120 -IIMCirKK, MOTIONS, REPRODUCTION. 



ity of the oval spore, as shown at a, Fig. 70, arid in other species the 

 exosporium is ruptured and the bacillus emerges from the side. as 

 seen at b. 



The considerable resistance of these endogenous spores to desic- 

 cation, to heat, and to various chemical agents is an important fact 

 both from a biological and from a hygienic point of view, and will 

 be fully considered in a subsequent chapter. The fact that certain 

 bacilli and spirilla do not withstand a temperature of 80 to 90 C. , 

 which does not destroy the vitality of known spores, leads to the in- 

 ference that they do not form similar reproductive bodies. But re- 

 productive elements of a different kind are described by some botan- 

 ists as being produced during the development of these bacteria, 

 and also of the micrococci. These are the so-called arthrospores. 

 In the process of binary division certain cells in a chain may be ob- 

 served to be somewhat larger than others and to refract light more 

 strongly. The same may be true of certain cells in a culture in 

 which the elements are not united in chains. These cells are believed 



a-- 



FIG. 76. 



by De Bary and others to have greater resisting power to desiccation 

 than the remaining cells in the culture, and to serve the purpose of 

 reproductive elements. 



It has generally been supposed that spore formation is most likely 

 to occur when the pabulum for supporting the growth of the vegeta- 

 tive form is nearly exhausted. But, as pointed out by Frankel, facts 

 do not support this view, as many species form spores when condi- 

 tions are most favorable for a continued development. An abundant 

 supply of oxygen favors the formation of spores in aerobic species, 

 and, in some instances at least, the temperature has an important in- 

 fluence upon spore formation. Thus the anthrax bacillus does not 

 form spores at temperatures below 20 C. or above 42 C. 



The very interesting fact has been demonstrated by Lehman and 

 by Behring that a species which usually forms spores may be so 

 modified by certain influences that it is no longer capable of spore 

 production, and that such an asporogenous variety may be cultivated 

 for an indefinite time without showing any return to the stage of 

 spore formation. This was effected in Behring's experiments by 

 cultivating the anthrax bacillus in a medium containing some agent 

 detrimental to the vitality of the vegetative cells, but not in suffi- 

 cient quantity to restrain their development. 



