BLOOD AND H^MOPOIETIC ORGANS 85 



that, when a demand for a greater number of cells of 

 one type occurs, the lymphoid cells in any blood-forming 

 organ are able to produce that type. 



The place of origin of the large mononuclears and of 

 the transitional cells derived from them is uncertain, 

 but it is probably in the bone marrow, though in part 

 also they may come from the lymph glands and the 

 spleen. 



Various origins have likewise been described for the 

 eosinophils, but they, too, seem to arise from the 

 eosinophil myelocytes of the marrow. 



The mast cells or basophils are probably also derived 

 from the bone marrow, and their number in the blood is 

 therefore increased as a result of the proliferation of the 

 marrow in leukaemia. 



It will thus be seen that the marrow is by far the 

 most important seat of blood formation, for not only 

 is it the sole producer of red corpuscles, but it also gives 

 origin to all the granular leucocytes, to the large mono- 

 nuclears, and probably also to some extent to lymphocytes, 

 whilst adenoid tissue can give rise to lymphocytes only. 



The functions of the different forms of white 

 coppuscle have been as much disputed as their genesis. 

 Metchnikoff's discovery of phagocytosis at once raised 

 the leucocytes to a position of great esteem as protectors 

 of the body against disease. They were regarded at first 

 as ' soldiers ' ready to rush out and destroy any invading 

 micro-organisms, and it became the fashion to speak of 

 them as if almost endowed with a sentient intelligence 

 and great discriminative powers. Further investigations 

 tended to throw doubt on the ability of the leucocytes 



