fete) 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 Spheroma, 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 
-Mid, Segineut 
Aut II 
Fig. 39.—THE Brain oF Spheroma serratum. (After BELLONCI.) 
Ant. I. and Ant. IT., nerves to 1st and 2nd antenne. /f.br.*., terminal fibre-layer of 
retina; Op. g. I., first optic ganglion; Op. g. IJ., second optic ganglion; O.n., 
optic nerve-fibres forming an optic chiasma. 
internal enlargement, as Bellonci calls them. The external ganglion 
(Op. g. [., 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 (cf. Fig. 46, in which 
the double nature of the retinal ganglion is indicated), the peripheral 
part of the optic ganglion—z.e, the retinal ganglion—may be. spoken 
