PHYSIOLOGY OF NERVOUS SYSTEM IN ARTHROPODS. 473 



ances is erroneous, as I am quite unable to stain the granules I 

 took for nuclei, except those which manifestly belong to the 

 basket-like fibres ; neither can I find any such cells in the 

 same organ in the pupa. I believe the rod-like fibres are either 

 packets of primitive fibrillae, or medullated nerve fibres. In 

 many specimens they exhibit the appearance of rods of 

 granules. Viallanes says : ' The external medullary mass in 

 the Wasp, as in the Dragon-fly, consists entirely of punctate 

 substance ;' but in his later work on the brain of the Cricket, 

 he adds : ' When sections strongly impregnated with osmic 

 acid are examined, the substance of this body is seen distinctly 

 divided into three zones. The internal and external zones are 

 strongly stained. Under low powers, two systems of striae 

 may be observed, one normal to and the other parallel with the 

 surfaces of the organ.' This is a little disappointing from 

 one who has apparently worked so long at the structure in 

 question. 



The structure of the optic ganglion is as well described by 

 Berger as by anyone since. In the intermediate layer the 

 radial fibres are thinner, and make way for those parallel with 

 the surfaces of the ganglion, and in young pupae the layer is 

 almost entirely cellular. 



In the pupa the reticular substance is of small amount, and 

 as development progresses it encroaches on the very thick 

 cortex, which is correspondingly reduced in thickness, so that 

 I conclude the reticular medulla originates from the cells of the 

 cortex arid from those of the intermediate layer, either by the 

 great development of certain cell processes, which become 

 nerve-fibres, or by the differentiation and fibrillation of the 

 cells themselves. 



2. GENERAL PHYSIOLOGY OF THE NERVOUS SYSTEM IN 



ARTHROPODS. 



Very little is known of the special functions of the several 

 parts of the nervous system in Arthropods. There is, however, 

 a general similarity between the functions of the ventral chain 



