THE LIFE-HISTORY OF THE LEUCOCYTE 263 



angle of 45 degrees. The film thus obtained is allowed to dry- 

 in the air and may then be stained with any appropriate 

 stain, such as Jenner, Giemsa, Leishman, or Ehrlich's triacid 

 stain. I have almost entirely employed Leishman's stain, the 

 staining being thus carried out : the air-dried film is flooded 

 with Leishman's stain, two minutes later double the amount 

 of distilled water is added, and after thorough mixing 

 is allowed to stand for a further period of eight minutes. 

 During this process care should be taken that the slide is not 

 allowed to dry. The film is then thoroughly washed in distilled 

 water and allowed to dry in the air. Microscopic examination 

 is then carried out, using a yV-inch oil-immersion lens. To 

 facilitate counting, and to avoid counting the same field twice 

 over, a movable stage should be used. 



By this method of staining the granules present in some 

 forms of leucocyte are brought into great prominence. From 

 the appearances thus presented Ehrlich classified the leucocytes 

 according to the affinity of their granules for aniline dyes into 

 (a) Acidophile, which stain readily with acid dyes such as 

 eosin ; (b) Basophile, which stain readily with basic dyes 

 such as methylene blue ; and (c) Neutrophile, which stain oilly 

 with a mixture of acid and basic dyes. 



According to Ehrlich's conception an acid dye is usually a 

 salt in which the part of an acid radicle is played by the 

 coloured substance. Thus, for example, the acid dye eosin 

 is the sodium salt of tetrabromfluorescin, which is a coloured 

 acid. A basic dye, on the other hand, is usually a salt with 

 a basic colour radicle. Thus methylene blue is the chloride 

 of tetramethyldiphenthiazine, which is a coloured base. The 

 conventionally termed " neutral " dyes consist of a coloured 

 base loosely combined with a coloured acid as in the so-called 

 " eosinate of methylene blue." 



In addition to the presence or absence of granules and the 

 number and affinity of the granules for dyestuffs, other factors 

 are employed in the classification of the leucocytes, namely the 

 size of the cell and the shape, size, and tenuity of the nucleus. 



Classified on this basis, normal human blood contains the 

 following varieties of leucocyte, together with the percentages 

 they form of the total number of leucocytes : 



1. Lymphocytes, large and small (22 per cent.). 



2. Polymorphonuclear neutrophilous leucocytes (70 per 

 cent.). 



3. Large hyaline mononuclear leucocytes (4 per cent.). 



4. Transitional leucocytes (i per cent.). 



5. Eosinophilous leucocytes (2*5 per cent.). 



6. Basophilous leucocytes or " mast-cells " (0-5 per cent.). 



