118 BULLETIN OF THE 
The termination of the fibrille of the optic nerve in the rhabdome 
supports Miiller’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 fibrille 
are distributed to the cone. I am therefore compelled to think that 
these authors are mistaken in their conclusion. 
THEORETIC CONCLUSIONS. 
4 
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 Stomatopods, 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 
