THE DEVELOPMENT OF THE COMPOUND EVE. 545 
b. The Development of the Compound Eye in the Blow-fly. 
It will be convenient to divide the development of the eye 
into four stages. 
First Stage of Development.—In the larva of the Blow-fly the 
optic disc is connected with the central nervous system by the 
optic stalk, Weismann’s Nervenstiel, so that it has the form of 
a mushroom, the optic stalk forming its stem. The relation 
of the optic stalk with the central nervous system indicates un- 
doubtedly that it isa rudimentary optic nerve; it consists chiefly 
of neuroblastic cells and their processes, and exhibits a distinct 
central cavity (Pls. III. and IV., os). It expands beneath the 
optic disc, and may represent a rudimentary retina; and it is 
covered by a thin layer of parablastic cells, its peritoneal 
covering. 
In this stage the eye disc differs in no respect from the other 
imaginal discs, so that there is no reason to suppose that its 
nervous stalk possesses any functional activity asa nerve of sight. 
The disc is neither pigmented nor has it any special end organs. 
I regard the stalk of the optic disc as the morphological repre- 
sentative of the optic nerve and retina of the larval eye in the 
Ametabola. 
Second Stage of Development.—This stage is seen in nymphs 
on the third day of the pupa. The neural disc of the optic 
stalk separates from the dermal disc except at its periphery 
(Fig. 71); and the intermediate space, the sub-dioptric space, 
is seen to be occupied by a delicate reticulum of branching 
cells, the inter-spaces of which are filled with blood and 
granule cells. 
The neural disc exhibits distinct fibres, resembling those 
of the stalk, and these either end in cells, which are im- 
bedded in the substance of the disc, or{in flattened epithelial 
elements which form a distinct layer on, the external surface 
of the disc. These cells are apparently continuous with the 
peritoneal investment of the optic stalk (Pl. XXXVIIL., 
Fig. 6, nd). 
The branching cells of the sub-dioptric space are connected 
36—2 
