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 numher 

 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 a , 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 



