THE EVIDENCE OF THE ORGANS OF VISION 89 
1. The optic ganglion of the Arthropoda consists of two parts, of 
which the one stands in direct inseparable connection with the 
facetted eye, and together with the layer of retinal rods forms the 
retina of the facetted eye, while the other part is connected rather 
with the brain, and is to be considered as an integral part of the 
brain in the narrower sense of the word. 
2. In all arthropods examined by him, the retina consists of five 
layers, as follows :— 
(1) The layer of rods and their nuclei. 
(2) The layer of nerve-bundles, 
(3) The nuclear layer. 
(4) The molecular layer. 
(5) The ganglion cell layer. 
Berger passes under review the structure and arrangement of 
the optic ganglion in a large number of different groups of arthropods, 
and concludes that in 
all cases one part of  , { I 
the optic ganglion is {Put MME 
always closely attached »!r-g. - MaMa 
to the visual end-cells, i 
and’ this combination oe CE i 
he calls the retina. N 
On the other hand, the 
nerve-fibres which con- 
nect the peripheral part Br 
of the optic ganglion 
with the brain, the so- 
called optic nerve, are 3 =F 
by no means homolo- 
gous in the different 
Fic. 38.-TuHr Retina or Musca. (After BERGER.) 
Br., brain; O.n., optic nerve; 2.1.0.g., nuclear layer of 
groups; for in some 
cases, aS in many of 
the stalk-eyed crusta- 
ganglion of optic nerve; m.l., molecular layer 
(Punktsubstanz) ; n.l.7.g.i, and n.1.7.9.0., inner and 
outer nuclear layers of the ganglion of the retina ; 
ftrr., terminal fibre-layer of retina; 7., layer of 
ceans, the whole optic 
ganglion is at the pe- 
riphery, while in others, as in the Diptera, only the retinal ganglion 
is at the periphery, and the nerve-stalk connects this with the rest 
of the optic ganglion, the latter being fused with the main brain- 
mass, ‘In the Diptera, in fact, according to Berger, the optic nerve 
retinal end-cells (indicated only). 
