482 EMBRYOLOGY. 



externa. Then there appear upon the latter numerous, small, 

 lustrous elevations, which have been secreted by the outer granules 

 or visual cells. The elevations, which consist of a protoplasmic 

 substance and are stained red in carmine, become elongated and 

 acquire the form of the inner limb of the retinal element. Finally 

 there is formed at their outer ends the outer limb, which MAX 

 SCHULTZE and W. MtiLLER compare to a cuticular product, on 

 account of its lamellate structure. 



Inasmuch as the rods and cones of the retinal cells grow out in 

 this way beyond the membrana limitans externa, they penetrate 

 into the closely applied outer lamella of the optic cup, which becomes 

 the pigmented epithelium of the retina (figs. 266, 267, 270 pi) ; 

 their outer limbs come to lie in minute niches of the large, hexagonal 

 pigment -cells, so that the individual elements are separated from 

 one another by pigmented partitions. 



A few additional words concerning the connective tissue enveloping 

 the fundament of the optic cup. It acqiUEsJiere, as on the ciliary 

 body and the iris, a special, and for this region characteristic, stamp. 

 It is differentiated into vascular [choroid] and fibrous [sclerotic] 

 membranes, which in Man are distinguishable in the sixth week 

 (KOLLIKER). The former is characterised by its vascularity at an 

 early period, and develops on the side toward the optic cup a special 

 layer, provided with a fine network of capillary vessels, the mem- 

 brana choriocapillaris, for the nourishment of the pigment-layer and 

 the layer of rods and cones, which have no blood-vessels of their 

 own. It further differs from the ciliary body in the fact that at 

 the fundament of the optic cup the choroid membrane is easily 

 separable from the adjoining membranes of the eye, whereas in the 

 ciliary body a firm union exists between all the membranes. 



If we now glance back at the processes of development last 

 described, one thing will appear clear to us from this short sketch : 

 that the changes in the form of the secondary optic cup are of 

 preeminent importance for the origin of the individual regions of the 

 eye. Through different processes of growth, which have received a 

 general discussion in Chapter IV., there have been formed in the cup 

 three distinct portions. By means of an increase in thickness and 

 various differentiations of the numerous cell-layers, there is formed 

 the retina ; by an increase of surface, on the contrary, is produced 

 an anterior, thinner part, which bounds the pupil and is subdivided 

 'into two regions by the formation of folds in the vicinity of the lens. 

 :Frorn the folded part, which joins the retina at the ora serrata, is 



