292 S. WATASE. 



rptiiuilii, then, astrccs with the corncai;en and vitrclla whicli lie 

 ahovo, in its capacity to secrete chitin on a part of its surface. 

 Thus the three strata of cells, a, h, and c, agree fundamentally 

 ■with one another in their capacities to secrete chitinous structures, 

 and those arc respectively known as the cornea (1), the crystal- 

 line cone (2), and the rhahdomere (3). 



Hence, aside from the general homology which exists between 

 the cells in the optic area, all being derived from a common 

 source, the ectoderm, there exists a speinal homology between 

 the cells in the three strata which constitute the important parts 

 of the ommatidiujn, all being characterized by the capacity for 

 secreting chitin from a part of their surface. 



In this respect these ommatidial cells are essentially like the 

 general ectodermal cells which sec^rete the chitinous covering of 

 the body from their distal extremities. 



In carrying this analysis further we find that tlic outer surface 

 of the corneagen is homologous with the axial stirface of the 

 vitrella, and that of the retinula and the three chitinous structures, 

 cornea, crystalline cone and rhahdomere, are homologous with 

 each other. 



If this interpretation be established by the further discussion 

 of the ommatidia of other forms, it follows that the crystalline 

 cone and rhahdomere are really homologous with the chitinous 

 investment of the body, and the narrow axial space of the 

 ommatidium represented by the line x must therefore have the 

 same significance as the space outside of the body. Morpholo- 

 gically speaking, the crystalline cone and rhahdomere are just 

 as much a part of external surface as the cornea itself. Diagram- 

 atically, the three strata of the ommatidium may be represented 

 as in Fig. 2, PI. XXIX. 



According to this view, the ommatidium of Serolis may be 

 regarded as an open pit of the ectoderm, the walls and margin 

 of which are covered by the product of their own secretion, the 

 chitin. The chitin in the walls and margin of the pit becomes 

 continuous with the general chitinous covering of the body, as 

 the rows of the ommatidial cells themselves become continuous 

 with the general ectoderm. 



At the bottom of the ommatidial ]>it there exists a pair of 

 colorless " hyaline cells" (Beddard), with tlieir upper processes 



