84 BACTERIA LESS. 



of a rod constricted in the middle. But it is only by using 

 the very highest powers of the microscope that its precise 

 form and structure can be satisfactorily made out. It is then 

 seen (Fig. 14) to consist of a little double spindle, showing 

 neither nucleus, vacuole, nor other internal structure. It 

 stains very deeply with aniline dyes, and from this and other 

 circumstances there is reason for thinking that the whole 

 cell consists of chromatin covered with a membrane of 

 extreme tenuity formed of cellulose. It may therefore be 

 considered as a cell consisting of cell-wall and nucleus only, 

 the cell-body being absent. At each end is attached a 

 flagellum about as long as the cell itself. 



Bacterium termo is much smaller than any organism we 

 have yet considered, so small in fact that, as it is always 



FIG. 14. Bacterium tcrmo ( x 4000), showing the terminal fiagella. 

 (After Dallinger.) 



easier to deal with whole numbers than with fractions, its 

 size is best expressed by taking as a standard the one- 

 thousandth of a millimetre, called a micromillimetre and 

 expressed by the symbol //. The entire length of the 

 organism under consideration is from i '5 to 2 /JL, i.e. about 

 the -$ mm. or the T O-TOTT inch. In other words, its entire 

 length is not more than one-fourth the diameter of a yeast- 

 cell or of a human blood-corpuscle. The diameter of the 

 flagellum lias been estimated by Dallinger to be about -| p 

 or ^o-jVuo mcn , a smallness of which it is as difficult to form 

 any clear conception as of the distances of the fixed stars. 



Some slight notion of these almost infinitely small dimen- 

 sions may, however, be obtained in the following way. Fig. 



