THE ORGANS OF SPECIAL SENSE. 583 



horizontally and unite to form these membranes. Other spongioblasts develop 

 into other types of glia cells, mainly spider cells, which are most numerous in 

 the layer of ganglion cells and in the layer of nerve fibers. 



The rod and cone cells are first recognizable as unipolar cells (Fig. 506, a, c). 

 The single process of each extends outward as far as the outer limiting mem- 

 brane. About as soon as these cells are recognizable, a differentiation between 

 the rod cells and the cone cells can be made by their reactions to the Golgi 

 silver stain, the cone cells impregnating much more completely than the rod 

 cells. Processes next grow out from the inner ends of the cells so that they 

 become bipolar (Fig. 506, b, d) . Both rod and cone cells are at first distributed 

 throughout the entire nuclear layer, but later they become arranged in a dis- 

 tinct layer just beneath the outer limiting membrane. Each cell next gives 

 rise to or acquires at its outer end an expansion which extends through 



Layer of nerve fibers 

 Layer of nerve cells 

 Inner molecular layer 

 Inner nuclear layer 



Outer undifferentiated layer 



FIG. 507. Vertical section through retina of a four months' human embryo. Modified from Langes. 



the outer limiting membrane into the pigmented layer. As the pigmented 

 cells give off pigmented processes which extend inward among the outer 

 ends of the rods and cones, the layer of retina just beneath the pig- 

 mented layer consists of the outer ends of the rod cells, the tips of the cone 

 cells, and the extensions of the pigmented cells. The nucleated portions of 

 the rod and cone cells form the outer nuclear layer. Though the layer of rods 

 and cones and the outer nuclear layer present the appearance in haematoxylin- 

 eosin stained specimens of two distinct layers, it is evident from their develop- 

 ment and structure that they should be regarded as a single neuro-epithelial 

 layer. The apparent separation into two layers is due to the interposition of the 

 outer limiting membrane, through tiny holes in which the rod and cone cells 

 extend. The inwardly directed processes of the rod and cone cells are their 

 axones. These cells constitute the first or distal optic neurone. 



The bipolar cells (Fig. 506, e), which with their processes constitute the 

 middle or second optic neurone, also develop from cells of the nuclear layer 



