CYTOPLASMIC INCLUSIONS 139 



the primary criteria (Gatenby, 1930), particularly since the discovery 

 that the supposedly typical reticular network is a relatively rare type 

 and in many cases is due to a temporary aggregation of granules (Hirsch- 

 ler, 1927) . Both the osmic and the silver methods involve the same prin- 

 ciple — the adsorption of the reduced metal on particular structures, 

 although the actual reduction of the metallic compound may take place 

 either where the deposits are located or in other parts of the cell (Owens 

 and Bensley, 1929). Since the osmic acid methods have been most 

 thoroughly investigated, this discussion will be restricted to these tech- 

 niques, but the fact should be emphasized that the same general principles 

 are involved in the silver techniques as well. 



The preliminary treatment of the cell strongly affects the subsequent 

 impregnation (Lison, 1936). Thus direct exposure of the protozoan 

 to osmic fumes (Hall, 1929) is not equivalent to the full technique 

 which involves preliminary fixation in such mixtures as Champy's fluid. 

 Likewise, the blackening of granules by exposure to osmic fumes after 

 they have been stained with neutral red is not the equivalent of the 

 standard Golgi methods, since it has been shown in the case of Flabel- 

 lula (Hopkins, 1938b) that the blackening is produced only in the 

 presence of neutral red. 



The Golgi methods are not specific for any single type of material, 

 since a series of different lipoids extracted from echinoderm eggs give 

 a typical Golgi reaction (Tennent, Gardiner, and Smith, 1931). Further- 

 more both lipoid and non-Iipoid bodies of various types in the Protozoa 

 react typically and identically to the Golgi techniques (MacLennan, 

 1940). Although the method is not specific in the sense that it reveals 

 a single known substance, or some unknown Golgi material, the method 

 is consistent in the sense that under proper conditions the results can 

 be reproduced. In general, the reputation of osmic acid methods as 

 carpricious and erratic is due to the slow penetration of tissues by 

 osmium tetroxide, with the result that the cells in a block vary in ex- 

 posure to the unreduced reagent because of their varying distances from 

 the free surfaces. Another source of difficulty is the fact that the com- 

 position of cytoplasmic granules changes during the growth of that 

 granule (Kedrowsky, 1931e; Mast and Doyle, 1935a, 1935b; Mac- 

 Lennan, 1936), and it is to be expected that the reducing power will 

 also vary. The Protozoa are admirably adapted to solve both of these 



