404 THE BET IN A. 



The Retina. At first the two walls of the optic cup do not greatly 

 differ in thickness. On the third day the outer or posterior becomes 

 much thinner than the inner or anterior, and by the middle of the 

 fourth day is reduced to a single layer of flattened cells (fig. 289 

 p.Gh). At about the 80th 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 mem- 

 brana 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 supported by the development, 

 the retina becomes divided in the 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 walls of the brain. According to Lowe, how- 

 ever, 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 statement.s (Kolliker, No. 298, ]). 693) the layer of 

 ganglion cells and the inner molecular layer are first differentiated, 

 wliile the remaining cells give rise to tlie 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. 



Lowe's account of the development of the retina in the Rabbit is in many 

 points different from the above. He finds that three stages in thediHeren- 

 tiation of the layers of the retina may be distinguished. 



In the first stage, in an embryo of four or five millimetres, the following 

 layers are present, commencing at the outer side, adjoining the external 

 "Wall of tlie secondary optic ou[>. 



