THE MEDIAN OCELLI, STEM M ATA, OR SIMPLE EYES. 511 



parent cylindrical rod, and a posterior nucleated protoplasmic 

 cell directly connected with a terminal fibre of the nerve. 

 The external segment varies from 10 fj, to 15 /JL, and the internal 

 from 15 /u. to 2O/JL in length. 



The rod cells of the retina of the simple eye are strikingly 

 similar to the layer of rods or bacilli to be hereafter described 

 in the compound eye, and also to the rods of Jacob's mem- 

 brane in the vertebrate eye. The external segments correspond 

 in their transparent appearance and in the manner in which 

 they are acted upon by various reagents with the external 

 ends of the rods of the vertebrate eye, but are not inverted 



FIG. 67. A vertical median section through the Ocellus of a Blow-fly Imago. 

 Seen with J objective. //, hypoderm ; ;;, nerve ; s, preretinal space. 



as they are in the vertebrate retina. The inner extremities 

 differ, however, in being nucleated both from the inner ex- 

 tremities of the bacilli in Vertebrates and from the inner 

 extremities of the rodlets in the compound eye which I regard 

 as retinal end organs. These organs are admitted by aH 

 previous writers to be the true nerve end organs of the simple 

 eye. 



The retina is surrounded by a layer of large but very thin 

 pigment cells, which are apparently continuous with the 

 nerve-sheath. These cells are pigmented with deep rose- 



34 



