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TEXT-BOOK OF EMBRYOLOGY. 



and are probably bipolar at the time that the rod and cone cells are in the 

 unipolar condition. Reference to the two bipolar cells shown in Fig. 506, e, e, 

 shows that at this stage in their development their outwardly directed processes 

 extend to the outer limiting membrane. These processes must either actually 

 shorten or else fail to grow in length proportionately as the retina increases in 

 thickness, for in the mature retina they end in relation with the centrally 

 (inwardly) directed processes (axones) of the rod and cone cells. According as 

 they are in relation with rod cells or cone cells, they are known as rod bipolars 

 or cone bipolars. The retinal layer in which the axones of the rod and cone 



Inner limiting membrane 



Layer of nerve fibers 

 Layer of nerve cells 



Inner molecular layer 



( horizontal cells 



Inner nuclear layer ^ bipolar cells 

 (amacrine cells 



Outer molecular layer 

 Outer nuclear layer 



Outer limiting membrane 

 Layer of rods and cones 



Layer of pigmented epithelium 



FIG. 508. Vertical section through retina of a five and one-half months' human embryo. 



Modified from Lange. 



cells and the dendrites of the rod and cone bipolars intermingle is the outer 

 molecular layer of the adult retina. It is first distinctly recognizable as a mo- 

 lecular layer about the end of the fifth month (Fig. 508). 



The development of the outer molecular layer separates the originally single 

 nuclear layer into two layers, an outer composed of the nuclei of the rod and cone 

 cells and an inner composed of the nucleated bodies of the rod and cone 

 bipolars, of the horizontal cells (Fig. 506, g) and of the amacrine cells (Fig. 506, 

 / and i), all of which can be recognized in Golgi specimens by the end of the 

 seventh month. The rod and cone bipolars and probably most of the other 

 cells of the inner nuclear layer send their axones centrally to lie in contact with 

 the dendrites and bodies of the ganglion cells. 



