THE COLORLESS BLOOD-CELLS. 97 



known as lymphocytes, in recognition of their origin from lymphoid tissue, 

 and the others as leucocytes. The genetic relations of these groups are still a 

 subject of discussion. According to one view, all forms of colorless cells are 

 derived from similar sources, the recognized varieties being only different 

 stages in the development of the same elements. The other view regards 

 the leucocytes and the lymphocytes as distinct in origin, the latter arising 

 within lymphoid tissue and the former chiefly within the bone- marrow. 

 A discussion of this mooted subject is beyond the purpose of these -pages. 

 Suffice it to note, that facts concerning the early development of these cells 

 lend support to the belief that there is a close primary relation between the 

 two groups. 



If a thin film of blood be fixed by heat and stained with a "triacid 

 stain ' ' the following varieties of colorless cells are distinguishable : 



1. Small Lymphocytes. These are distinguished by a large com- 

 pact deeply staining nucleus that occupies almost the entire cell. The 

 meagre cytoplasm, reduced to a mere narrow peripheral zone, is devoid of 

 granules. The small lymphocytes measure about 7.5 /* in diameter and 

 constitute from 20-30 per cent, of all the colorless cells. 



2. Large Lymphocytes. These are presumably parent cells of the 

 preceding variety, from which they differ in their larger size (12-15 /*) 

 and relatively small oval nucleus. The cytoplasm is nongranular and com- 

 paratively plentiful. 



3. Large Mononuclear Leucocytes. These elements, from 10- 

 15 fi in diameter, exhibit clear pale nuclei, which are usually eccentrically 

 placed and oval or slightly indented. The cytoplasm, generous in amount, 

 appears uniform save for fine neutrophilic granules which are often present. 



4. Polymorphonuclear Leucocytes. These are by far the most 

 common type of white cells, of which they constitute approximately 70 per 

 cent. Their diameter is about 10 p., hence they are somewhat larger than 

 the erythrocytes. The cytoplasm is relatively abundant and contains fine 

 neutrophilic granules. The nuclei are very conspicuous on account of their 

 great diversity of form. At first sight they appear multiple, but on closer 

 examination are seen to consist of irregular nuclear segments connected 

 by delicate processes. Occasionally, however, two or more actually isolated 

 nuclei exist, in such cases the cells being truly polynuclear. 



5. Eosinophiles. The blood-cells of this type resemble the poly- 

 morphonuclear leucocytes in size (lo/^) and in the tabulated form of their 

 nuclei. Their distinguishing feature is the presence of coarse, highly 

 refracting granules within the cytoplasm that display an especial affinity for 

 eosin and other acid dyes. The eosinophiles are prone to rupture, the pale 

 nucleus then lying in the midst of a swarm of brightly tinged granules. 



Mast-cells, with coarse basophilic granules, and the finely granular 

 basophiles are other granular colorless cells that are occasionally observed, as 

 are also the myelocytes derived from the bone-marrow. They are, however, 

 rarely present in normal blood. 



The Blood-Plates. In addition to the erythrocytes and colorless 

 cells, the blood of man, and perhaps of other mammals, constantly contains 

 small bodies, the blood-plates or plaques. They are exceedingly prone to 

 change, or indeed to disappear altogether, when exposed to the air; hence, 

 to insure their presence in an unaltered condition, the blood must be drawn 

 directly into a drop of .8 per cent, salt solution, or, still better, into one of 

 osmic acid solution. After such precautions, the blood-plates appear as 

 round or oval disks, from 2-4 it in diameter, commonly somewhat less than 



