VI.] THE RETINA. 145 



On the fourth day, the inner (anterior) wall of the 

 optic cup (Fig. 51, i?) is perfectly uniform in structure, 

 being composed of elongated somewhat spindle-shaped 

 cells, with distinct nuclei. On its external (posterior) 

 surface a distinct cuticular membrane, the membrana 

 limitans externa, early appears. 



As the wall increases in thickness, its cells multiply 

 rapidly, so that it soon appears to be several cells thick : 

 each cell being however probably continued through 

 the whole thickness of the layer. The wall at this 

 stage corresponds closely in its structure with the brain, 

 of which it may properly be looked upon as part. Ac- 

 cording to the usual view, which is not however fully 

 supported by recent observations, the retina becomes 

 divided in its subsequent growth into (1) an outer 

 part, corresponding morphologically to the epithelial 

 lining of the cerebro-spinal canal, composed of what 

 may be called the visual cells of the eye, i. e. the cells 

 forming the outer granular (nuclear) layer and the rods 

 and cones attached to them ; and (2) an inner portion 

 consisting of the inner granular (nuclear) layer, the 

 inner molecular layer, the ganglionic layer and the 

 layer of nerve-fibres corresponding morphologically to 

 the substance of the brain and spinal cord. 



The actual development of the retina is not thoroughly 

 understood. According to the usual statements (Kolliker^) the 

 layer of ganghon cells and the inner molecular layer are first 

 differentiated, while the remaining cells give rise to the rest 

 of the retina proper, and are bounded externally by the membrana 

 limitans externa. On the inner side of the ganghonic layer the 

 stratum of nerve-fibres is also very early established. The rods 



' Entwick. d. Menschen, etc., 1879. 

 F. & B. 10 



