142 Mr. P. A. Buxton on the 



I cannot see that anything is gained by leaving material 

 in it for so long a time, though it is well known that all 

 fixatives containing osmic acid penetrate slowly even 

 through small pieces of tissue. Forty-eight hours is quite 

 sufficient, according to my experience. Borrel's fluid also 

 gives good results very similar to those obtained with other 

 osmic acid fluids. Micropteryx tends to float in this and 

 other fixatives ; if it cannot be caused to sink with the aid 

 of shaldng it may be lightly painted with 90 % alcohol in 

 order to reduce the surface tension. All these fixatives 

 are extremely useful, though they occasionally tend to 

 shrink the cytoplasm of the larger nerve cells. The nerve 

 fibres (axons) stand out from one another with great clear- 

 ness, and in this way the sections are well suited for study : 

 they are never distorted, and there is no tendency for 

 the ganglion cells to break away in masses from the under- 

 lying fibrillar substance. Great care must be exercised 

 in washing the material very thoroughly in water after 

 fixation, or the staining will be unsatisfactory. 



Formalin. — Formalin has been recommended in various 

 rather high percentages (10 %, 20 %, etc.) by more than one 

 worker. It is customary to leave the heads in it for some 

 days. I anticipate that the use of formalin will soon be 

 discontinued, for though it gives a distinctly good demon- 

 stration of the tracts of axons, there is a great tendency for 

 the formation of vacuoles in the fibrillar substance. The re- 

 sult of this is that the tracts are pushed to one side and 

 distorted. This vacuolisation is not invariable, but it 

 constitutes a grave defect in the method, which is one that 

 I found unreliable. 



Picro-Chlor- Acetic Mixture. — I do not know to 

 whom we are indebted for this very useful fixative; but 

 it appears that it has not previously been used by insect 

 neurologists. My own experience is that it is the best 

 general fixative 1 have ever employed, and I trust that the 

 workers of the future will be as satisfied with it as I am 

 myself. It possesses very great powers of penetration, 

 and can be relied on to fix small insects completely without 

 decapitation or any other precautions. Insect histologists 

 will find that it is an exceedingly fine preservative of the 

 details of cell-structure; as far as the brain is concerned 

 this fluid demonstrated the tracts of axons with particular 

 clearness, and in this respect it does not fall far short of 

 osmic acid. The nerve cells are also well preserved, and 



