THE EVIDENCE OF THE ORGANS OF VISION IOI 



Langerkaiis and carefully figured l>y 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 Sphgeroma, 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 Sphaeroma 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. II., 

 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 retiua 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 Gotte'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 



