VIII 



AETHEOPODA 



275 



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

 remain during life as closed pigmented vesicles attached to the 

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

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



Pig. 221. — Two stages in the development of the compound eye of Dytiscus Tnarginalis 

 as seen in longitudinal section. (After Gtinther. ) 



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

 tion. &.C, basal cell ; l.c, cells which form the lens ; pig, pigment-secretiDg cell ; rfc, rhabdome 

 rt, rudiment of retinnla ; vit.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 



