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TEXT-BOOK OF EMBRYOLOGY. 



the optic stalk as one of the fibers of the optic nerve. The layer of ganglion cells 

 is thickest in an area situated somewhat lateral to the attachment of the optic 

 stalk and known as the area centralis. It is distinguishable about the end of the 

 fourth month. In the center of the area centralis the retinal layers become 

 thin to form the fovea centralis which develops toward the end of foetal life. 

 The macula lutea with its yellow pigment does not develop until after birth. 

 The retina at this stage thus consists of four layers which from within out- 

 ward are (i) the layer of nerve fibers, (2) the layer of ganglion cells, (3) the 

 nuclear layer, (4) the pigmented layer (see Fig. 507). 



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Fig. 506. — Diagram of the development of the retinal cells. Kallius, after Cajal. 

 a, Cone cells in unipolar stage; b, cone cells in bipolar stage; c, rod cells in unipolar stage; d, rod cells 

 in bipolar stage; e, bipolar cells; / and i, amacrine cells; g, horizontal cell; h, ganglion cells; 

 k, Muller's cells or fibers; I, external limiting membrane. 



The further development of the retina consists largely of a differentiation of 

 the cells of the nuclear layer. This is extremely complex and our knowledge 

 of it meager. From the cells of this layer develop (1) the rod and cone cells, (2) 

 the bipolar cells, (3) the tangential or horizontal cells, (4) the amacrine cells, (5) 

 Muller's cells or fibers. The differentiation of these cells and their processes 

 also results in the demarcation of the following layers of the adult retina; (1) the 

 layer of rods and cones, (2) the outer limiting membrane, (3) the outer nuclear 

 layer, (4) the outer molecular layer, (5) the inner nuclear layer, (6) the inner 

 molecular layer, (7) the inner limiting membrane (see Fig. 508). 



Muller's cells or the sustentacular cells (Fig. 506, k) develop from spongio- 

 blasts which lie toward the inner limit of the nuclear layer. This accounts 

 for the location of the nucleated portions of Muller's cells. Processes of these 

 cells grow toward both surfaces of the retina until they reach the positions of the 

 future outer and inner limiting membranes where they are believed to spread out 



