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. 



