THE EVIDENCE OF THE ORGANS OF VISION  IOl 
Langerhans and carefully figured by Kohl, a crossing of the fibres of 
the optic nerve occurs as the nerve leaves the retina, just as is so uni- 
versally the case in all compound retinas. To this crossing Kohl has 
given the name chiasma nervi optici, in distinction to the cerebral 
chiasma, which he calls chiasma nervorum opticorum. Further, we 
find that even this latter chiasma is well represented in the arthro- 
pod brain; thus Bellonci in Spheroma, Berger, Dietl, and Krieger in 
Astacus, all describe a true optic chiasma, the only difference in 
opinion being, whether the crossing of the optic nerves is complete or 
not. Especially instructive are Bellonci’s figures and description. 
He describes the brain of Spheroma as composed of three segments 
—a superior segment, the cerebrum proper, a middle segment, 
and an inferior segment; the optic fibres, as is seen in Fig. 39, 
after crossing, pass direct into the middle segment, in the ganglia of 
which they terminate. From this segment also arises the nerve to 
the first antenna of that side—i.e. the olfactory nerve. The optic 
part, then, of this middle segment is clearly the brain portion of the 
optic ganglionic apparatus, and may be called the optic lobes, in 
contradistinction to the peripheral part, which is usually called the 
optic ganglion, and is composed of two ganglia, Op.g. I. and Op. g. IL, 
as already mentioned. These optic lobes are therefore homologous 
with the optic lobes of the vertebrate brain. 
The resemblance throughout is so striking as to force one to the 
conclusion that the retina of the vertebrate eye is a compound retina, 
composed of a retina and retinal ganglion of the type found in arthro- 
pods. From this it follows that the development of the vertebrate 
retina ought to show the formation of (1) an optic plate formed 
from the peripheral epidermis and not from the brain ; (2) a part of 
the brain closely attached to this optic plate forming the retinal 
ganglion, which remains at the surface when the rest of the optic 
ganglion withdraws; (3) an optic nerve formed in consequence of 
this withdrawal, as the connection between the retinal and cerebral 
parts of the optic ganglion. 
This appears to me exactly what the developmental process does 
show according to Gétte’s investigations. He asserts that the retina 
arises from an optic plate, being the optical portion of his ‘Sinnes- 
platte.’ At an early stage this is separated by a furrow (Furche) 
from the general mass of epidermal cells which ultimately form the 
brain, This separation then vanishes, and the retina and brain-mass 
