STRUCTURE, MOTIONS, REPRODUCTION. 123 
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 observed 
‘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 ele- 
ments are not united in chains. These cells are believed by De Bary 
and others to have greater resisting power to desiccation than the re- 
maining cells in the culture, and to serve the purpose of reproductive 
elements. 
Hueppe groups all rod-shaped bacteria which do not form endo- 
spores under the generic name Arthrobactertum. This author be- 
lieves that, as a rule, bacilli which do not form endospores under 
certain circumstances produce more resistant cells which “take 
charge of the perpetuation of the species under the guise of a resting 
stage or spore.” According to Hueppe, true arthrospores are spheri- 
cal in form. 
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 aérobic 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. 
The pseudo-branching of the filaments in the genus Cladothrix 
has been referred to in the chapter on Morphology. Recent re- 
searches show that other bacteria heretofore included in the genus 
Bacillus may also present branching forms. This is especially true 
of the tubercle bacillus, which when obtained from cases of fowl 
tuberculosis not infrequently exhibits a sort of branching. Hueppe 
and Fischel have also demonstrated the presence of branching forms 
of the bacillus of mammalian tuberculosis, and as a result of his ob- 
