Fig. 284. Section through the head of an embryo Teleostean, to shew the formation of the optic vesicles, etc. (From Gegenbaur; after Schenk.)
c. fore-brain; a. optic vesicle; b. stalk of optic vesicle; d. epidermis.
After the establishment of the optic nerves, there take place (1) the formation of the lens, and (2) the formation of the optic cup from the walls of the primary optic vesicle.
The external or superficial epiblast which covers, and is in most forms in immediate contact with, the most projecting portion of the optic vesicle, becomes thickened. This thickened portion is then driven inwards in the form of a shallow open pit with thick walls ([fig. 285] A, o), carrying before it the front wall (r) of the optic vesicle. To such an extent does this involution of the superficial epiblast take place, that the front wall of the optic vesicle is pushed close up to the hind wall, and the cavity of the vesicle becomes almost obliterated ([fig. 285] B).
The bulb of the optic vesicle is thus converted into a cup with double walls, containing in its cavity the portion of involuted epiblast. This cup, in order to distinguish its cavity from that of the original optic vesicle, is generally called the secondary optic vesicle. We may, for the sake of brevity, speak of it as the optic cup; in reality it never is a vesicle, since it always remains widely open in front. Of its double walls the inner or anterior ([fig. 285] B, r) is formed from the front portion, the outer or posterior ([fig. 285] B, u) from the hind portion of the wall of the primary optic vesicle. The inner or anterior (r), which very speedily becomes thicker than the other, is converted into the retina: in the outer or posterior (u), which remains thin, pigment is eventually deposited, and it ultimately becomes the tesselated pigment-layer of the choroid.
Fig. 285. Diagrammatic sections illustrating the formation of the eye. (After Remak.)
In A the thin superficial epiblast h is seen to be thickened at x, in front of the optic vesicle, and involuted so as to form a pit o, the mouth of which has already begun to close in. Accompanying this involution, which forms the rudiment of the lens, the optic vesicle is doubled in, its front portion r being pushed against the back portion u, and the original cavity of the vesicle thus reduced in size. The stalk of the vesicle is shewn as still broad.
In B the optic vesicle is still further doubled in so as to form a cup with a posterior wall u and an anterior wall r. In the hollow of this cup lies the lens l, now completely detached from the superficial epiblast xh.
By the closure of its mouth the pit of the involuted epiblast becomes a completely closed sac with thick walls and a small central cavity ([fig. 285] B, l). At the same time it breaks away from the external epiblast, which forms a continuous layer in front of it, all traces of the original opening being lost. There is thus left lying in the cup of the secondary optic vesicle, an isolated elliptical mass of epiblast. This is the rudiment of the lens. The small cavity within it speedily becomes still less by the thickening of the walls, especially of the hinder one.
At its first appearance the lens is in immediate contact with the anterior wall of the secondary optic vesicle ([fig. 285] B). In a short time however, the lens is seen to lie in the mouth of the cup ([fig. 288] D), a space (vh) (which is occupied by the vitreous humour) making its appearance between the lens and anterior wall of the vesicle.