THE WHITE BLOOD-CORPUSCLES 921 



either of eosinophile cells or of the neutrophile cells, and either ondi- 

 tion is associated with changes in the red bone-marrow. We may 

 therefore provisionally arrange the leucocytes of the blood according 

 to their origin as follows : 



(1) Small lymphocyte derived from lymphoid tissue. 



(2) Large mononuclear or hyaline corpuscle : doubtful whether 

 derived by a growth of (1) or from a myelocyte. 



(3) Polymorphonuclear leucocyte formed in bone-marrow. 



(4) Eosinophile cell derived from similar cells in the bone-marrow. 

 This origin of the eosinophile corpuscle is rendered more probable by 

 the fact that the shape of the granule, which differs from one species 

 to another, is the same whether the cell be derived from the blood or 

 the bone-marrow. 



The intermediate or transitional cell may be derived either from 

 tho lymphocyte or from a myelocyte. In many cases of leukaemia the 

 myelocyte passes into the blood in large numbers without undergoing 

 the changes necessary to convert it into the typical blood-cell. We 

 find then mononuclear cells which are either free from granules or 

 contain eosinophile or basophile granules. 



FUNCTIONS OF THE LEUCOCYTES 



PHAGOCYTOSIS. We have seen that the leucocytes from whatever 

 animal they be taken present two phenomena, viz. that of amoeboid 

 movement and that of ingesting foreign particles which may be pre- 

 sented to them. On account of this power of eating up foreign 

 particles they are frequently spoken of as ' phagocytes/ in this respect 

 resembling unicellular organisms and the undifferentiated cells of 

 many kinds of tissue. All the phenomena connected with the process 

 of inflammation in higher animals are directed to the assemblage of 

 leucocytes at the spot which is the seat of injury or of infection, so 

 that they may devour and remove either the injured tissue or the 

 invading micro-organisms. This process plays therefore an important 

 part in determining the immunity of any animal against infection ; 

 though in the higher animals it is assisted by a number of other 

 mechanisms directed towards the same end, which we shall have to 

 discuss in a subsequent chapter. The use of phagocytosis is not, 

 however, confined to the protection of the organism against infection. 

 Wherever any effete or dead tissue has to be cleared away, whether as 

 the result of injury or in the course of metamorphosis of organs, the 

 leucocytes play an important part. Thus in the great rearrangement 

 of tissues which occurs in the larval state of insects, the removal of the 

 muscle fibres which are no longer required is effected by the accu- 

 mulation of phagocytes around them. The phagocytes may send 



