160 BULLETIN OF THE 



granular substance of the mass also closely resembles the granular ma- 

 terial of a cone. For these reasons it seems probable that the granular 

 nucleated masses in the retinal region of C. pellucidus are the degen- 

 erate representatives of the cones in normal eyes. 



The fact that, of all the ectodermic elements of the retina, only the 

 granular nucleated masses continue to be differentiated, throws them 

 into strong contrast with the surrounding structures. The retention of 

 these masses may mean that on account of their extreme differentiation 

 they have had time to respond only incompletely to the influence of 

 degeneration ; or it may imply that phylogenetically they were among 

 the earliest retinal structures differentiated. Admitting them to be 

 degenerated cone-cells and merely dioptric in function, one can scarcely 

 conceive how they could have been differentiated before the sensory 

 cells which they serve. But even if they cannot be regarded as more 

 primitive structures than retinulse, their retention still may be signifi- 

 cant, as an indication that the ommatidia of primitive crustaceans con- 

 tained cone-cells as well as retinulse. 



Former studies have led me to believe that the difference in the 

 ommatidia of various crustaceans could be explained on the assump- 

 tion that the number of elements has been gradually increased from 

 lower to higher forms by cell-division. The simplest conceivable rep- 

 resentative of an ommatidium in the Crustacea might then be a sin- 

 gle cell. This would be of course a sensory cell ; by its division, 

 the more complicated ommatidia might subsequently be derived from it. 

 In such an event, the cone-cells must be modified sensory cells ; but 

 the fact that these cells persist in so rudimentary a retina as that of 

 C. pellucidus points rather to the conclusion, that they are probably 

 almost as old, phylogenetically, as the retinulas themselves, and that 

 primitive ommatidia consisted of at least two kinds of cells, sensory 

 cells or retinulse, and cone-cells, derived not from degenerated sensory 

 cells, but from the undifferentiated hypodermis. 



As I have already shown, the results which Newport, Leydig. and 

 Packard arrived at are not always in agreement. This might be ex- 

 plained by the fact that the organ under consideration is a degenerated 

 one, and consequently subject to considerable individual variation. This 

 supposition, however, is not supported by anything I have observed. 

 The preceding account of the eye in C. pellucidus is based upon the 

 examination of three individuals. These were respectively 6.5 cm., 

 5.6 cm., and 4.4 cm. long. Figure 2 was drawn from the optic stalk of 

 the shortest individual. In all essential features the eyes of the two 



