FIXING AND HARDENING AGENTS. 41 



serving hyaloplasm and its inclusions, secretions, etc., much 

 better than chromic acid. 



For an elaborate study of the action of chrome salts on 

 nucleus and cytoplasm, see BUECKHAEDT, La Cellule, xii, 1897, 

 p. 335. He finds that the bichromates of sodium, ammonium, 

 magnesium, strontium, and zinc have the same destructive 

 action on nuclei that the bichromate of potassium has ; but 

 that the bichromates of barium, calcium, and copper have 

 not. He concludes that acetic add ought always to be added, 

 not only to ensure the correct fixation of nuclei, but also to 

 enhance penetration and the good preservation of cytoplasm. 



The following is recommended by him as a good combina- 

 tion for the fixation both of cytoplasm and nucleus : 



Bichromate of barium, 4 per cent, solution . 60 vols. 



Bichromate of potassium, 5 per cent, solution oO 



Glacial acetic acid . . . . 5 



(Instead of the barium you may take 4 per cent, solution 

 of bichromate of calcium, or 6 per cent, solution of bichromate 

 of copper.) 



For the demonstration of the achromatic figure of cell 

 division he recommends 



Chromic acid, 1 per cent, solution . . 60 vols. 



Bichromate of potassium, 5 per cent, solution 30 ,, 



Glacial acetic acid . . . . 5 



51. Bichromate of Potash. Perhaps the most important of 

 all known hardening agents, sensu slricto. It hardens slowly, 

 much more so than chromic acid, but it gives an incom- 

 parably better consistency to the tissues. They may remain 

 almost indefinitely exposed to its action without much hurt. 



The strength of the solutions employed is from 2 to 5 per 

 cent. As with chromic acid, it is extremely important to 

 begin with weak solutions and proceed gradually to stronger 

 ones. About three weeks will be necessary for hardening a 

 sheep's eye in solutions gradually raised from 2 to 4 per cent. 

 Spinal cord requires from three to six weeks ; a brain at least 

 as many months. 



After hardening, the objects should be well soaked out in 

 water before being put into alcohol, or be treated as directed 

 for chromic acid, 37. They had better be kept in the dark 

 when in alcohol (see 37). (Bomi and OPPEL [Taschenlucli , 



