8 GENERAL MORPHOLOGY AND BIOLOGY. 



spore formation only occurs at temperatures specially 

 favourable for growth and multiplication. There is often a 

 temperature below which, while vegetative growth still takes 

 places, sporulation will not occur, and in the case of B. 

 anthracis, if it be kept at a temperature above the limit at 

 which it grows best, not only are no spores formed, but the 

 species may lose the power of sporulation. Furthermore, 

 in the case of bacteria preferring the presence of oxygen for 

 their growth, an abundant supply of this gas may favour 

 sporulation. It is probable that even among bacteria pre- 

 ferring the absence of oxygen for vegetative growth, the 

 presence of this gas favours sporulation. Most bacterio- 

 logists are, however, of opinion that when a bacterium forms 

 a spore, it only does so when its surroundings, especially 

 its food supply, become unfavourable for vegetative growth ; 

 it then remains in this condition until it is placed in more 

 suitable surroundings. Such an occurrence would be 

 analogous to what takes place under similar conditions in 

 many of the protozoa. Often sporulation can be prevented 

 from taking place for an indefinite time if a bacterium is 

 constantly supplied with fresh food (the other conditions of 

 life being equal). The presence of substances excreted by 

 the bacteria themselves plays, however, a more important 

 part in making the surroundings unfavourable than the mere 

 exhaustion of the food supply. A living spore will always 

 develop into a vegetative form if placed in a fresh food 

 supply. With regard to the rapid formation of spores when 

 the conditions are favourable for vegetative growth, it must 

 be borne in mind that in such circumstances the conditions 

 may really very quickly become unfavourable for a continu- 

 ance of growth, for not only will the food supply around 

 the growing bacteria be rapidly exhausted, but the excre- 

 tion of effete and inimical matters will be all the more 

 rapid. 



We must note that the usually applied tests of a body 

 developed within a bacterium being a spore are (i) its 

 staining reaction, namely, resistance to ordinary staining 

 fluids, but capacity of being stained by the special methods 



