86 DEVELOPMENT OF THE EYE. 



The development of the retina from the inner layer of the optic cup, has not 

 been fully worked out. In its earlier stages it closely resembles in structure the 

 wall of the cerebral vesicles, consisting of elongated epithelium-like cells, apparently 

 arranged in several interlocking layers. Of these cells some become developed into 

 nerve-fibres and nerve-cells (inner granules and gangliomc layer), others into susten- 

 tacular tissue, similar to the neuroglia of the central nervous system (molecular 

 layers, Miillerian fibres), whilst the outermost layer forms the sense- epithelium (W. 

 Miiller), or layer of outer granules, which is sharply marked off against the 

 layer of hexagonal pigment cells by the membrana limitans externa, as is the nerve- 

 fibre layer from the vitreous humour by the membrana limitans interna. For a long 

 time there is no trace of the rods and cones. These begin to appear some little time 

 before birth in man and most animals, but in animals which are born blind, such as 

 kittens, not until after birth (M. Schultze), in the shape of small protuberances of 

 the sense-epithelium cells growing beyond the limitans externa, and forming at first 

 the inner segments of the rods and cones, and subsequently the outer segments also. 

 The latter as they are developed become imbedded in the inner surface of the 

 hexagonal pigment cells, which have become developed from the outer layer of the 

 optic cup. 



The anterior third of the optic cup does not undergo the changes above 



ri 



>.c.n 



Fig. 102. SECTION THROUGH THE EYE OF A RABBIT EMBRYO, MORE ADVANCED IN DEVELOPMENT THAN 



THAT SHOWN is FIG. 98. (Balfour.) 



c, epithelium of cornea; I, lens; me.c, mesoblast growing in to form the substantia propria of the 

 cornea ; o.n, optic nerve ; rt, retina ; a.c.r, mesoblast for the formation of the vitreous humour, and 

 the arteria centralis retinae. 



described. Its two layers become here developed into the comparatively simple pars 

 ciliaris retinee, and in front of the ciliary, region they extend forwards and inwards 

 in front of the lens and in close contact with the back of the iris, where they form 

 the thickly pigmented epithelium, which is known as the uvea and terminates at the 

 margin of the pupil. 



Further development of the lens. The hollow epiblastic vesicle from 

 which the lens develops is composed of a thick posterior and a thin anterior layer 

 which pass into one another at the equator of the lens, and enclose a clear fluid. 

 In mammals, the vesicle when first formed also contains a small mass of epithelium 

 cells which have become separated off from the posterior wall (fig. 100), but these 



