Protocerebrum of Micropteryx. 147 



Another valuable stain is Picro-nigrosin. This brings 

 into special prominence the tracts of fibres which run 

 through the brain. Counter-staining should be avoided, 

 and also over-staining, because picro-nigrosin washes out 

 only with difficulty in acid alcohol. The best results are 

 obtained with material fixed in osmic acid or Flemming's 

 solution. 



Other haematoxylin methods have little to recommend 

 them. The Weigert-Pal method cannot be used because 

 it is specific for myelin sheaths, which are never found on 

 the nerve fibres of insects. 



5% HEMATOXYLIN CONTAINING LITHIUM CARBONATE 



is a stain for nerve fibres, but picro-nigrosin gives similar 

 though more distinct results. Staining with Mallory's 

 HEMATOXYLIN is Very strongly recommended by Kenyon 

 and by Altens. The process is complicated. At first 

 sight the stain appears much too general to be of use. 

 Every tissue is stained a heavy dark blue, and differentia- 

 tion in acid alcohol is useless. I have discovered that the 

 sections may be quickly dift'erentiated in a solution of 

 sodium bicarbonate in distilled water. The stain becomes 

 bright sky blue and most of the tissues are partly decolor- 

 ised. The nerve fibres retain the stain. This complicated 

 method produces results which are not really superior to 

 those produced by staining with picro-nigrosin. The blue 

 colour appears to be permanent. The cytoplasm of the 

 giant cells retains even more of the stain than do the nerve 

 fibres. The ordinary small ganglion cells are very much 

 decolorised in the alkali. 



Mallory's Anilin Blue. — This has been used by 

 Bretschneider in his work on the brain of Periplaneta. The 

 method is one of great complexity, especially with the addi- 

 tion of the modifications which he introduces. The results 

 are extremely beautiful, but most workers will probably 

 consider them hardly worth the trouble and time expended 

 on them. 



Mann's Stain. — I am much indebted to Dr. D. Keilin 

 for insisting on my giving a trial to this stain, which 

 will be found of great value. I find it best to stain first, 

 lightly, with Delafield, but this is not necessary or desir- 

 able except for nerve cells; the stain is widely known 

 to insect histologists, and is strongly recommended to 

 neurologists. It is seen at its best when applied to 

 material fixed in the fluids of Bouin, or Gilson, or in the 



