9 o 



THE ORIGIN OF VERTEBRATES 



and retina are most nearly comparable to those of the vertebrate. 

 For this reason I give Berger's picture of the retina of Musca 

 (Fig. 38), 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 description of 

 Branchipus, and Bellonci's of Spha^roma, it would almost appear 

 as though the cerebral part of the retina in the higher forms 

 originated from two ganglionic enlargements, an external and 



Sup. Segment Ant I 



Ant II 

 Inf. Segment 



Fig. 39. — The Brain op Sphceroma scrratum. (After Bellonci.) 



Ant. I. and Ant. II., nerves to 1st and 2nd antennae, f.br.r., terminal fibre-layer of 

 retina; Op. g. I., first optic ganglion; Op. g. II., second optic ganglion; O.n., 

 optic nerve-fibres forming an optic chiasma. 



internal enlargement, as Bellonci calls them. The external ganglion 

 (Op. g. I., Fig. 39) may be called the ganglion of the retina, the cells 

 of which form the nuclear layer of the higher forms, and the internal 

 ganglion (Op. g. II., Fig. 39), from which the optic nerve-fibres to the 

 brain arise, may therefore be called the ganglion of the optic nerve. 

 Bellonci describes how in this latter ganglion cells are found very 

 different to the small ones of the external ganglion or ganglion of 

 the retina. So also in Branchipus, judging from the pictures of 

 Berger, Claus, and from my own observations (ef. Fig. 46, in which 

 the double nature of the retinal ganglion is indicated), the peripheral 

 part of the optic ganglion — i.e. the retinal ganglion — may be spoken 



