140 PROCEEDINGS OF THE ACADEMY OF [Feb., 



cells secrete on their distal surface while the retinular cells secrete on 

 their lateral surfaces. My observations show that both cone cells and 

 retinular cells form their secretions intracellularly and from their very 

 positions they could not secrete on their distal surfaces, but this does 

 not seem to me to be any objection to the theory of Watase, since in 

 the invagination of the cells and the taking on of new functions new 

 forms of metabolic activity might easily be acquired. 



Since, however, in the embryonic development of the ommatidium of 

 the bee we find a stage in which the retinula is formed without cone 

 cells on the distal end and with the rhabdome partly formed, the only 

 inference, it seems to me, is that the cone arises from lateral cells, and 

 the corneal and outer pigment cells are, of course, still more peripheral. 

 From this, then, it seems to follow that the conception of Watase con- 

 cerning the morphology of the ommatidium is the correct one. There 

 is, so far as has been observed, no real invagination, but such a thing 

 would scarcely be expected in so compact an organ; neither have I 

 observed the actual overgrowing of the cone cells, but the conclusion 

 seems inevitable that the retinula is the centre of the ommatidium. 



Some compound eyes have been described in which, in the adult eye, 

 the retinular cells extend outside the cone to the lens. Such cases are 

 found when the number of pigment cells is reduced or when they are 

 entirely wanting, and it is safe to assume that the distal lengthening 

 has taken place secondarily, late in development. From the migration 

 of the corneal pigment cells of the ommatidium of the bee, to be de- 

 scribed later, we see that a late rearrangement is possible, and it seems 

 more plausible to assume that such cases are a secondary modification 

 rather than that there are two ground plans of ommatidia, one of 

 which has its retinula centrally placed, the other has the cone cells 

 inside the retinula as the axis. 



The retinular spindle of the larva resembles in appearance various 

 sense buds throughout the animal kingdom, such as taste buds and 

 lateral line organs of vertebrates, the aesthetes of Chitons, etc. These 

 sense buds often have some marked differentiation of the cytoplasm 

 internally to enable the peripheral organ to perform its function. This 

 similarity is more than superficial, however, for the method of innerva- 

 tion which will be described in detail later is from the sense cell toward 

 the central nervous system, and this is the method for many of these 

 sense buds, although the opposite direction of fibres is described for 

 some (e.g., taste buds). 



It is-^safe to assume that these sense buds are accumulations of 

 single sensory cells, such as are widely known (e.g., sensory epithelial 



