44 CELL STRUCTURES 



Flagella are too fine and delicate to be seen on the living 

 organism, or even on bacteria which have been colored by 

 the ordinary stains. They are rendered visible only by 

 certain methods which cause a precipitate on both bacteria 

 and flagella which are thereby made thick enough to be 

 seen (Chapter XIX, p. 194). The movement of liquid 

 around a bacterium caused by vibrations of flagella can 

 sometimes be observed with large forms and the use of 

 "dark-field" illumination. 



Flagella are very delicate and easily broken ofl^ from the 

 cell body. Slight changes in the density or reaction of the 

 medium frequently cause this breaking off, so that prepara- 

 tions made from actively motile bacteria frequently show 

 no flagella. For this reason and also on account of their 

 fineness the demonstration of flagella is not easy, and it is 

 not safe to say that a non-motile bacterium has no flagella 

 except after very careful study. 



The motion of bacteria is characteristic and a little prac- 

 tice in observing will enable the student to recognize it and 

 distinguish between motilitj- and "Brownian" or molecular 

 motion. Dead and non-motile bacteria show the latter. 

 In fact, any finely divided particles suspended in a liquid 

 which is not too viscous and in which the particles are not 

 soluble show Brownian motion or "pedesis." This latter is 

 a dancing motion of the particle within a very small area 

 and without change of place, while motile bacteria move 

 from place to place or even out of the field of the microscope 

 with greater or less speed. There is a marked difference in 

 the character of the motion of different kinds of bacteria. 

 Some rotate around the long axis when moving, others 

 vibrate from side to side. 



Among the higher thread bacteria there are some which 

 show motility without possessing flagella. Just how they 

 move is little understood. 



Spores. — Under certain conditions some bacterial cells 



undergo transformations which result in the formation of 



so-called spores. If the process is followed under the micro- 



. scope, the changes observed are approximately these : A very 



minute point appears in the protoplasm which seems to act 



