9 



in glycerin or Canada balsam. The slides are now ready to be examined 

 with the oil immersion lens. 



Examination. Examination in various stages shows that the strep- 

 tococci in the leucocytes multiply, burst open the cell and form a small 

 colony surrounding the cell which contained them. Figures 2 and 3 are 

 photomicrographs of such preparations at different stages. In Figure 2 

 the leucocyte has just been burst open and the streptococci are beginning 

 to grow out of it; in Figure 3 a considerable colony has already formed 

 around the ruptured leucocyte. These colonies may be found in all stages, 

 by varying the time of incubation. 



Fig. 2. Photomicrograph x^W- Multiplication of streptococci after their 

 ingestion (in vivo) by a guinea-pig's leucocyte. 



The leucocytes do, however, kill streptococci if the conditions are 

 right, as is shown by the following two facts: i. Organism No. 300 is 

 not virulent for guinea-pigs, and we have seen that the leucocytes take' 

 up the cocci alive. The guinea-pig must destroy these organisms some- 

 where, and we can find no substance in its body that is capable of doing 

 this except the leucocytes and the bone marrow. We have seen in Table 

 2 that the leucocytes destroy streptococci in vitro, and the inference is- 

 that they also destroy them in vivo. 2. Plates made with three loopfuls 

 of the exudate when taken from the guinea-pig contained 2,540 colonies 

 of streptococci after twenty-four hours' incubation, while plates made 



