i] FIXATION AND STAINING 11 



(mitochondria, Golgi apparatus, etc.). In cells which have 

 begun to be differentiated to form tissues of any sort, some 

 kind of cell-structure is almost always present, but this must 

 be regarded as the structure of the cell and characteristic 

 of the particular tissue rather than as protoplasmic struc- 

 ture as such. Some examples of such cell-structures will be 

 referred to later, but in general they are the concern of the 

 histologist rather than of the cytologist, and will not be 

 dealt with fully. It will also be convenient to postpone any 

 further consideration of cytoplasmic structures until some 

 account has been given of the nucleus. First, however, it 

 will be advisable to devote a short space to the different 

 kinds of stains commonly used to differentiate the various 

 structures in the cell. 



A very large variety of stains has been used in cytological 

 work, but in general these may be classified into two groups, 

 known as acid and basic. "Acid" stains do not necessarily 

 give an acid reaction with indicators such as litmus, but 

 tend to combine with substances which have the chemical 

 constitution of bases, while basic stains combine with 

 organic acids. In general basic dyes are used chiefly for 

 staining the chromatin of the nucleus, but stain also certain 

 kinds of inclusions in the cytoplasm, while acid stains are 

 taken up by the non-chromatinic parts of the nucleus and by 

 most cytoplasmic structures. It should be noted, however, 

 that the staining reactions depend to a considerable extent 

 on the fixatives used, and especially on whether the tissues 

 have previously been treated with acid or alkaline solution. 

 Most fixatives are acid, and the staining effects mentioned 

 are those which normally follow the use of such fixatives. 

 When only one stain is used, it is commonly taken up to 

 some extent by almost all parts of the cell, though it usually 

 stains some parts more intensely than others; but when 



