Fig. 287. Diagrammatic representation of the eye of the Chick of about the third day as seen when the head is viewed from underneath as a transparent object.
l. the lens; l´. the cavity of the lens, lying in the hollow of the optic cup; r. the anterior, u. the posterior wall of the optic cup; c. the cavity of the primary optic vesicle, now nearly obliterated. By inadvertence u has been drawn in some places thicker than r, it should have been thinner throughout. s. the stalk of the optic cup with s´ its cavity, at a lower level than the cup itself and therefore out of focus; the dotted line indicates the continuity of the cavity of the stalk with that of the primary vesicle.
The line z z, through which the section shewn in fig. 288 F is supposed to be taken, passes through the choroidal fissure.
With reference to the above description, taken with very slight alterations from the Elements of Embryology, Pt. 1., two points require to be noticed. Firstly it is extremely doubtful whether the invagination of the secondary optic vesicle is to be viewed as an actual mechanical result of the ingrowth of the lens. Secondly it seems probable that the choroid fissure is not simply due to an inequality in the growth of the walls of the secondary optic cup, but is partly due to a doubling up of the primary vesicle from the side along the line of the fissure, at the same time that the lens is being thrust in in front. In Mammalia, the doubling up involves the optic stalk, which becomes flattened (whereby its original cavity is obliterated) and then folded in on itself, so as to embrace a new central cavity continuous with the cavity of the vitreous humour. And in other forms a partial phenomenon of the same kind is usually observable, as is more particularly described in the sequel.
Before describing the development of the cornea, aqueous humour, etc. we may consider the further growth of the parts, whose first development has just been described, commencing with the optic cup.
During the above changes the mesoblast surrounding the optic cup assumes the character of a distinct investment, whereby the outline of the eyeball is definitely formed. The internal portions of this investment, nearest to the retina, become the choroid (i.e. the chorio-capillaris, and the lamina fusca; the pigment epithelium, as we have seen, being derived from the epiblastic optic cup), and pigment is subsequently deposited in it. The remaining external portion of the investment forms the sclerotic.
The complete differentiation of these two coats of the eye does not however take place till a late period.
The cavity of the original optic vesicle was left as a nearly obliterated space between the two walls of the optic cup. By the end of the third day the obliteration is complete, and the two walls are in immediate contact.
The inner or anterior wall is, from the first, thicker than the outer or posterior; and over the greater part of the cup this contrast increases with the growth of the eye, the anterior wall becoming markedly thicker and undergoing changes of which we shall have to speak directly ([fig. 289]).
In the front portion however, along, so to speak, the lip of the cup, anterior to a line which afterwards becomes the ora serrata, both layers cease to take part in the increased thickening, accompanied by peculiar histological changes, which the rest of the cup is undergoing. Thus a hind portion or true retina is marked off from a front portion.