CHAPTER XVII. 205 



endings. He therefore soaks pieces of tissue in fresh lemon juice 

 until they become transparent (five or ten minutes in the case of 

 muscle). They are then rapidly washed in water, brought for about 

 twenty minutes into 1 per cent, gold chloride solution, washed again 

 in water, and brought into a mixture of 50 c.c. of distilled water and 

 2 drops of acetic acid. They are exposed to the light for twenty-four 

 to forty-eight hours. The preparations thus obtained are good for 

 immediate study, but are not permanent, the reduction of the gold 

 being incomplete. In order to obtain perfectly reduced, and there- 

 fore permanent, preparations, the reduction should be done in the 

 dark in a few cubic centimetres of dilute formic acid (1 part acid to 

 4 of water), which takes about twenty-four hours. 



367. VIALLANE'S Osmic Acid Method (Hist, et Dev. des Insectes, 

 1883, p. 42). The tissues are treated with osmic acid (1 per cent, 

 solution) until they begin to turn brown, then with 25 per cent, 

 formic acid for ten minutes ; they are then put into solution of 

 chloride of gold of 1 : 5000 (or even much weaker) for twenty-four 

 hours in the dark, then reduced in the light in 25 per cent, formic 

 acid. I find this an excellent method. 



KERSCHNER (Arch. mik. Anat., Ixxi, 1908, p. 522) puts till brown 

 into a mixture of 10 parts 5 per cent, formic acid with 1 part 2 per 

 cent, osmic acid, washes, puts for two to six hours into 1 per cent, 

 gold chloride in the dark, washes, puts for twelve hours into 25 per 

 cent, formic acid in the dark and then for twenty-four in the light, 

 and mounts in 50 per cent, glycerin with 1 per cent, of formol. 



368. Other Methods. The numerous other methods that have 

 been proposed differ from the foregoing partly in respect of the 

 solutions used for impregnation, but chiefly in respect of details 

 imagined for the purpose of facilitating the reduction of the gold. 



Thus BASTIAN employed a solution of gold chloride of a strength 

 of 1 to 2000, acidulated with HC1 (1 drop to 75 c.c.), and reduced in 

 a mixture of equal parts of formic acid and water kept warm. 



HENOCQUE (Arch, de I'Anat. et de la Physiol, 1870, p. Ill) impreg- 

 nates in a 0-5 per cent, solution of gold chloride, washes in water for 

 twelve to twenty-four hours, and reduces in a nearly saturated 

 solution of tartaric acid at a temperature of 40 to 50 C. Reduction 

 is effected very rapidly, sometimes in a quarter of an hour. 



HOYER (Arch. mik. Anat., ix, 1873, p. 222) says that the double 

 chloride of gold and potassium has many advantages over the simple 

 gold chloride. He impregnates in solutions of 0-5 per cent, strength, 

 and reduces in water containing 1 or 2 drops of a pyrogallic add 



