8 GENERAL MORPHOLOG Y AND BIOL OG Y. 



favourable for growth and multiplication. There is often a 

 temperature below which, while vegetative growth still takes 

 place, 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. On the other hand, however, most bacterio- 

 logists are 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. Sporulation can be prevented from 

 taking place for an indefinite time if a bacterium is con- 

 stantly supplied with fresh food (the other conditions of 

 life being equal). In old growths, where the food supply 

 is exhausted the bacteria have either become spores or have 

 died. Further, a spore will always develop into a vegeta- 

 tive form if placed in a fresh food supply. With regard to 

 the rapid formation of spores when the conditions are favour- 

 able for vegetative growth, it must be borne in mind that 

 in such circumstances the conditions may really very quickly 

 become unfavourable for a continuance of growth, for not 

 only will the food supply around the growing bacteria be 

 rapidly exhausted, but we have evidence that in such con- 

 ditions effete products are excreted which are inimical to 

 the life of the organisms excreting them. 



We must note that the 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 devised for the 

 purpose (vide p. 103); (2) the fact that the bacterium con- 

 taining the spore has higher powers of resistance against 

 inimical conditions than a vegetative form. It is important 

 to bear these tests in mind, as in some of the smaller 



