BACTERIUM TERMO 83 



Fig. 14, A : it is like a minute finger-biscuit, i.e. has the form 

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

 very highest powers of the microscope that its form and 

 structure can be satisfactorily made out. It is then seen 

 (Fig. 15) to consist of a little double spindle, showing neither 

 nucleus, vacuole, nor other internal structure. It is com- 

 posed of a particular variety of protoplasm, and is sur- 

 rounded by a membrane of extreme tenuity formed of 

 cellulose. 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 

 easier to deal with whole numbers than with fractions, its 

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



FIG. 15. Bacterium termo (X 4000), showing the terminal flagella. 

 (After Dallinger.) 



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 /z, i.e. about 

 the y^Q- mm. or the jo-Jo-Q- 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 has been estimated by Dallinger to be about | //. 

 or 2 orVoir 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. 

 15 shows a Bacterium termo magnified 4000 diameters, the 

 scale above the figure representing -^ mm. magnified to the 

 same amount. The height of this book is a little over 18 cm.; 



G 2 



