THE DEVELOPMENT OF THE COMPOUND EYE. 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 is a rudimentary optic nerve; it consists chiefly 

 of neuroblastic cells and their processes, and exhibits a distinct 

 central cavity (Pis. 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 as a 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 (PI. XXXVIII. , 

 Fig. 6, nd). 



The branching cells of the sub-dioptric space are connected 



362 



