CHAPTER IV. 39 



Flemming at one time employed solutions of 40 to 50 per cent, for the 

 ova of Invertebrates. 



TELLYESNICZKY (Arch. mik. Anat., lii, 2, 1898, p. 222) thinks that " for 

 general cell-fixing " the proper strength is 2 to 2| per cent., as stronger 

 grades act too energetically on the superficial layers. 



MAYER has had good results with 5 per cent, solution. 



Nitric acid has the valuable property of hardening yolk without making 

 it brittle. 



Pure water should in no case be used for washing out ; the prepara- 

 tions should be brought direct into alcohol. Some persons take absolute, 

 but I should say 70 per cent, is more generally indicated. Rabl has 

 employed a 1 or 2 per cent, solution of alum. 



For prolonged hardening, strengths of from 3 to 10 per cent, are 

 sometimes employed. A strength of 12 per cent., allowed to act for 

 two or thr>e weeks, is said to afford very tough preparations of the 

 encephalon. 



BENDA (Verh. Anal, Ges., 1888 ; Ergeb. d. Anat., i, 1891, p. 7) fixes for 

 twenty-four to forty -eight hours in 10 per cent, nitric acid, and then 

 hardens in bichromate of potash. 



Fol's Mixture (verbally communicated to me). Three vols. of nitric 

 acid, with 97 vols. of 70 per cent, alcohol. 



48. Chromo-nitric Acid (PERENYI'S formula, Zool. Anzeig., v, 1882, 

 p. 459) : 



4 parts 10 per cent, nitric acid. 



3 parts alcohol. 



3 parts 0-5 per cent, chromic acid. 



Fix for four to five hours and pass into alcohol of 70 per cent. 

 This mixture has been criticised (see previous editions) as irrational, 

 the alcohol reducing the chromic acid and itself becoming etherised by 

 the nitric acid. Some workers reject it, especially for ova, for which it 

 is specially intended. But others speak highly of it. I myself have 

 used it extensively for preparing objects for dissection and museum 

 specimens, and found it admirable for these purposes. But preparations 

 made to test its value from a cytological point of view have given me 

 only second-rate results. It is now little used. 



49. Chromic Acid and Platinum Chloride (MERKEL'S Macula lutea 

 des Menschen, Leipzig. 1870, p. 19). Equal volumes of 1400 

 solution of chromic acid and 1400 solution of platinum chloride. 

 Objects should remain in it for several hours or even days. After 

 washing out with alcohol of 50 to 70 per cent., objects stain 

 excellently. If objects that have been fixed by osmic acid be 

 put into it for some hours, blackening is said to be effectually 

 prevented. 



This is an excellent hardening medium for delicate objects. 

 Merkel allowed from three to four days for the action of the fluid 

 for the retina ; for Annelids Eisig employs an immersion of three to 

 five hours, and transfers to 70 per cent, alcohol ; for small leeches 



