130 BULLETIN OF THE 



ticular lens. Although there cannot be the least doubt that in this case 

 each pit is a hypodermal involution, the belief that each one is homolo- 

 gous with an ommatidium is by no means so well founded. In structure 

 the wall of the pit differs considerably from that of an ommatidium ; it 

 contains no cells which can be definitely denominated, either as cone 

 cells or as cells of the corneal hypodermis, and it does contain a large 

 ganglionic cell, which is only questionably homologous with any element 

 in an ommatidium. In most respects in which these pits differ from 

 ommatidia, they resemble simple eyes, and I therefore regard them as 

 such, rather than as representatives of an early condition in the forma- 

 tion of an ommatidium. 



When to the objections raised in the preceding paragraphs the state- 

 ment is added, that in both Homarus and Gammarus — representatives 

 of the extremes of organization — the ommatidia are developed without 

 showing any trace of infolding, Watase's theory of the formation of om- 

 matidia by means of involutions appears in a still less favorable light. 

 I therefore regard ommatidia, not as the result of involutions, but as 

 differentiated clusters of cells in a continuous unfolded epithelium. 



I have not observed anything that would lead to the conclusion re- 

 cently expressed by Patten ('90), that an ommatidium is a hair-bearing 

 sense bud. I believe, on the contrary, that they have had a very differ- 

 ent origin. 



In conclusion, I may add, that if my idea of the origin of ommatidia 

 be correct, it supports Grenacher's opinion, that compound eyes are 

 not derived directly from aggregations of simple eyes, but from groups 

 of optic organs which were even more primitive in their structure than 

 simple eyes. Possibly such primitive organs were the antecedents of 

 both the compound and simple eyes of Arthropods, as Grenacher sug- 

 gests ; but possibly the two kinds of eyes may have had totally different 

 origins. 



