THE COMPOUND EYE OF THE BLOW-FLY. 527 



appear to consist of a thin capsule, from which a fluid of high 

 refractive power may be seen escaping. 



From the above facts I arrive at the following conclusions : 

 (i) The cornea consists of a cuticular membrane with lenses 

 beneath it. (2) That the lenses are each developed from four 

 cells. (3) That the lenses consist of a stroma infiltrated with 

 an oil-like fluid. Hickson [237], without troubling himself 

 about my facts, takes exception to this view, and says he sees 

 nothing to justify it. 



The Nuclei of Semper. The ' Nuclei of Semper ' play a very 

 important part in many of the descriptions of the compound 

 eye hitherto published, although it is difficult in all cases to 

 know what nuclei are referred to. 



Claparede, in 1860, wrote : [203] ' In the spring of last year 

 I travelled from Altona to Paris with Carl Semper, who was 

 about to depart for the Philippine islands '; and we learn that 

 Semper gave him a box of slides of insects' eyes, and pointed 

 out to him ' four nuclei between the cornea and crystalline 

 cones,' which Claparede determined to name ' Semper's 

 nuclei,' 



Now, fortunately, Claparede figured these so-called nuclei 

 [203, Fig. i], and it is evident from his figure that he had not 

 nuclei before him, but a lens split into four parts. 



Claparede also figured the partially-developed eye of Sphinx 

 Euphorbias, and represented the nuclei of the cells from which 

 the corneal lens is developed, he also called these ' Semper's 

 nuclei ' and these nuclei have been confounded with the four 

 segments of a split lens ever since. 



Nor is this all. As Claparede has shown, and as is perfectly 

 clear in my preparations, numerous nuclei are seen in relation 

 to the cone, cornea and surrounding cells at one period or other 

 of development, and it is exceedingly difficult to identify 

 ' Semper's nuclei,' so that various nuclei have been described 

 as ' Semper's '; this has led to considerable confusion. I shall 

 therefore cease to speak of ' Semper's nuclei,' and designate 

 the nuclei as ccrneal or nuclei of the cone, or of the pigment 

 cells. 



35 



