32 BACTERIOLOGY 



and the anthrax bacillus considerably larger than this, and 

 in the accompanying figure will be found a diagrammatic 

 representation of these organisms compared, as regards size, 

 with red blood-corpuscles, each being magnified about 2500 

 diameters. 



In this connection we may recall the fact already mentioned 

 on page 10 that, of certain other organisms, the filter-passers, 

 the majority are still invisible even with this high magnifica- 

 tion ; though at all events one of those so-called filter-passers, 

 the organism of contagious pleuro-pneumonia in cattle, is just 

 visible under the highest powers at present available. This 

 organism is a little over a tenth of a micromillimetre in 

 diameter, which measurement, i.e. O'l ft, has been calculated 

 to be the limit beyond which our present-day microscopes 

 cannot render objects visible. 



Structure of Bacteria. 1 Owing to the minute size of 

 bacteria it is very difficult to make out their finer structure. 

 They consist of semi-fluid, jelly-like protoplasm, clear, re- 

 fractile, more or less homogeneous, or sometimes slightly 

 granular in appearance. The outer layer is condensed into a 

 firmer " skin " the cell-wall or cell-membrane which does 

 not consist of cellulose as is the case with vegetable cells. There 

 is no nucleus present, as there is in the higher animal and 

 vegetable cell, the nuclear material or chromatin, so called 

 from its affinity for various dyes, being scattered diffusely or 

 in granules, or sometimes aggregated towards certain parts 

 especially the ends of the bacterial body. The presence of 

 droplets of fat and granules of various substances such as 

 sulphur, starch, glycogen, etc., may in some cases be demon- 

 strated by their chemical reactions; and pigment granules 

 or particles of coloured material are sometimes found. 



Young and actively growing bacteria usually show a well- 

 defined and regular contour, but when old, or placed in un- 

 favourable circumstances, they show various degenerative 

 changes, such as irregularities in size and shape, vacuolation 

 (i.e. the appearance of small areas like empty holes in the 

 protoplasm not in reality empty as the name would imply, 

 but filled with clear, colourless fluid), alterations in their 

 staining capacity, and other changes. 



Many bacteria are actively motile, i.e. can themselves travel 

 in a fluid medium ; others do not possess this capacity. Such 

 motility must not be confused with passive convection or 

 carriage of contained bacteria by currents in a fluid, or by the 

 fluid itself travelling from place to place, as in the case of a 



1 For many of the points here described, see Frontispiece. 



