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. 
