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Mr. B. T. Lowne on the Modifications of [Mar. 28, 



confounded with sub-corneal nuclei of the eye in the Diptera and some 

 other insects, but they are, I believe, very different structures, mor- 

 phologically speaking. The other four form the hard cone, or, as I 

 shall call it, the scleral cone. These remain distinct throughout the 

 life of the insect, as the cone always splits most readily into four equal 

 segments corresponding to the four primitive cells. 



An examination of the eye in moths has shown that the segments of 

 the scleral cone are prolonged as threads into the rhabdion. The threads 

 are apparently viscous immediately after death, as they are liable to 

 contract and form globular or pear-shaped drops, as the inner segments 

 of the rods and cones of some vertebrates do, according to the observa- 

 tions of Max Schultze.* 



The chamber in which the cone lies is -usually lined with pigment 

 cells. These are very remarkable in structure, reminding me of the 

 retinal pigment cells of vertebrates ; they are generally arranged round 

 the apex of the cone, and give off numerous thread-like processes which 

 surround the cone. 



Although the compound eye in insects exhibits very considerable 

 modifications in different families, the researches of Claperedef show 

 that these are all developed from a condition closely resembling the 

 type I have just described. The primitive zone consists of eight cells : 

 four of which Claperede speaks of as the ceils of " Semper," and four 

 of which he terms cells of the cone. There is every reason to believe 

 that the scleral structures are formed in the interior of the latter, 

 which, in moths, become entirely converted into scleral tissue ; the same 

 occurs, according to Ley dig, J in some Coleoptera, as Cantharis 

 melanura and Elater noctiluca : I have observed the same thing in 

 the eye of Dytiscus, and, as is well known, this condition appertains 

 in Hyperia, amongst the Crustacea. 



I think there is little doubt, from the observations of Leydig and 

 Dr. Grenadier, that the eyes of most, if not of all, the pentamerous 

 Coleoptera retain the primitive structure of the cone throughout life, 

 and those of many Crustacea present cones which are either partially 

 or entirely converted into scleral tissue. § 



I propose to term these two forms of eye, proto- and sclero- conic. 



Starting from the proto-conic eye : There are two extreme forms 

 of deviation ; in the one, the cone disappears, and its place is occupied 

 by a slightly coagulable fluid. This is the case in the eyes of the hete- 

 rocerous Diptera. In these insects four portions of protoplasm remain 

 attached to the outer extremity of the rhabdion ; further observations 

 are needed in the development of these structures, and as to the 

 origin of the fluid ; but analogy, with the conditions in some of the 



* " Arcliiv.," Bd. iii. f " Kol. Zeitsch.," Bd. viii. 



t " Miiller's Archiv.," 1855. 

 § Newton, " Eye of the Lobster," " Quarterly Journ. Mic. Sc.," 1874. 



