212 MICROBES, FERMEN'TS, AND MOULDS. 
It may be useful to retrace here the curious history 
of vaccine, since it is directly interesting to us all. 
cy aay 
Fig. 91.—Section of skin covering a small-pox pustule. a, horny layer of the 
epidermis; d, adenoid tissue; m, m, micrococci stained with methyl violet (x 
850 diam.). 
Before vaccine was discovered, inoculation with 
small-pox was practised as a preventive measure, 
