THE DEVELOPMENT OF THE COMPOUND EVE. 543 
different description of the manner in which the compound eye 
is developed, and their observations agree very closely with my 
own. Claparéde, 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 
antennz, 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-cesophageal 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 (Jappen- 
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 ‘o5 mm., but it contains all 
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