THE MEDIAN OCELLI, STEMMATA, OR SIMPLE EVES. Sir 
parent cylindrical rod, and a posterior nucleated protoplasmic 
cell directly connected with a terminal fibre of the nerve. 
The external segment varies from 10 » to 15, and the internal 
from 15 to 20m 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 
Fic. 67.—A vertical median section through the Ocellus of a Blow-fly Imago. 
Seen with } objective. 4, hypoderm ; 2, 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 all 
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 eells are pigmented with deep rose- 
34 
