterior and posterior walls of the primary optic vesicle come into close contact, and the cavity of the vesicle is obliterated. The two layers of the vesicle are now cup-shaped, and receive the name of the secondary optic vesicle or the optic cup. This ultimately becomes the retina, and the optic stalk, losing its cavity, is transformed into the optic nerve.

Meanwhile, the local involution of the epiblast over the optic cup, which is the rudiment of the crystalline lens, becomes gradually separated from the general epiblast, and is finally detached from its point of origin. It now lies as a somewhat spherical body in the cavity of the optic cup within the superficial mesoblast, which has closed over it.

The secondary optic vesicle grows (except at its lower part, just at the junction of the optic stalk) so as to deepen the optic cup, which contains the rudimentary lens. At the lower part an interval is left, which receives the name of the choroid fissure. Through this gap in the secondary optic vesicle the mesoblast enters and separates the lens from the optic cup, forming the vitreous humor.

Section through the head of a chick at the third day, showing the origin of the lens.

Fig. 310. Section through the head of a chick at the third day, showing the origin of the lens.

a. Epiblast thickened at c, which is the point of origin of the lens. o. Optical vesicle. V\. Anterior cerebral vesicle. V2. Posterior cerebral vesicle.

Diagrammatic sections of the primitive eye, showing the choroidal fissure.

Fig. 311. Diagrammatic sections of the primitive eye, showing the choroidal fissure. {Foster and Balfour).

D. Horizontal section. E. Vertical transverse section just striking the posterior part of the lens. F. Vertical longitudinal section through the optic stalk, and the fissure through which the mesoblast passes to form the vitreous humor.

h. Superficial epiblast. x. Point of origin of the lens. v. h. Vitreous humor, r. Anterior layer of the optic vesicle, u. Posterior layer of the optic vesicle, c. Cavity of the optic vesicle, f. Choroidal fissure. s. Opticstalk. s'. Cavity of the optic stalk. l. Lens. l'. Cavity of the lens.

The mesoblast surrounding the optic cup develops two coverings of the eye, an outer fibrous capsule called the sclerotic coat, and a vascular coat, the choroid.

Later stages in the development of the lens. (Cad/at.) a. Epiblast. c. Rudimentary lens. o. Optic vesicle.

Fig. 312. Later stages in the development of the lens. (Cad/at.) a. Epiblast. c. Rudimentary lens. o. Optic vesicle.

In front of the lens, beneath the epiblast, the mesoblast forms the corneal tissue proper. The epiblast forms the epithelial or conjunctival covering of the eyeball.

The involution of mesoblast through the choroidal fissure, which forms the vitreous humor, indents the optic stalk, and forms the central artery of the retina. The choroidal fissure is gradually obliterated, and its position may sometimes be marked by a permanent fissure in the iris (coloboma iridis). The rudimentary lens is a spherical body, hollow in the centre, made up of an anterior and posterior wall, each of which is formed of columnar cells. The posterior wall of the lens increases greatly in thickness,and approaching the anterior obliterates the original cavity of the lens.

A farther stage of the development of the lens. (Cadi'at ).

Fig.313. A farther stage of the development of the lens. (Cadi'at ).

a. Elongating epithelial cells forming lens; b. Capsule; c. Cutaneous tissue becoming conjunctiva; d, e. Two layers of optic cup forming retina f. Cell of mucous tissue of the vitreous humor; g. Intercellular substance; h. Developing -optic nerve; i. Nerve fibres passing to retina.

The cells forming this wall become very much elongated, and develop into long fibre-like columnar cells. Those of the anterior walls from being a columnar, are modified to a flattened epithelium, and finally become the layer lining the anterior surface of the capsule of the lens. The capsule of the lens has been variously considered as arising from the cells of the lens substance, or as originating from a thin layer of mesoblast, which forms not only the lens capsule, but also the hyaloid membrane, which is continuous with it.

The optic cup gives origin to the retina. The inner or anterior layer of the cup becomes thickened, and from it are differentiated the various layers of the retina, except that layer of pigment cells which lies next to the choroid. The posterior layer develops this layer of pigment cells, which, from their intimate connection to the choroid, were formerly considered as part of that membrane.

The thickening of the inner or anterior layer of the optic cup ceases at the ora serrata. The outer layer with its contiguous choroid is thrown into a number of folds - the ciliary processes - and passing in front of the lens, helps to form the iris.

In front of the ora serrata the anterior layer of the cup is no longer differentiated into the special retinal elements, but joins with the posterior to form a layer of columnar cells, - the pars ciliaris retina. In front of this the interior rim of the optic cup passes forward and lines the posterior surface of the iris, forming the uvea of that organ, and terminating at the margin of the pupil.

The rest of the substance of the iris is developed from the mesoblast, from which also arise the choroid, the cornea and the sclerotic.

The development of the eye may be thus briefly described. An offshoot of nervous matter from the fore-brain forms the retina and the uvea, and its stalk, or connection with the brain, develops into the optic nerve.

An involution of epiblast which grows into the nervous cup forms the lens, while from the adjacent mesoblast arise the surrounding parts of the eye. The vitreous is produced by the mesoblast growing through a fissure in the lower part of the optic cup to fill its cavity.