IMMUNITY AGAINST MICROBES. 443 



The amoeboid cells which enter into the formation of the 

 nodule are derived from small round mononucleated cells, which 

 exactly resemble ordinary leucocytes. The nucleus of such a 

 cell is round, stains deeply with hsematoxylin or alum carmine, 

 and shows no trace of intra-nuclear network. The delicately 

 mottled protoplasm surrounding the nucleus varies somewhat 

 in amount, whilst the whole cell is sometimes quite round, and 

 occasionally slightly irregular in shape. I cannot say for 

 certain whether these small round cells are all derived from 

 emigrating leucocytes, or whether some are not derived from 

 pre-existing connective-tissue cells. Both modes of origin are 

 probably the rule, for at the periphery of the new growth — in 

 places where no parasites can be discovered — the connective- 

 tissue cells are often in a state of active proliferation. The 

 blood-vessels, their walls, and their immediate neighbourhood^ 

 on the other hand, are frequently crammed with leucocytes. 



It is quite easy to trace the development of epithelioid and 

 giant cells from leucocytes in the nodule of actinomycosis. 

 The nucleus of some of the small lymphocytes is occasionally 

 composed of a dark outer border, and a clear, but somewhat 

 swollen, centre (fig. M, a). In others the centre of the 

 nucleus becomes clearer, and the chromatin almost entirely 

 disappears in places, thus leaving a fine intra-nuclear network 

 possessing one or more nucleoli (fig. M, b). Owing to the 

 gradual disappearance of the chromatin in the centre and 

 periphery, the nucleus becomes clear and bladder-like, and 

 possesses a beautiful intra-nuclear network (fig. M, c) ; but 

 the protoplasm in this stage has as yet undergone no change, and 

 the whole cell may not be larger than an ordinary lymphocyte. 



The nucleus now slowly becomes larger, the protoplasm 

 increases enormously in size, becomes coarser and vacuolated, 

 with extremely irregular contours, and the cell finally presents 

 the appearance characteristic of an epithelioid cell (fig. M, 

 d, e). The nucleus now gradually changes its place and 

 approaches one of the poles of the cell. 



Just as in the healthy lymphoid tissues of animals the 

 larger amoeboid cells or macrophages take into their interior 



