The front portion, accompanied by the mesoblast which immediately overlies it, is behind the lens thrown into folds, the ciliary ridges; while further forward it bends in between the lens and the cornea to form the iris. The original wide opening of the optic cup is thus narrowed to a smaller orifice, the pupil; and the lens, which before lay in the open mouth of the cup, is now inclosed in its cavity. While in the hind portion of the cup or retina proper no deposit of black pigment takes place in the layer formed out of the inner or anterior wall of the vesicle; in the front portion forming the region of the iris, pigment is largely deposited throughout both layers, though first of all in the outer one, so that eventually this portion seems to become nothing more than a forward prolongation of the pigment epithelium of the choroid.
Fig. 288.
D. Diagrammatic section taken perpendicular to the plane of the paper, along the line yy, fig. 287. The stalk is not seen, the section falling quite out of its region. vh. hollow of optic cup filled with vitreous humour; other letters as in fig. 285 B. (After Remak.)
E. Section taken parallel to the plane of the paper through fig. 287, so far behind the front surface of the eye as to shave off a small portion of the posterior surface of the lens l, but not so far behind as to be carried at all through the stalk. Letters as before; f. the choroidal fissure.
F. Section along the line zz, perpendicular to the plane of the paper, to shew the choroidal fissure f, and the continuity of the cavity of the optic stalk with that of the primary optic vesicle. Had this section been taken a little to one side of the line zz, the wall of the optic cup would have extended up to the lens below as well as above. Letters as before. The external epiblast is omitted in this section.
Thus, while the hind moiety of the optic cup becomes the retina proper, including the choroid-pigment in which the rods and cones are imbedded, the front moiety is converted into the ciliary portion of the retina, covering the ciliary processes, and into the uvea of the iris; the bodies of the ciliary processes and the substance of the iris, their vessels, muscles, connective tissue and ramified pigment, being derived from the mesoblastic choroid. The margin of the pupil marks the extreme lip of the optic vesicle, where the outer or posterior wall turns round to join the inner or anterior.
The ciliary muscle and the ligamentum pectinatum are both derived from the mesoblast between the cornea and the iris.
Fig. 289. Section of the eye of Chick at the fourth day.
e.p. superficial epiblast of the side of the head; R. true retina: anterior wall of the optic cup; p.Ch. pigment-epithelium of the choroid: posterior wall of the optic cup. b is placed at the extreme lip of the optic cup at what will become the margin of the iris. l. the lens. The hind wall, the nuclei of whose elongated cells are shewn at nl, now forms nearly the whole mass of the lens, the front wall being reduced to a layer of flattened cells el. m. the mesoblast surrounding the optic cup and about to form the choroid and sclerotic. It is seen to pass forward between the lip of the optic cup and the superficial epiblast.
Filling up a large part of the hollow of the optic cup is seen a hyaline mass, the rudiment of the hyaloid membrane, and of the coagulum of the vitreous humour, y. In the neighbourhood of the lens it seems to be continuous as at cl with the tissue a, which appears to be the rudiment of the capsule of the lens and suspensory ligament.