ON THE ORIGIN OF VERTEBRATES. 



231 



Berger passes under review the structure and arrangement of 

 the optic ganglion in a large number of different groups of 

 animals, and concludes that in all cases one part of the optic 

 ganglion is always closely attached to the visual end cells, and 

 this combination he calls the retina. On the other hand, the 

 nerve fibres which connect the peripheral part of the optic 

 ganglion with the brain, the so-called optic nerve, are by no 

 means homologous in the different groups ; for in some cases, as 

 in many of the stalk-eyed Crustaceans, the whole optic ganglion 

 is at the periphery, while in others, as in the Diptera, only the 



Fig. 1. — The retina of Mnsca (after Berger). Br., brain; O.n., optic nerve; 

 n.l.o.g., nuclear layer of ganglion of optic nerve ; vi.l., molecular layer 

 (Punktsubstanz) ; n.l.r.g.i. and n.l.r.g.o., inner and outer nuclear layers of 

 the ganglion of the retina ; f.hr.r., terminal fibre layer of retina ; r., layer 

 of retinal end cells. 



retinal ganglion is at the periphery, and the nerve stalk connects 

 this with the rest of the optic ganglion, which latter is fused with 

 the main brain mass. In the Diptera, in fact, according to 

 Berger, we have an optic nerve and retina most nearly compar- 

 able to that of the Vertebrate, For this reason I give Berger's 

 picture of the retina of Musca, in order to show the arrangement 

 there of the retinal layers. 



In Branchipus and other primitive Crustacea, Berger also 

 finds the same retinal layers, but is unable to distinguish in the 

 brain the rest of the optic ganglion. Judging from Berger's 



