WHITE CORPUSCLES. 149 



in the adult are found normally only in the marrow and are called myelo- 

 cyles. They enter the blood when their protoplasm is full of the granules 

 which develop gradually, and when their nuclei are polymorphous. Only 

 in disease are myelocytes and erythroblasts found in the blood of adults 

 but they circulate normally in the blood of young embryos. The important 

 question, whether the leucocytes arise directly from the mesenchymal 

 tissues of lymph gland and bone marrow, or from cells which have emi- 

 grated into them from the blood vessels, has not been determined. 



The large mononuclear cells with round nuclei are thought by some 

 to be cells from which both lymphocytes and granular forms arise. The 

 granules may be secretory products. Eosinophilic granules were once 

 thought to be transformations of the neutrophilic, occurring in old cor- 

 puscles. Lately they have been regarded as the ingested fragments of 

 red corpuscles, but the fact that they rarely, if ever, are mixed with 

 neutrophilic granules is against this view. The form of granule seems 

 to be determined by unknown factors early in the differentiation of the 

 leucocytes, and to be fixed for a given cell after the first granules have 

 appeared. 



In connection with the terms applied to leucocytes it should be noted 

 that those with basophile granules are no.t called basophiles as would be 

 consistent, but mast cells. The non-granular lymphocytes and large mono- 

 nuclear cells are, however, sometimes called basophiles because their 

 protoplasm takes a pale basic stain. This is undesirable. Mast cells 

 were originally called plasma cells, a term now applied to oval cells derived 

 from lymphocytes by an increase in their protoplasm (Fig. 49, p. 47). 

 They have eccentric nuclei, and their non-granular protoplasm stains deeply 

 with basic dyes. Plasma cells occur in connective tissue, but probably 

 not in the blood; they are of pathological importance. 



The varieties of leucocytes may be reviewed as follows : 



Lymphocytes, 22 to 25% of the leucocytes, are small (about the size 

 of a red corpuscle) or large (perhaps twice the diameter of a red corpuscle), 

 non-granular, with round checkered nuclei. 



Large mononuclear leucocytes, i to 3%, may be two or three times the 

 diameter of red corpuscles. They are non-granular, or with few granules, 

 and have pale vesicular nuclei, round or crescentic. 



Polymorphonuclear leucocytes, larger than red corpuscles, are gran- 

 ular, with nuclei variously constricted or bent. They include, 



Mast cells, 0.5%, with very coarse basophilic granules obscuring 



the nucleus. 



Eosinophiles, 2 to 4%, with coarse eosinophilic granules. 

 Xeutrophiles, 70 to 72%, with fine neutrophilic granules. 



