152 PROCEEDINGS OF THE ACADEMY OF [Feb., 



Hesse seems more probable. As Hickson says, there is nothing mor- 

 phologically wrong with the supposition that certain cells are multi- 

 nucleate; but since the explanation of Hesse helps us to complete the 

 homologies of the cells of the ommatidia of the various groups, it 

 seems to have more weight. 



The question as to the method of modification in number of retinular 

 cells during the course of evolution is an interesting one, but it must be 

 admitted that as yet very little is known concerning it. It seems not 

 unlikely that the ommatidium of the bee is changing cither from eight 

 to nine retinular cells, or from nine to eight, since it is rather rare for 

 the number of these elements to be variable. The thought has sug- 

 gested itself that possibly this one proximal nucleus was one which was 

 in the process of delamination from the ommatidial epidermis, and was 

 therefore tending toward a reduction of retinal elements, but this does 

 not seem to be as probable an explanation as that of Hesse. It may 

 be said, however, that Johansen describes the ommatidium of Vanessa 

 as having seven retinular cells and two retinal ganglion cells, while 

 in Apis there is probably but one retinal ganglion cell to each omma- 

 tidium and at least one more cell in each retinula. 



VII. HOMOLOGIES OF COMPONENT PARTS. 



The question of homologies of the various eyes of the invertebrates 

 has excited much discussion, but since only compound eyes have been 

 investigated in this paper, this problem will not be taken up here. 

 The question of the homology of the different kinds of compound eyes 

 is worthy of consideration. Such eyes occur in Crustacea and insects, 4 

 and a comparison of the groups indicates that there is here either uni- 

 formity of origin and plan or one of the most remarkable cases of con- 

 vergence known in the animal kingdom. The essential part of the 

 ommatidium is the retinula, and this may be considered as a sense bud, 

 formed by the accumulation of cells sensitive to light, which has been 

 modified internally to aid in light perception. Since such groups of 

 cells occur throughout the whole animal kingdom and associated with 

 all the senses, there is nothing remarkable about the similarity so far. 

 In addition to the retinula, an ommatidium consists of a cone and a 



4 No account of the so-called compound eyes of myriapods and arachnids is 

 taken here, since their plan is so different that they cannot readily be homologized 

 with those of crustaceans or insects. Until we know more of the comparative 

 embryology of these forms it may be as well to suspend judgment. I do not feel 

 qualified to include these in the present discussion, but evidently from the re- 

 searches of numerous investigators we may conclude that the homology is not as 

 close as in the forms under discussion. 



