ON THE ORIGIN OF VERTEBRATES. 233 



retinal ganglion — may be spoken of as composed of two ganglia. 

 The external of these is clearly the ganglion of the retina ; its 

 cells form the nuclear layer, the striking character of which, 

 and close resemblance to the corresponding layer in Vertebrates, 

 is shown by Glaus' picture, which I reproduce. The internal 

 ganglion with which the optic nerve is in connection contains 

 large ganglion cells, which, together with smaller ones, form 

 the ganglionic layer of Berger. 



Glaus agrees with Berger that the whole optic ganglion 

 does not belong to the retina, but only that part which 

 is called the retinal ganglion. He says : — " In the Edriop- 

 thalmen, such as Phronima, Gammarus, Asellus, in which a 

 stalked eye does not exist, the optic ganglia are attached to 

 the brain lobes along their whole breadth, and give the impres- 

 sion of forming lateral brain ganglia, while the nerve fibres 

 which pass to the retinal ganglion represent a more or less 

 elongated optic nerve. These nerve tracts are to be compared 

 with the optic nerves of vertebrates. The fact that their fibres 

 cross as they enter the retinal ganglion is an entirely sub- 

 ordinate peculiarity, and has no weight as an argument against 

 considering them as equivalent to the vertebrate optic nerve." 



If Glaus had only known the nature of the optic nerve in 

 Ammoccetes, where an exactly similar crossing of the fibres of 

 the optic nerve as they enter the retina has been universally 

 observed, as will be pointed out when I compare the Ammoccetes 

 retina with that of the invertebrate, he would have seen that, 

 far from such crossing having no weight, it is an additional 

 argument in favour of the justice of the very comparison which 

 he was making. 



The most recent observations of the structure of the compound 

 retina of the Grustacean eye are those of Parker,^ who, by the 

 use of the methylene blue method, and Golgi's method of staining, 

 has been able to follow out the structure of the compound retina 

 in the Arthropod on the same lines as had already been done 

 for the Vertebrate. These two methods have led to the con- 

 clusion that the arthropod central nervous system and the 

 vertebrate central nervous system are built up in the same 

 manner — viz., by means of a series of ganglia connected together, 



1 Mitth. a. d. Zool. Stat. z. Neapel., vol. xii. [). 1, 1895. 



