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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

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A further stage of the development of the lens. (Cadiat) a. Elongating epithelial cells forming lens ; b. Capsule ; c. Cutaneous tissue becoming con.

junctiva; 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.

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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

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in thickness, and approaching the anterior obliterates the original cavity of the lens.

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 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 processesand 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 retinæ. 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.

DEVELOPMENT OF THE EAR. The ear is developed chiefly from the epiblast, a special and independent involution of which forms both its essential nervous structures and the general epithelium lining the membranous

FIG. 314.

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GO Transverse section throngh the head of a fatal sheep in the region of the hind-brain.

(Boettcher.) H.B. Hind-brain. CC. Canal of the cochlea. RV. Recessus vestibuli. VB. Vertical

semicircular canal. G.C. Auditory ganglion. G'. Auditory nerve. N. Connection of auditory nerve to the hind-brain.

labyrinth. The mesoblast supplies the surrounding firmer structures, such as the fibrous substance of the inner ear, and the bony parts in which the organ lies. The auditory nerve grows as a bud from the neural tissue forming the hind-brain, and connects it with the delicate specialized auditory cells.

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The process begins by the appearance of a depression of the general epiblast covering the head, which forms a tubular diverticulum, lying in the mesoblast at the side of the hind-brain.

This diverticulum becomes separated from the epiblast by the obliteration of its outer extremity, which united it to the superficial epiblast, and is converted into a cavity receiving the name of the otic vesicle. It soon becomes somewhat triangular in shape, the base of the triangle lying upward.

From the lower angle arises a projection, which is the rudimentary canal of the cochlea. The angle lying next to the neural epiblast similarly gives off a tubular process, which forms the recessus vestibuli.

Elevations in the primitive vesicle indicate the origin of the semicircular canals, which become tubular, opening at their ends into the general cavity of the vesicle. The two superior canals are the first to appear, the horizontal arising somewhat later.

The part of the otic vesicle in connection with the canal of the cochlea becomes separated from the latter by a narrow constriction which forms the canalis reuniens, the part of the vesicle beyond this developing into the saccule.

The utricle arises from that part of the vesicle which is in connection with the semicircular canals. It is at first in direct connection with the saccule, but after a time it only communicates by means of a narrow canal with a similar one from the saccule; these two canals are connected with a third, which lies in the aqueductus vestibuli.

The canal of the cochlea is at first a straight tube, but as it develops it becomes coiled upon itself.

The walls of the primitive otic vesicle, formed from the epiblast, become developed into the epithelium lining the internal

The mesoblast immediately surrounding the vesicle forms a supporting capsule of fibrous tissue, which completes the membranous parts of the internal ear.

Part of the mesoblast around the optic vesicle becomes liquefied, and gives origin to the canals and spaces in which the membranous labyrinth lies; the neighboring mesoblast is changed into cartilage which ossifies and forms the bony parts of the ear.

ear.

The auditory nerve is developed from the hind-brain, and grows through the mesoblast toward the otic vesicle. It is recognizable from its having some ganglion cells in its growing extremity from a very early period of its development.

The Eustachian tube and the tympanum are formed in connection with the inner part of the first visceral cleft, and the

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Section through the head of a fatal sheep. (Boettcher.) R. V. Recessus vestibuli. V. B. Vertical semicircular canal. H. B. Horizontal semicir.

cular canal. G. Auditory ganglion. C.C. Canal of the cochlea.

ossicles are developed from the corresponding visceral arch-hyomandibular.

The membrana tympani is formed at the surface of the embryo, the adjacent parts grow outward and give rise to the external auditory meatus.

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