118 Mr. P. A. Buxton on the 



parts of the j)rotocerebrum it should be reahsed that this 

 portion of the brain is not merely a complex but also an 

 entity. We might comj)are it physiologically to the cere- 

 brum of a vertebrate. Both are known to be the highest 

 or governing centres of the organism ; both possess a soli- 

 darity or unity of action ; both consist also of parts, each 

 of which in its turn is not only an anatomical, but also a 

 physiological entity. We are quite justified in regarding 

 the protocerebrum of an insect as the headquarters from 

 which are directed all those complicated reactions and 

 instincts of the organism which give to its activities what 

 at any rate appear to be purpose, and adaptation to the 

 surroundings. There is too great a tendency to lay 

 emphasis on the potentialities for independent action 

 which are undoubtedly possessed by the lower nerve 

 centres of the insect. In all but the very lowest insects 

 there is a marked degree of specialisation in the structure 

 and function of the protocerebrum, and this fact of its 

 solidarity should not be allowed to pass from the mind while 

 we study in detail the structure and perhaps something of 

 the function of its parts. 



THE PEOTOCEREBRUM OF MICROPTERYX. 



I. The Neurilemma and Ganglion Cells. 



The whole brain of Micropteryx, that is to say the axonic 

 parts and the cells, is included in a limiting membrane or 

 Neurilemma (Plate X). This is a very fine sheet of 

 substance which stains well with the acid stains. It is 

 certainly a syncytium in Micropteryx, and probably in all 

 insects. In places where no ganglion cells intervene be- 

 tween the neurilemma and the axonic part of the brain 

 the two are closely applied to each other, and the neuri- 

 lemma can hardly be distinguished, though in material 

 fixed in the picro-chlor-acetic mixture it can generally be 

 seen. Occasionally the nuclei of the neurilemma can be 

 seen even when the layer itself is indistinguishable. The 

 neurilemma is somewhat thickened mid-dorsally, partly 

 owing to the fact that a number of tracheal tubes (Plate X) 

 lie in it in this positioji, partly OAving to the presence of a 

 immber of the nuclei, the cells corresponding to which have 

 fused to form the syncytium of which the neurilemma con- 

 sists. These nuclei are elongate and smaller than those of 



