MATERIAL WITH WHICH TO BEGIN WORK. 263 



the naked eye. In a few cases we may still see nothing 

 more than a smooth, non-characteristic surface ; while 

 in others minute, sometimes regularly arranged tiny 

 corrugations may be observed. In one colony they may 

 appear as tolerably regular lines, radiating from a cen- 

 tral spot; and again they may appear as concentric 

 rings; and if by. the methods which have been de- 

 scribed we obtain from these colonies their individual 

 components in pure culture, we shall see that this 

 characteristic arrangement in folds, radii, or concentric 

 rings, or the production of color, is characteristic of the 

 growth of the organism under the conditions first 

 observed, and by a repetition of those conditions may 

 be reproduced at will. 



So much for the simplest naked-eye experiment that 

 can be made in bacteriology, and wliich serves to furnish 

 the beginner with material upon which to commence his 

 studies. It is not necessary at this time for him to bur- 

 den his mind with names for these organisms ; it is suffi- 

 cient for him to recognize that the}' are mostly of differ- 

 ent species, and that they possess characteristics which 

 will enable him to differentiate the one from the other. 



In order now for him to proceed it is necessary that 

 he should have familiarized himself with the methods 

 by which his media are prepared and the means em- 

 ployed in sterilizing them and retaining them sterile — 

 i. e., of preventing the access of foreign germs from 

 without — otherwise his efforts to obtain and retain his 

 organisms as pure cultures will be in vain. 



Exposure and Contact. — Make a number of plates 

 from bits of silk used for sutures, after treating them 

 as follows : 



Place some of the pieces (about 5 centimetres long) 



