154 PROCEEDINGS OF THE ACADEMY OP [Feb., 



from embryological evidence. The problem is, which of the two 

 methods of formation previously mentioned is the method which actu- 

 ally exists in ontogeny, and are all compound eyes formed by the same 

 method? From this work on Apis and that of Johansen on Vanessa it 

 is evident that the differentiation of cells outside the retinula to form 

 cone and lens layers is what occurs in insects, and the whole question 

 hinges on the development of the crustacean eye. Reichenbach and 

 Kingsley describe the eye as arising by an invagination; and if either 

 of these investigators is right, although they differ as to the fate of 

 the three layers formed, then the compound eyes of these crustacean.s 

 are not homologous with the compound eyes of iasects. On the other 

 hand, Herrick insists that the compound eye of Alpheus arises from a 

 single layer of epidermis, and according to this view the homology 

 holds. Herrick's view, that even if an invagination does occur it is 

 of no importance, does not seem tenable, for if an invagination occurs 

 then cone and retinula do not come from contiguous cells, and that I 

 beheve to be a matter of great importance. 



From the striking similarity in position and function of the parts 

 of the ommatidium, and from the observations of Herrick, we are safe 

 in concluding that the eyes of the various groups under consideration 

 are distinctly homologous, and there must be some other explanation 

 for the invaginations observed by the other writers mentioned. 



The interpretation of the formation of the ommatidium which is 

 held from an examination of the eye of Apis makes possible a very 

 close homology of the elements of the compound eye with the ocelli 

 of insects, such as was held by Grenadier; and this homology seems 

 materially strengthened since an homology can be shown between 

 the corneal pigment cells of insect ommatidia with the chitin-secreting 

 cells of the ocelli. An objection that might be raised is that the 

 vitreous body of the ocellus arises from cells which are all to one side of 

 the retina rather than from all sides, but since they are adjoining cells 

 this might be a secondary change. From sections of ocelli of the pupae 

 of the bee which have been examined, it is evident that the middle 

 ocellus arises from a double invagination, indicating a fusion of two 

 organs, while the lateral ocelli arise from single invaginations. 



VIII. — Summary. 



The primitive arrangement of ommatidia is tetragonal (p. 130). 

 The hairs over the lens are secreted by bi-nucleated hair cells with 

 intracellular ducts which lie between the ommatidia (p. 131). 



The ommatidium arises as a group of cells with superimposed nuclei, 



