92 THE ORIGIN OF VERTEBRATES 
of four distinct ganglia; the retina is connected with the first of 
these by means of the retinal fibres, and the optic nerve extends 
proximally from the fourth ganglion to the brain. Each ganglion con- 
sists of ganglion-cells, nerve-fibres, and ‘neuropil,’ and, in addition, 
supporting cells of a neuroglial type. By means of the methylene 
blue method and the Golgi method, it is seen that the retinal end- 
cells, with their visual rods, are connected with the fibres of the 
optic nerve by means of a system of neurones, the synapses of 
which take place in and help to form the ‘neuropil’ of the various 
ganglia. Thus, an impulse in passing from the retina to the brain 
would ordinarily travel over five neurones, beginning with one of 
the first order and ending with one of the fifth, He makes five 
neurones although there are only four ganglia, because he reckons 
the retinal cell with its elongated fibre as a neurone of the first 
order, such fibre terminating in dendritic processes which form 
synapses in the ‘neuropil’ of the first ganglion with the neurones of 
the second order. 
Similarly the neurones of the second order terminate in the 
‘neuropil’ of the second ganglion, and so on, until we reach the 
neurones of the fifth order, which terminate on the one hand in the 
‘neuropil’ of the fourth ganglion, and on the other pass to the optic 
lobes of the brain by their long neuraxons—the fibres of the optic 
nerve. 
He compares this arrangement with that of Branchipus, Apus, 
Estheria, Daphnia, etc, and shows that in the more primitive 
crustaceans the peripheral optic apparatus was composed, not of 
four but of two optic ganglia, not, therefore, of five but of three 
neurones, viz.— 
1. The neurone of the first order—i.e. the retinal cell with its 
fibre terminating in the ‘ neuropil’ of the first optic ganglion (ganglion 
_ of the retina). 
2. The neurone of the second order, which terminates in the 
‘neuropil’ of the second ganglion (ganglion of the optic nerve). 
3. The neurone of the third order, which terminates in the optic 
lobes of the brain by means of its neuraxons (the optic nerve). 
We see, then, that the most recent researches agree with the 
older ones of Berger, Claus, and Bellonci, in picturing the retina of 
the primitive crustacean forms as formed of two ganglia only, and 
not of four, as in the specialized crustacean group the Malacostraca. 
