BLOOD. 1 1 5 



present in the circulating blood, especially at the periphery of 

 the stream i. e., near the vessel walls. They can thus easily 

 leave the vessel by pushing in between the endothelial cells and 

 reaching the connective tissue (diapedesis). 



There are many kinds of leucocytes to be distinguished, 

 and these may be divided according to their general form and 

 the character of their nuclei into the following groups : 



1. Small Mono nuclear Leucocytes (Lymphocytes}. These are 

 cells about the same size as a red blood-corpuscle, sometimes a 

 little smaller and sometimes a little larger. Their protoplasm 

 is very small in quantity, slightly granular, and arranged like 

 a ring around the nucleus. It stains not very deeply in acid 

 dyes. The nucleus is large in proportion to the size of the cell 

 and takes a deep stain. The chromatin is in large masses so 

 arranged that the nucleus seems to be mapped out in areas. 

 This has given rise to the term checker-board nucleus. These 

 cells are recognized readily in stained preparations by their 

 deeply staining characteristic nucleus, and their almost invisible 

 protoplasm. They form about 20-30 per cent, of all the leu- 

 cocytes in the blood. They are identical also with the lympho- 

 cytes which make up the lymphoid masses in the intestine, 

 lymph glands, tonsils, thyrnus, and spleen. 



2. Large mononuclear leucocytes are cells very much larger 

 than lymphocytes. The protoplasm is very abundant, usually 

 clear, and capable of taking only a faint eosin coloring. 

 Occasionally granules of various kinds are present. The 

 nucleus is large, oval, and somewhat vesicular. It is situated 

 usually peripherally, and stains very faintly in basic dyes. 

 These cells form about 4-8 per cent, of all the leucocytes in 

 normal blood. 



3. Transitional leucocytes are not usually so large as the 

 large mononuclears. They are characterized mainly by the 

 horseshoe-shaped nucleus. This is a nucleus staining fairly 

 deeply, not constricted in any place, but bent either slightly or 

 in the form of the letter U. These cells may contain various 

 kinds of granules in their protoplasm. 



4. Poly morpho nuclear leucocytes are so named from the 



