DOUBLE-IMPREGNATION 505 



They can, however, be made to impregnate by one or other 

 of Golgi's so-called processes of rejuvenation. These can be 

 carried out in various ways given here with sufficient detail, as 

 they may be of great use not only for rejuvenating ordinary pieces 

 of central nervous system, but also, and particularly, for the 

 staining of nerve-endings in glandular and other tissues, internal 

 apparatus, spiral filaments of pcriplieral nerve-fibres, etc. 



Golgi at first suggested washing the over-hardened pieces in 

 a half-saturated solution of copper acetate until they no longer 

 give a precipitate, afterwards putting them back again for five 

 or six days into the osmium-bichromate mixture, and subse- 

 quently transferring them into the silver nitrate solution. 



Later he advised leaving tissues in 3 to 4 per cent, copper 

 sulphate or 1 to 2 per cent, arsenic acid. After one, two and 

 three days some pieces are brought back into the osmium- 

 bichromate mixture in which they have been hardened, or into 

 a weaker one, proceeding further as in the rapid process, viz., 

 as if the pieces had been freshly fixed in the osmium-bichromate 

 mixture. 



More recently Golgi appears to have preferred mixtures of 

 equal parts of 2 or 3 or 4 per cent, copper sulphate or acetate 

 and 4 to 5 per cent, potassium bichromate, filtering them if 

 copper acetate was used, and treating the pieces as stated above. 

 As a rule these copper acetate and potassium bichromate mixtures 

 ought to be tried first and in preference to others. As with other 

 points of Golgi's methods, so also in this case, one must proceed 

 by tentative experiments, according to the purpose of one's 

 investigation and the quality of the material in hand, but chiefly 

 according to the length of time during which the tissues have 

 been left in the osmio-bichromate solution. 



See on this subject Sacerdotti, Intern. Monatschr. Anat., xi, 1894, 

 p. 326 ; Golgi, Cinquant. Soc. Biol., 1899, p. 514, and Opera Omnia, II, 

 1903, p. 677 ; Fusari, Trait. Elem. Istol. Teen. Istol., Torino, 1909 ; 

 Sala, G., Anat. Anz., xviii, 1900, p. 176 ; Gemmelli, Anat. Anz., 1913, 

 p. 444. 



1036. Formaldehyde Modifications of Golgi's Bichromate and 

 Nitrate of Silver Method. Many investigators have found that 

 formaldehyde can take the place of the osmic acid in the osmio- 

 bichromate mixture of the rapid process. This has certain 

 advantages : A cheap reagent is employed instead of the expensive 

 osmic acid. Pieces much larger than by Golgi's original process 

 may be used. The stage of hardening favourable for a good 

 impregnation lasts longer, i.e., formalin-bichromate mixtures do 

 not over-harden. Moreover, the formaldehyde modifications can 

 be usefully resorted to for impregnating nervous tissues of adult 

 or young subjecl^, as well as for material which after repeated 

 attempts has been found impervious to the osmic mixtures. How- 



