402 MARGARET ROBINSON. 



who was followed by Huxley, considered the eyes as the first 

 pair of appendages. (Lankester, 1904.) 



Reichenbach (188G) and Nusbauni (1887) found first traces 

 of a nervous system in the crayfish and in My sis similar to 

 those above described^ and they too looked upon the optic 

 tract as the first body segment. 



If this be sOj then the eye region forms the first segment of 

 the body not only in all the Malacostraca, but also in all the 

 Crustacea, and in the rest of the Arthropoda. 



Evidence against this opinion has been given by Claus in 

 his papers on the development o£ Branchipus (1873 and 

 1886). Since he referred to the facts there recorded at 

 great length in his last paper on Nebalia (1889) one feels 

 bound to mention them, but it seems that his observations 

 were not made on sufficiently early stages. 



My observations on the development of the nervous system 

 in Nebalia are merely additional evidence in favour of the 

 opinion that in the Crustacea the optic region forms the first 

 segment of the body. This is the view taken by Heymons 

 (1901), and the one to which, lately, Prof. Lankester has 

 given his support. (Lankester, 1904.) 



In this paper I am using the terminology of Viallanes and 

 Heymons (1892) for the different parts of the brain. Proto- 

 cerebrum for that in the first or optic segment, deutero- 

 cerebrum for that belonging to the second segment, and 

 tritocerebrum for the pair of antennary ganglia in the third 

 segment. 



To continue the description of the nervous system at this 

 stage. The cells of the four pairs of rudimentary ganglia, 

 although they form part of the general ectoderm of the body- 

 wall, differ from the other ectoderm cells in size, and also in 

 the size and colour of their nuclei. They are larger than the 

 cells of the rest of the ectoderm, and have larger and paler 

 nuclei, which stain very slightly. 



The ganglia of each pair are separated from each other by 

 a median narrow band (one cell deep) of the cells of the 

 ordinary ectoderm. The ganglia of the first pair are in 



