162 Animal Microlot/i/ 



any stain will follow them. Even such difficult objects as the 

 eggs of Ascaris may be fixed by the second mixture. The reagent 

 should be washed out in absolute or at least in strong alcohol. 



A mixture of absolute alcohol, glacial acetic acid and chloro- 

 form, equal parts, saturated with corrosive sublimate (formula of 

 Carnoy and Lebrun) becomes even more valuable for the fixation 

 of difficult objects. According to Lee, isolated ova of Ascaris 

 are fixed in 30 seconds, entire oviducts in 10 minutes, in this 

 liquid. 



3. Alcohol. Alcohol is used especially for gland cells and for 

 preserving the brain and spinal cord for Nissl's method of staining 

 nerve cells. See chap, iii, alcohol fixation; also chap, i, reagents 

 i and 2. 



Alcohol and Chloroform. See 2ft. 



Bichloride of Mercury. See corrosive sublimate. 



4. Bichromate of Potassium. Bichromate of potash is one of 

 the oldest and best-known fixing reagents. At present it is more 

 commonly used in mixtures than alone. It is widely used in 

 hardening nervous tissue. Its fixation of nuclei is imsatisfactory 

 unless it is properly corrected through the addition of acetic acid. 

 It acts very slowly, about three weeks being necessary to harden 

 properly a sheep's eye, and from three to six months for a good- 

 sized brain. A weak solution (2 per cent.) should be used at 

 first, to be replaced gradually by stronger solutions up to 5 per 

 cent. When hardening is completed the object should be thor- 

 oughly washed in running water and then put into alcohol; begin 

 with low percentages of alcohol and gradually increase the strength 

 up to 70 or 80 per cent. Change the alcohol as often as it 

 becomes yellow. After the object has been placed in alcohol, 

 keep it in the dark in order to prevent a precipitate forming on 

 the surface. Either carmine or hematoxylin may be used as a 

 stain after bichromate of potash. In case carmine is used, the 

 staining is best done before the object is placed in alcohol. 

 Tissues which do not stain well should be placed for 3 hours in 

 acid alcohol and then washed in alcohol before staining. 



