CYTOLOOICAL FIXTXO AGENTS. 331 



of picric acid with osmic acid or with osmic and acetic acid 

 (proportions of the latter as in the chromo-acetic-osmic mix- 

 ture [ 35], but of picric acid about 50 per cent.) fix quite as 

 well as the chromic mixtures, but precise staining is even 

 more difficult than with pure osmic acid preparations. Flem- 

 ming concludes that the beneficial effects of the osmium in 

 all these mixtures are to be ascribed to the instantaneous 

 rapidity with which it kills, the function of the other acids of 

 the mixture being to render the structures distinctly visible. 



Mixtures containing osmic acid should therefore be em- 

 ployed whenever it is desired to fix the chromatic figures as 

 faithfully as possible; whilst pure chromic acid should be taken 

 whenever very sharp staining is the more important point. 



For the study of the achromatic figures he recommends the 

 chrorno-acetic acid mixture ( 31), followed by staining in 

 haematoxylin (anilins do not give so good results for this 

 purpose) . 



For the study of polar corpuscles he recommends the 

 osmium mixtures, or pure chromic acid followed by staining 

 with gentian-violet. 



The above account stands nearly as it stood in the first 

 edition. The state of things at present is as follows : It is 

 admitted by all competent observers that the chromo-aceto- 

 osmic mixture is, with at most one or two possible exceptions, 

 by far the best fixing agent for nuclei. But some observers 

 have stated that it does not always preserve the cell-body 

 well. This is a question that has been already discussed in 

 35 and 36. I will only add here that after considerable 

 experience I see no reason to distrust Flemrning's mixture as 

 a preservative of any kind of protoplasm, provided it be used 

 in the proper way. It must be taken of the proper strength, 

 it must be used with very small objects, so that it may act on 

 all parts of them with its full strength, and not be filtered and 

 diluted through thick walls of tissue before coming into con- 

 tact with the object of study; and it must be allowed to act 

 for the proper time. 



This brings us to another point. There are two chromo- 

 aceto-osmic mixtures the old weaker one, and the new stronger 

 one. Flemming recommended the strong one primarily as 

 affording a means of differentiating kinetic chromatin from 

 resting chromatin. He did not recommend it as a reagent for 



