588 BIOLOGY OF THE MICRO-ORGANISMS. 



for the simple multiplication of cells. "What the con- 

 ditions are which must be present in order to set up the 

 phenomena of spore formation, a process on the whole 

 of rare occurrence, have not as yet been fully worked out. 

 From analogy we may suppose that the exhaustion or 

 vitiation of the nutrient medium forms the necessary 

 condition for the commencement of this act ; and as a 

 matter of fact this seems to be the case in many species. 

 Apparent exceptions have as yet been observed in bacillus 

 butyricus, bacillus subtilis, anthrax bacilli, &c. ; never- 

 theless, in the spore-bearing cultivations of these bacteria 

 it is possible that one or other of the nutrient materials 

 has become diminished to such an extent as to suffice 

 for the introduction of the process of spore formation, 

 or the nutrient substrata may have become sufficiently 

 saturated with the noxious products of tissue change of 

 the bacteria to render them imperfect as nutrient media. 

 It is possible that further observations will enable us 

 to formulate the conditions of spore formation in the 

 bacteria in a similar manner to those of the yeast fungi ; 

 in that case, however, the marked differences between 

 the individual species of bacteria must of course be 

 taken into account. The oxygen exerts a peculiar effect 

 on the spore formation of the bacteria. While in 

 the case of the mould and yeast fungi oxygen must have 

 free access, the bacteria are divided in this respect into 

 two groups. The majority also appear to require 

 oxygen for the formation of spores, and Prazrnowski 

 pointed out that it is characteristic of these forms that 

 they are non-motile during the stage of fructification. 

 The true anaerobes, however this has been demonstrated 

 with certainty as regards bacillus butyricus can only 

 fructify in the absence of oxygen, and continue to move 

 during the stage of fructification. 



The temperature exerts a marked influence on the 

 process. Koch* has shown, in the case of anthrax bacilli, 

 that a temperature of at least 16 C. is necessary for the 

 formation of spores; and under these circumstances 

 limited formation of spores did not occur till after seven 



* Mittheilimr/en a, d. Kais, Ges. Amt. } p. 05. 



