246 



THE DEVELOPMENT OF THE VASCULAR SYSTEM 



2. Large mononuclear leucocytes (1 to 3 per cent, of the leucocytes) are devel- 

 oped from the endothehal cells hning the medullary sinuses of the lymph glands 

 (Evans, Anat. Record, vol. 8, 1914), or from the reticulum of lymph glands (Kyes, 

 1915). 

 II. Granular or Polymorphonuclear Leucocytes: 



The blood-forming cells lodged in the red bone marrow are known as pre- 

 myelocytes. They give rise to myelocytes, cells with round or crescentic nuclei and 

 granular cytoplasm. Similar cells are developed in the lymphoid organs. By 

 undergoing changes (1) in the form and structure of their nuclei, (2) in the size 

 and staining quahties of their cytoplasmic granules, the myelocytes give rise to 

 three types of granular leucocytes: 



^P Q 





-J^. 



"\ 



P 



Fig. 253. — Giant cell from the bone marrow of a kitten, showing pseudopodia extending into a blood 

 vessel (F), and giving rise to blood plates {bp) (Wright). 



1. Neutrophiles, or leucocytes with a finely granular cytoplasm which is 

 neutral in its staining reactions, coloring sKghtly with both acid and basic stains. 

 In development, their nuclei take up an eccentric position and become crescentic, 

 horse-shoe shaped, or in the older stages moniliform (three or four pieces linked 

 together) . As it changes in form the nucleus undergoes pyknosis and stains in- 

 tensely. Neutrophiles are produced in the bone marrow of the embryo during 

 the fifth month. In the human adult they form 70 to 72 per cent, of the leuco- 

 cytes in normal circulation. 



2. Eosinophiles, or coarsely granular leucocytes, are characterized by their 

 large cytoplasmic granules which stain intensely red with eosin. In develop- 

 ment the nucleus becomes bilobed. Eosinophiles form 2 to 4 per cent, of the 

 leucocytes in normal human blood. 



