ORGANS OF VISION OF THE VERTEBRATA. 491 



cells (fig. 28g,fl.C/i). At about the 8oth hour its cells commence 

 to receive a deposit of pigment, and eventually form the so-called 

 pigmentary epithelium of the choroid ; from them no part of the 

 true retina (or no other part of the retina, if the pigment-layer in 

 question be supposed to belong more truly to the retina than to 

 the choroid) is derived. 



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

 (fig. 289, R} has a perfectly uniform 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 becomes 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. 

 According to the usual view, which is not however fully sup- 

 ported by the development, the retina becomes divided in the 

 subsequent growth into (i) an outer part, corresponding morpho- 

 logically 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 walls of the brain. According to Lowe, 

 however, only the outer limbs of the rods and cones, which he 

 holds to be metamorphosed cells, correspond to the epithelial 

 layer of the brain. 



The actual development of the retina is not thoroughly understood. 

 According to the usual statements (Kolliker, No. 298, p. 693) the layer of 

 ganglion 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 ganglionic layer the stratum of nerve-fibres is also very early 

 established. The rods and cones are formed as prolongations (Kolliker, 

 Babuchin), or cuticularizations (Schultze, W. Miiller) of the cells which 

 eventually form the outer granular layer. The layer of cells external to 

 the molecular layer is not divided till comparatively late into the inner 

 and outer granular (nuclear) layers, and the interposed outer molecular 

 layer. 



