CLASSIFICATION AND MORPHOLOGY 7 



A single cell usually produces but one spore, which may be located 

 in the centre or at the extremity of the mother-cell. The cell which 

 produces a spore may assume the shape of a drum-stick or a spindle 

 (known as a Clostridium). Spores cannot be stained by the ordinary 

 methods employed for bacteria (for special methods of staining Spores 

 see 26, 27). 



A spore about to develop into a bacillus exhibits the following 

 phenomena : 



1. It gradually loses its highly refractive appearance, enlarges, 



and appears to assume a consistency approaching to that 

 of the bacillus. 



2. The membrane of the spore is ruptured, and the bacillus 



grows out of the aperture. 



3. In some organisms the separation of the spore capsule is more 



evident than in others, and in this case the remains of the 

 capsule may adhere for some time to the young bacillus. 

 It is on account of the resistance of the spores that the 

 elaborate means of sterilization about to be described are 

 necessary for obtaining media free from germs, by which 

 in turn we can obtain pure cultures that is, separate culti- 

 vations of single species of bacteria. The resistance of 

 the spores is due to a very dense and impenetrable mem- 

 brane, as well as to the fact that the protoplasm contains 

 less water than that of the vegetative form. More water 

 in conjunction with protoplasm lowers the temperature at 

 which it coagulates. 



METHODS OF STERILIZATION. 

 STERILIZATION BY HEAT. 



Sterilization may be accomplished by subjecting the articles to be 

 treated to high temperatures, either in a moist or dry state. Success- 

 ful sterilization by dry heat cannot usually be accomplished at a 

 temperature lower than 150 C., and the objects should be subjected 

 to this temperature for not less than one hour. The apparatus used 

 for hot-air sterilization is double-jacketed, and made of strong sheet- 

 steel, preferably with a copper bottom, and having two perforated 

 movable shelves. On the top there is a regulating slide, by which a 

 current of air through the apparatus may be secured, and two tubes, 



