NUCLEAR STAINS WITH COAL-TAR DYKS. 173 



279. Other Progressive Stains. — Most of the basic tar colours used 

 for regressive staining will also give by the progressive method a nuclear 

 stain of greater or less purity if used in solutions acidified with acetic 

 acid. Amongst these may be mentioned tliionin, which need not even 

 be acidified ; also, for fresh tissues especially, gentian violet, dahlia, and 

 tolnidin hluc. 



B. Regressive Stains. 



280. The Practice of Regressive Staining: : The Staining Bath. — 



Sections only, or material that is tliin enough to behave like 

 sections, such as some membranes, can be stained by this 

 method. 



The solutions employed are made with alcohol, water, or 

 anilin, or sometimes other menstrua, according to the solubility 

 of the colour. There seems to be no special object in making 

 them with alcohol if water will suffice, the great object being 

 to get as strong a solution as possible. Indeed, the solutions 

 made with strong alcohol are found not to give quite such 

 good results as those made with water or weak alcohol. 

 Alcohol of 50 per cent, strength, however, may be said to 

 constitute a very generally desirable medium. The sections 

 must be very thoroughly stained in the solution. As a general 

 rule they cannot be left too long in the staining fluid. With 

 the powerful solutions obtained with anilin a few minutes or 

 half an hour will usually suffice, but to be on the safe side 

 it is frequently well to leave the sections twelve to twenty- 

 four hours in the fluid. Up to a certain point the more the 

 tissues are stained the better do they resist the washing-out 

 process, which is an advantage. Some workers, indeed, 

 prefer weak solutions ; so Hbidenhain, Encycl. mih. 

 Technik, i, pp. 433, 434 ; but the nature of the fixing agent 

 should be taken into account. 



Material fixed in chromic or chromo-osmic mixtures gives 

 a sharper and more selective stain than Eiaterial fixed in 

 sublimate or the like. In fact, to ensure the best results, only 

 material fixed in cJiromic mixtures (or Hermann's fluid) .'should 

 he employed. 



During the staining the tissues become overstaiiied, that 

 is, chai'ged with colour in an excessive and diffuse manner. 

 The stain must now l^e differentiated by removal of the excess 

 of colour. 



