238 
SYDNEY J. HICKSON. 
is the fact that Ley dig (11) discovered that they are possessed 
of a true retina purple. As this substance is not known to 
occur anywhere in the animal body but in the visual nerve- 
end cells, and as the retinulae of Arthropods possess this 
substance and Lowne’s bacillar layer does not, the retinulae 
must be the nerve-end cells of the Arthropod eye. 
This being the case, the terms “Dioptron,” and “ Neuron,” 
“ Retina/’ and “ Great rods,” introduced by Lowne, must be 
rejected, and the nomenclature used which has been introduced 
by Grenacher (7), and Lankester and Bourne (15). 
The first important paper upon the optic tract of Arthro- 
poda was by Emile Berger (2), who described and figured this 
region in the larva of Libellula, in Musca, Dytiscus Apis, 
Pieris, Squilla, Astacus, &c. 
He was the first to describe the true position of the optic 
nerve, namely, between the opticon and the cerebrum ; and 
he also figured, fairly accurately, the epi-opticon and the optic 
nerve-fibrils decussating between this and the peri-opticon. 
The rest of the optic tract he divided as follows. The optic 
nerve-fibrils, after they have decussated, enter a ganglion -cell 
layer, this is followed by a molecular layer (Marksubstanz), 
which is situated immediately behind a nuclear layer (Korner- 
schicht), which is separated from the basement membrane of 
the ommateum by a “ nerve-bundle sheath.” These various 
layers, however, are not so constant as he supposed, nor are 
they in all cases just as he described. The ganglion-cell layer is 
not always present, and sometimes it is only represented by a 
few scattered cells as in Eristalis, nor can any distinction be 
drawn between the cells situated in this region and the region 
situated in front of the peri-opticon. In both cases the cells 
are nerve-cells and not ganglion-cells, according to the distinc- 
tion I have drawn between the two ; that is, in neither region 
are cells found containing a considerable quantity of true cell- 
protoplasm. 
The molecular layer of Berger corresponds with my peri- 
opticon. He does not seem to have noticed that it is composed 
of a number of elements, but his idea that it corresponded in 
