THE DEVELOPMENT OF THE COMPOUND EYE. 543 



different description of the manner in which the compound eye 

 is developed, and their observations agree very closely with my 

 own. Claparede, describing the development of the eye in 

 Vanessa and in some Hymenoptera, stated that it is formed 

 from two layers one derived from the supra -cesophageal 

 ganglion, from which the nervous elements are formed, and the 

 other from the cutaneous hypoderm, which becomes the non- 

 nervous part of the eye. He says : ' The eye is formed from 

 two parts, which remain for a long time distinct, of which one, 

 derived from the hemisphere of the larva, becomes the purely 

 nervous part, the optic ganglion ; whilst the other forms the 

 faceted cornea, the chamber, the crystalline cone, and the 

 prismatic so-called nervenstab and its envelopes. All these 

 parts originate from the same cell mass, out of which the 

 antennae, the proboscis, and the whole head are developed.' 



Weismann, describing the development of the eye in Musca, 

 says : ' On the fifth day of the pupa the optic ganglion consists 

 of a nearly spherical cell mass, which is about double the size 

 of the rest of the supra-oesophageal ganglion, from which it 

 is developed by the formation of a constriction. I term this 

 purely nervous part of the eye the bulbus. The bulbus is 

 attached by a broad base to the supra-cesophageal ganglion, 

 and is covered on its outer surface by the lappet-like (lappen- 

 formigen) eye disc. 



' Between the bulbus and the disc a thin layer of fat and 

 granule cells penetrates, from which very probably the cells 

 are developed, which unite the two surfaces. 



' Let us first follow the development of the eye disc, which is 

 a thin cellular layer of considerable extent, enveloping the 

 anterior part of the central nervous system like a watch-glass. 

 Its outer surface at the time of going into the pupa exhibits 

 already an arrangement of cells, representing the corneal facets. 

 They are like the cells of the other imaginal discs but larger, 

 spheroidal, very transparent, and covered by a thin membrane. 



' The thickness of the eye disc, even on the twelfth day, in 

 Sarcophaga is so little that it is easy to believe that it only 

 forms the cornea. It is then only '05 mm., but it contains all 



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