Appendix B : Some Standard Reagents and Their Uses 169 



up to 1 cm. in size, in from 24 to 36 hours. They may be left a 

 much longer time, however, without injury. Large objects may 

 require weeks for proper fixation. After fixing, tissues should be 

 washed in 70 per cent, alcohol until the alcohol is no longer 

 colored by the picric acid. The tissue should not pass, during 

 subsequent treatment (with a few exceptions in case of staining), 

 into an aqueous medium or into an alcohol of less than 70 per cent, 

 strength, because such media seem to undo the work of fixation. 



22. Picric Alcohol. Gage recommends a 0.2 per cent, solu- 

 tion of picric acid in 50 per cent, alcohol as an excellent fixer and 

 hardener for almost any tissue or organ. Time required, 1 to 3 

 days. Entire objects which have been fixed in picric acid or 

 in picric alcohol stain readily in borax-carmine or paracarmine. 



23. Picro-Acetic. Saturate a 1 per cent, aqueous solution of 

 acetic acid with picric acid. This liquid is widely used as a 

 general reagent, and is to be preferred for most purposes to 

 picric acid alone. For washing, etc., see remarks under 21- The 

 author has found that his preparations are improved if about 25 

 parts of formalin are added to each 100 parts of the picro-acetic. 



24. Picro-Sublimate.- 

 RaWs- 



Picric acid, saturated aqueous solution ... 1 vol. 

 Corrosive sublimate, saturated aqueous solution 1 vol. 

 Distilled water : 2 vols. 



This mixture has been especially recommended for embryos. 

 They are left in the fluid for 12 hours, then washed in weak 

 alcohol and transferred to gradually increasing strengths of 

 alcohol. 



O. vom Rath's 



Picric acid, cold saturated solution 1 vol. 



Corrosive sublimate, hot saturated solution . . 1 vol. 

 Glacial acetic acid 0.5 to 1 vol. 



After fixing for several hours, transfer the material directly 

 into alcohol. 



25. Picro-Sulphuric (Kleinenberg's).- 



Picric acid, saturated aqueous solution . . 98 vols. 



Sulphuric acid 2 vols. 



Water . 200 vols. 



