168 Mr. H. M. Bernard on 



common type of sensory epithelium, and, as far as the retina 

 is concerned, describes fairly well the undifferentiated portion 

 of that structure as seen in the eyes of tadpoles round the 

 rim of the retinal cup, although the layers of nuclei are 

 even here already numerous. For some little distance round 

 the outside of tliis rim the granule-bearing cells of the 

 choroidal epitlielium are in close contact witli the external 

 limiting membrane of the retina — that is to say, there are no 

 rods or cones keeping them at a distance. As we go further 

 from the rim these begin to appear, and the manner of their 

 appearance seems to me to be very significant. 



What first strikes one is that they are far less numerous 

 than the nuclei, which at this point have still further in- 

 creased in number without any very marked signs of differen- 

 tiation among them. So many nerve-cells with so few 

 terminal structures involuntarily suggest that these end- 

 structures, the " rods and cones," must contain many separate 

 nerve-fibrils. This suggestion finds support in the irregular 

 thickness of the rods and cones. Some receive far more 

 nerve-fibrils than others. There would thus be no morpho- 

 logical difference between these structures, i. e. between 

 rods and cones, whatever difference there might be among them 

 as to length or thickness. The rod-like structures, however, 

 are not composed wholly of nerve-fibrils ; they may be 

 described as cuticular outgrowths from the sensory layer 

 projecting into the granule-cells lying in contact with it. 

 The cuticular outgrowths appear to rise from that layer of the 

 sensory cells which lies just within the external limiting 

 membrane, while from the crowd of undifferentiated sensory 

 cells in the deeper layers (lying nearer the centre of the 

 eye) fine nerve-fibrils descend from all sides and run out 

 along (? within) the supporting cuticular rods prepared for 

 them. This will explain the fibrous character of the proto- 

 plasm ascending from each rod and surrounding its own 

 special nucleus, which can be clearly seen under the micro- 

 scope in sections of the retina of the frog. It might also be 

 associated with the longitudinal striation which has long been 

 attributed to these structures. 



From all that has been said above as to the connexion 

 between the granules in the wandering-cells and integu- 

 mentary protective structures, the development of cuticular 

 rods is not surprising even in such a place. We have but 

 to assume that a certain number of granules succeed in forcing 

 their way into the outer layer of cells in the opposing sensory 

 tissue-layer, and that these cells work them up into protective 

 structures. The structures lake the form of rods projecting into 

 and still further impeding the advancing stream of the granules. 



