MUSEUM OF COMPARATIVE ZOOLOGY. 121 



retinula of Idotea robusta represents, for reasons already stated, the 

 seventh cell present as a functional structure in Porcellio. In Idotea 

 irrorata the retinula?, with very few exceptions (Plate V. Fig. 54), contain 

 only six cells, showing no trace of the seventh cell. This condition, I 

 believe, is to be interpreted as one in which a cell has been completely 

 suppressed. In Stomatopods, Schizopods, and Decapods the retinula? 

 have been shown to contain, in addition to the nuclei of the seven func- 

 tional cells, an eighth nucleus, which may represent a rudimentary cell. 



In all of the cases thus far cited, it might be maintained that what I 

 have considered rudimentary cells are really cells newly acquired by the 

 ommatidia, and not old cells gradually undergoing suppression. The con- 

 dition in Idotea, however, where the body in question apparently contains 

 no nucleus, would be difficult to explain on this assumption, whereas, if 

 it be considered a cell undergoing reduction, its condition can be easily 

 accounted for. In Stomatopods, Schizopods, and Decapods, the con- 

 stancy in the number of cells and in the position of the eighth nucleus, 

 the small amount of protoplasm which surrounds it, and the striking 

 resemblance which it has to the other retinular nuclei, are facts difficult 

 to explain on the assumption that it represents a newly acquired cell, 

 but easily accounted for on the supposition that it is the remnant of a 

 partially suppressed cell. For these reasons, I believe that the instances 

 cited are valid cases of partial suppression, and that this must be regarded 

 as one of the actual means employed in the modification of ommatidia. 



That ommatidia have been modified by an increase in the number 

 of their cells by cell division, is a proposition not easily established. 

 The difficulty of obtaining conclusive evidence on this point can be 

 made clear by an example. Let it be assumed that cones composed 

 of two cells are converted by the division of the cells into cones com- 

 posed of four cells. This step, even when first taken, would probably 

 be accomplished during the embryonic growth of an animal, and there- 

 fore before the cones themselves had begun to be differentiated. . What 

 would actually happen would probably be this : the two cells, the 

 homologues of which in all previous animals had given rise to two cone 

 cells, would in this case each divide, thus producing a group of four 

 cells, which ultimately would form a cone of four segments. If we 

 could compare the adult animal in which such a process had occurred 

 for the first time with its immediate ancestors, the only important 

 difference that would be observed would be in the number of the 

 cells in each cone, and if the genetic relations of the two individu- 

 als were not known, it could not be stated with certainty whether in 



