200 fatten: [Vol. I. 



phorae are rather large, round nuclei belonging to other pigment- 

 cells of the ommatidia. Although there are, at first, but two retino- 

 phorae for each ommatidium, this number is increased to four 

 just before the first formation of pigment in the eye, by the 

 appearance of two very small cells just below the nuclei of the two 

 retinophorae already present. These two cells continue to 

 grow until all four are equal in size and configuration, forming 

 the cup-like swelling of the outer ends of the retinophorae, which 

 contains eventually the crystalline cone, but which is now almost 

 filled by the four nuclei. 



Pigment first appears in the outer ends of the retinula cells, 

 as a pair of dark spots on either side of the retinophorae, just 

 below the neck of the calyx. Each dark spot belongs to a single 

 retinula cell. The other retinula cells subsequently develop 

 pigment in the same way. But for a long time the pigment re- 

 mains as two distinct semicircular areas with the neck of the 

 calyx in the middle. These two patches of pigment finally unite, 

 so that the base of the calyx is completely enclosed in a circle of 

 pigment. 



Pigment is next deposited in the inner ends of the retinula 

 cells. Finally, pigment appears diffusely distributed in the 

 cells whose expanded outer ends surround the calyx. 



The folloiving facts are worthy of special notice: — i. The 

 crystalline-cone cells, or any of the eventually piginejited cells sur- 

 rounding them, do not for Jti a layer of cells distinct f'om and 

 superimposed on the retinulcB; on the contrary, the crystalline-cone 

 cells, the retinulcB, and the other pigmented cells are derived from, 

 and remain, a single layer of cells, 2. The rhabdom is not a pro- 

 diict of the retinulcB; it is merely the inward prolongatioji, or stalk, 

 of the crystalliiie cone-cells. 3. The layer of cells from which 

 the ommateum arises is the inner wall of an optic vesicle formed 

 by an invagination of the ectoderm, consequently the ommateal 

 cells are upriglit. 4, The retinophorcs which, in the adult, are 

 grouped in fours, in the youngest stages are arranged in twos, 

 thus repeating the permanent condition of the retinophorce found 

 in the ocelli of most Insects, and in the simpler compound eyes 

 of Crustacea. 5. TJie pigment first appears as paired patches 

 around the paired retinophorce and is retained until after the 

 retinophorce have increased to four. This transitory condition of 

 the ommatidial cells in the compound eye probably corresponds 



