ON THE ORIGIN OF VERTEBRATES. 235 



primitive crustaceans the peripheral optic apparatus was 

 composed, not of four but of two optic ganglia, not, therefore, of 

 five but of three neurons, viz. : — 



1. The neuron of the first order — i.e., the retinal cell with its 

 fibre terminating in the Punktsubstanz of the first optic 

 ganglion (ganglion of the retina). 



2. The neuron of the second order, which terminates in the 

 Punktsubstanz of the second ganglion (ganglion of the optic 

 nerve). 



3. The neuron 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 Eellonci, 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. 



The comparison of the arthropod compound retina with that 

 of the vertebrate shows, as one would expect upon the theory 

 of the origin of vertebrates put forward m this series of papers, 

 that the latter retina is built up of two ganglia, as in the 

 more primitive less specialized crustacean forms. The modern 

 description of the vertebrate retina, based upon the Golgi 

 method of staining, is exactly Parker's description of the 

 simpler form of crustacean retina in which the Punktsubstanz 

 of the first ganglion is represented by the external molecular 

 layer, and that of the second ganglion by the internal molecular 

 layer; the three sets of neurons being, according to Parker's 

 terminology : — 



1. The neurons of the first order — viz., the visual cells — the 

 nuclei of which form the external nuclear layer, and their long 

 attenuated processes form synapses in the external molecular 

 layer with 



2. The neurons of the second order, the cells of which form 

 the internal nuclear layer, and their processes form synapses in 

 tlie internal molecular layer with 



3. The neurons of the third order, the cells of which form the 

 ganglionic layer and their neuraxons constitute the fibres of 

 the optic nerve which end in the optic lobes of the brain. 



