Protocerehrum of Micropteryx. 143 



all the dift'erent types can easily be distinguished, though 

 for a special study of tlie cells it is certainly best to have 

 some material fixed for that purpose in Bouin's fluid. 



Bouin's Fluid. — This fixative is only of use for a study 

 of the nerve cells, and for this purpose it is unrivalled. It 

 fixes material in such a way that the tracts of fibres cannot 

 be distinguished at all, but that is immaterial provided it is 

 realised that the fluid is essentially a special fixative. 



Acetic Sublimate Solution. — This is simply a satu- 

 rated solution of mercuric chloride in dilute alcohol to which 

 a small percentage of acetic acid has been added. It has 

 been used by other authors but there is nothing to 

 recommend it ; the tracts or bundles of axons are shown in 

 much the same way as they are in material fixed in the 

 picro-chlor-acetic mixture, the cells are shrunken and the 

 different types cannot be distinguished, and the fluid has 

 poor power of penetration. 



Perenyi's Fluid. — This is a fixative ^\^th very small 

 power of penetration, even when used hot. Even if penetra- 

 tion is secured the tracts of axons cannot be distinguished 

 from one another, and the cells are swollen and matted 

 together. 



CtIlson's Fluid. — The penetrating power of this fluid 

 is so great that insects may be fixed in it whole. It is an 

 excellent fixative of ganglion cells, and shows the differences 

 between the types very clearly : for this purpose it is 

 valuable but it fails entirely to define the axons. 



Bichromate. — Potassium bichromate, apart from its 

 use in the Golgi method, is quite useless as a fixative of 

 insect nerve tissue, first because details of structure and 

 the course of axons are not well preserved, secondly because 

 material so fixed stains most intensely and generally with 

 the aniline dyes, thirdly because these stains can scarcely 

 be washed out or differentiated, and fourthly because of the 

 great brittleness of sections which have been exposed to 

 the action of these fluids. 



The Golgi Method. — This method has been applied 

 by Kenyon to the brain of the bee, and with it he has ob- 

 tained some very remarkable results; his original paper 

 (Kenyon, 1896, I) should be consulted for a full account 

 of his procedure. It is almost impossible to apply it to 

 insects which are not available for the greater part of the 

 year because it is extremely precarious, and even Kenyon 

 himself only obtained good results with an occasional 



