VIII 



ARTHROPODA 



275 



the place the ocelli originally occupied. The remnants of the ocelli 

 remain during life as closed pigrnented vesicles attached to the 

 optic nerves. The horseshoe-shaped pigmented area is, of course, 

 the " imaginal disc " of the adult compound eye. The ectoderm here 



b.c. 



FIG. 221. Two stages in the development of the compound eye of Dytiscus marginalia 

 as seen in longitudinal section. (After Giinther. ) 



A, in the stage of first differentiation of the retinulae. B, in the stage of crystalline cone-forma- 

 tion, b.c, basal cell ; l.c, cells which form the lens ; pig, pigment-secreting cell ; rh, rhabdome 

 rt, rudiment of retinula ; vlt.c, crystalline cone cell. 



becomes thick and consists of narrow columnar cells, whose nuclei are 

 disposed at several levels though they still constitute a single layer 

 of cells. 



At one end of the horse-shoe there is a spot where the cell limits 

 cannot be distinguished, and the ectoderm appears to consist of a 



