318 CYTOLOGICAL METHODS. 



Champy's fluid was invented for amphibia and gives very satis- 

 factory results ( 43). For mammalian tissues a preliminary 

 fixation in osrnic acid fixatives is not generally indicated ; the 

 tissues of mammals are far more " fatty " than those of invertebrata 

 or amphibia, and one finds that the OsO^ becomes reduced very 

 rapidly and penetration is very poor. For mammalian tissues 

 formalin-chrome (Regaud, Bensley-Cowdry), formalin-corrosive or 

 formalin alone are indicated as a preliminary treatment at the least. 

 Formalin does not destroy lipoids, and by subsequently placing small 

 pieces of chrome-formalin fixed tissues in osmic acid (post-osmica- 

 tion), a fixation of lipoids and fats is obtained (Schridde) ; the same 

 result may be got by fixing tissues in chrome salts and then trans- 

 ferring to osmic acid. It should be noted, however, that previous 

 fixation in a chrome salt prevents the blackening of the Golgi 

 apparatus ; the formalin-osmic acid method (Sjovall) is indicated 

 especially for mammalian tissues, when an impregnation of the Golgi 

 apparatus is required by means of an osmic method, but the formalin 

 silver nitrate methods of Golgi, Cajal and Da Fano are always 

 clearly indicated for work on the Golgi apparatus of mammalian 

 tissues. So far as possible intra vitam and fresh smear preparations 

 should be used, as these nearly always give valuable results. 



675. Specificity of Techniques for Cytoplasmic Inclusions, Fats, 

 and Lipoids. As a rule the lipoid granules, vacuoles, and cell organs 

 containing fats or lipoids are formed not of one pure substance, but 

 of a mixture of several. Consequently it is necessary to proceed 

 with caution in claiming a specificity for the techniques for various 

 lipoid substances : properly used, however, microchemical methods 

 may give valuable evidence as to the precise chemical nature of any 

 special body : microchemical methods, which depend for their 

 application on the use of complicated fixing and staining methods 

 are to be used cautiously. For example, Benda ( 683) and Altmann- 

 Bensley methods ( 680 and 686), will stain granules other than 

 mitochondria, while the Cajal formalin uranium and silver nitrate 

 technique impregnates bodies apart from the Golgi apparatus. In all 

 these cases, however, the number of such exceptions to a specificity is 

 small, and suitable differentiation between two doubtful bodies can 

 be made by some other method. See Bayliss on " Specificity " of 

 staining methods, 211. 



676. On Killing Animals for Cytological Purposes. So far as 



possible avoid narcotics of any sort. Either cut off the heads of 

 invertebrates, or, if delicate like some worms, drop them whole in 



