126 PROCEEDINGS OF THE ACADEMY OF [Feb., 



III. — Adult Form of Eve and Ommatidium. 



Before takint^ up the emljryonic development of the eye, it may l)e 

 well to give a short description of the adult form so that what follows 

 concerning the embryology may be more clear. 



The compound eye is made up of great nimibers of similar ommatidia 

 surroimded ])y pigment cells. Among the pigment cells lying between 

 the ommatidia are large numbers of hair cells which secrete the long 

 unbranched haii's which cover the surface of the eye. The entire eye 

 is covered by a layer of chitin, continuous with the chitin of the body 

 wall. 



Each ommatidium is composed of an external lens, a crj^stalline 

 cone and a rhabdome surrounded by eight or occasionally nine long 

 retinular cells which extend from the cone to the basement membrane. 

 Besides these are two pigment cells, the corneal pigment cells, which 

 surround the crystalline cone and about twelve which run the entire 

 length of the ommatidium from the lens to the basement membrane, 

 making in all about twenty-eight cells to each ommatidium. It might 

 be well to include in this enumeration a retinular ganglion cell, since 

 there is probably one to each ommatidium. The lens is chitinous and 

 quite thick and complicated in structure, as will be shown later. The 

 crystalline cone is composed of four highly modified cells, whose nuclei 

 have almost entirely disappeared, and the cells are modified internally 

 by the accumulation of an intercellular secretion which is more dense 

 than the original cytoplasm, but not so dense as the lens. The retinu- 

 lar cells begin at the apex of the cone, and from their proximal ends at 

 the basement membrane send in neural fibres to the retinular ganglion. 

 Each of the retinular cells has decome differentiated on its inner face 

 to form a part of the rhabdome which extends from the apex of the 

 cone to the distal side of the basement membrane. This rhabdome 

 is not chitinous, but is perfectly transparent and slightly more dense 

 than the surrounding cytoplasm. The corneal pigment cells and the 

 outer pigment cells which surround the entire ommatidium serve to 

 prevent the reflection of light in the ommatidium and the passage of 

 rays of light from one ommatidium to another. 



IV.— Embryology and Structure in Detail. 



1. The Entire Eye. 



The eye arises as a thickening of the hj^podermis on the side of the 

 head just posterior to the place where the antennae arise and just 

 over the optic ganglia. This thickening takes place before the larva 



