Fig. 315—Face of a human embryo, seven weeks old. (From Kollmann.) Joining of the nasal processes (e outer, i inner) with the upper-jaw process (o), n nasal wall, a ear-opening.
Immediately under the sclerotic we find a very delicate, dark-red membrane, very rich in blood-vessels—the choroid coat—and inside this the retina (o), the expansion of the optic nerve (i). The latter is the second cerebral nerve. It proceeds from the optic thalami (the second cerebral vesicle) to the eye; penetrates its outer envelopes, and then spreads out like a net between the choroid and the corpus vitreum. Between the retina and the choroid there is a very delicate membrane, which is usually (but wrongly) associated with the latter. This is the black pigment-membrane (n). It consists of a single stratum of graceful, hexagonal, regularly-joined cells, full of granules of black colouring matter. This pigment membrane clothes, not only the inner surface of the choroid proper, but also the hind surface of its anterior muscular continuation, which covers the edge of the lens in front as a circular membrane, and arrests the rays of light at the sides. This is the well-known iris of the eye (h), coloured differently in different individuals (blue, grey, brown, etc.); it forms the anterior border of the choroid. The circular opening that is left in the middle is the pupil, through which the rays of light penetrate into the eye. At the point where the iris leaves the anterior border of the choroid proper the latter is very thick, and forms a delicate crown of folds (g), which surrounds the edge of the lens with about seventy large and many smaller rays (corona ciliaris.)
Fig. 316—Face of a human embryo, eight weeks old. (From Ecker.)
At a very early stage a couple of pear-shaped vesicles develop from the foremost part of the first cerebral vesicle in the embryo of man and the other Craniotes (Figs. 155 a, 297 au). These growths are the primary optic vesicles. They are at first directed outwards and forwards, but presently grow downward, so that, after the complete separation of the five cerebral vesicles, they lie at the base of the intermediate brain. The inner cavities of these pear-shaped vesicles, which soon attain a considerable size, are openly connected with the ventricle of the intermediate brain by their hollow stems. They are covered externally by the epidermis.
At the point where this comes into direct contact with the most curved part of the primary optic vesicle there is a thickening (l) and also a depression (o) of the horny plate (Fig. 318, I). This pit, which we may call the lens-pit, is converted into a closed sac, the thick- walled lens-vesicle (2, l), the thick edges of the pit joining together above it. In the same way in which the medullary tube separates from the outer germinal layer, we now see this lens-sac sever itself entirely from the horny plate (h), its source of origin. The hollow of the sac is afterwards filled with the cells of its thick walls, and thus we get the solid crystalline lens. This is, therefore, a purely epidermic structure. Together with the lens the small underlying piece of corium-plate also separates from the skin.
As the lens separates from the corneous plate and grows inwards, it necessarily hollows out the contiguous primary optic vesicle (Fig. 318, 1–3). This is done in just the same way as the invagination of the blastula, which gives rise to the gastrula in the amphioxus (Fig. 38 C–F). In both cases the hollowing of the closed vesicle on one side goes so far that at last the inner, folded part touches the outer, not folded part, and the cavity disappears. As in the gastrula the first part is converted into the entoderm and the latter into the ectoderm, so in the invagination of the primary optic vesicle the retina (r) is formed from the first (inner) part, and the black pigment membrane (u) from the latter (outer, non-invaginated) part. The hollow stem of the primary optic vesicle is converted into the optic nerve. The lens (l), which has so important a part in this process, lies at first directly on the invaginated part, or the retina (r). But they soon separate, a new structure, the corpus vitreum (gl), growing between them. While the lenticular sac is being detached and is causing the invagination of the primary optic vesicle, another invagination is taking place from below; this proceeds from the superficial part of the skin-fibre layer—the corium of the head. Behind and under the lens a last-shaped process rises from the cutis-plate (Fig. 319 g), hollows out the cup-shaped optic vesicle from below, and presses between the lens (l) and the retina (i). In this way the optic vesicle acquires the form of a hood.