118 BULLETIN OF THE 



The termination of the fibrillac of the optic nerve in the rhabdome 

 supports Muller's belief that the nerve fibres terminate in a region near 

 the proximal ends of the cones, and Grenacher's more specific view that 

 they are connected with the retinular cells, and that the rhabdome is the 

 terminal organ. This method of termination is not consistent with the 

 opinion of Gottsche and Leydig, that the cone is the terminal organ, 

 nor with Patten's rather similar belief that the ultimate nerve fibrilhe 

 are distributed to the cone. I am therefore compelled to think that 

 these authors are mistaken in their conclusion. 



Theoretic Conclusions. 



In attempting to account for the variation in the number of cells in 

 different types of ommatidia, two courses naturally suggest themselves. 

 Either the different kinds of ommatidia vary in the number of cells 

 which they contain, because they have had separate origins, or they are 

 different because in some or all of them the ancestral ommatidium has 

 suffered modification. An examination of the table on page 115 shows 

 conclusively, I think, that in Crustaceans even the most extreme types 

 are so little removed from one another that it is much more probable 

 that the different kinds of ommatidia are genetically connected, than 

 that they have been produced independently. Granting this statement, 

 the question naturally arises, What are the means by which the primi- 

 tive ommatidium was modified? I believe that a close scrutiny of the 

 cellular structure of the ommatidia in living Crustaceans will disclose 

 some of the factors in this process. There are at least three of these to 

 be distinguished : the differentiation of cells, the suppression of cells, 

 and the increase in the number of cells by cell division. 



By the differentiation of cells, I do not mean the process by which 

 hypodermal cells have become converted into retinular or cone cells, 

 but that by which an element already differentiated in the ommatidium 

 is secondarily modified to subserve another function. The only instance 

 of this kind with which I am acquainted occurs among the retinular cells. 

 In the majority of the simpler Crustaceans, the sides of the cones are 

 covered with pigment, which is almost always contained in the distal ends 

 of the retinular cells. In Serolis, among the Isopods, and apparently 

 in all the genera of Ptomatopods, Schizopods, and Decapods, the cones 

 are surrounded by special pigment cells. These are always twice as 

 numerous as the ommatidia, and represent, I believe, retinular cells 

 which have become differentiated for the special purpose of sheathing 



