MUSEUM OF COMPARATIVE ZOOLOGY. 47 
| central axis. With very few exceptions, the different ommatidia in the 
retina of any given Crustacean agree with one another in the number 
and arrangement of their cells ; in other words, in a given retina any 
ommatidium is the structural duplicate of any other. This uniformity 
suggests the idea of a structural type, and already a number of such 
types have been described. Some of these find representatives appar- 
ently only in the ommatidia of a single species, but more frequently the 
type characterizes a genus, family, or even a sub-order. Types differ 
from one another, either in the number of their cells or in the arrange- 
ment of these cells. Of these differences, the one which involves a 
variation in the number of cells is the more fundamental. This dif- 
ference, however, has probably arisen by the gradual modification of 
an ancestral type, and, granting this, it follows that the ommatidia of 
one type are genetically connected with those of other types. This 
leads directly to the statement of the principal question, namely, What 
are the means by which ommatidial types are modified, and what is the 
significance of the changes through which these types pass ? 
This question, although easily stated, is not so easily answered ; the 
facts presented in the following pages cannot be said to settle it, and 
yet they seem to me to increase materially the possibilities of its 
solution. 
A partial answer to at least the first portion of the question has al- 
ready been suggested (Parker, ’90*, pp. 56-58) ; it can be briefly stated 
as follows. There is reason for believing that those ommatidia which are 
composed of a small number of cells more closely resemble the ancestral 
type than those composed of many cells. Granting this statement, one 
would naturally expect that the more complex ommatidia had been de- 
rived from the simpler ones by an increase in the number of their ele- 
ments. Perhaps the most natural method by which this increase could 
be accomplished would be by the further division of the cells already 
forming the ommatidium. Consequently, cell division in this sense 
seemed to me to afford a sufficient means for the modification of om- 
matidial types. In the present paper it is in part my purpose to show 
precisely to what extent cell division can be said to have modified om- 
matidia, and to determine whether any other factors have been involved 
in this process, 
THe Retina. 
The retina in those Crustaceans in which its development has been 
studied originates as a thickening in the superficial ectoderm. At least 
