BLOOD 
115 
in pure methyl alcohol, is now poured carefully upon the horizontal 
slide until the film is just covered by it. After standing thus for 
one minute the stain is diluted by adding drop by drop from a pip¬ 
ette enough distilled water to equal two or three times the volume 
of the stain. The diluted stain should then remain from five to 
ten minutes and should then be washed off gently with water. 
The slide should next be carried over in a horizontal position 
into a wide dish of distilled water for the purpose of differentiating 
the stain, and the washing continued until the more evenly spread 
portions of the film are yellowish or reddish in color. This should 
take from one to three minutes. The excess of water should then 
be drained off and absorbed by means of filter paper, and the 
preparation may then be put aside to dry. Since it is advisable 
to dry it as rapidly as posisble, the application of a very gentle 
warmth is recommended. When the drying is completed the slide 
is ready for examination. 
The differential staining makes it possible to distinguish 
between the varieties of leucocytes, which will be found scattered 
among the masses of pinkish orange erythrocytes. 
The following types should be identified (Cf. plates in refer¬ 
ence books showing the differences between the various types): 
a. Small mononuclear leucocyte (lymphocyte), with clearly 
defined, dark purplish-blue nucleus and a small amount of bluish- 
green cytoplasm in which a few reddish granules may be seen. 
These leucocytes vary in size from one to two times that of an 
erythrocyte, and constitute about 2o%-? > o% of all the leucocytes 
of normal blood. 
b. Large mononuclear leucocyte with the same general staining 
reaction as the small ones but less intensely so. The nuclei are 
proportionately smaller and are more variable in form from 
rounded to irregular. In size this type is from two to three times 
that of the erythrocytes. They constitute a very small percent¬ 
age, i%-8%, of the leucocytes of normal blood. 
c. Three varieties of polymorphonuclear leucocytes, all with 
very variable, irregularly lobed or subdivided nuclei, which may even 
appear to be fragmented into separate masses. These cells vary 
in size from two to three times that of an erythrocyte. The three 
varieties, distinguished from each other mainly by the differential 
