114 STRUCTURE, MOTIONS, REPRODUCTION. 



cells may remain attached to each other, forming chains (strepto- 

 cocci) or articulated filaments (scheinfdden of the Germans). 



The bacilli and spirilla divide only in a direction transverse to the 

 long diameter of the cells, but among the micrococci division may 

 occur either in one direction, forming chains ; or in two directions, 

 forming tetrads ; or in three directions, forming ' ' packets " of eight 

 or more elements. The staphylococci, in which the cells do not re- 

 main associated, divide indifferently in any direction. 



The rapidity of multiplication by binary division varies greatly in 

 different species, and in the same species depends upon conditions re- 

 lating to the culture medium, age of the culture, temperature, etc. 

 Under favorable conditions bacilli have been observed to divide in 

 twenty minutes, and it is a matter of common laboratory experience 

 that colonies of considerable size and containing millions of bacilli 

 may be developed from a single cell in twenty-four to forty-eight 

 hours. A simple calculation will show Avhat an immense number of 

 cells may be produced in this time as a result of binary division oc- 

 curring, for example, every hour. The progeny of a single cell 

 would be at the end of twenty-four hours 16,777,220, and at the end 

 of forty-eight hours the number would be 281,500,000,000. 



Some of the earlier observers have noted the presence of oval or 

 spherical refractive bodies in cultures containing bacilli ; but that 

 these were reproductive elements, although suspected, was not de- 

 monstrated until a comparatively recent date. Pasteur was one of 

 the first to point out the fact that certain bacteria have two modes of 

 reproduction by fission and by the formation of endogenous spores ; 

 but the first careful study of the last-mentioned method was made by 

 Koch in his classical study of the anthrax bacillus (1878), and by 

 Cohn, who studied the formation of spores in Bacillus subtilis. 



These reproductive bodies serve the same purpose in the preserva- 

 tion of species as the seeds of higher plants. They resist desiccation 

 and may retain their vitality for months or years until circumstances 

 are favorable to their development, when, under the influence of heat 

 and moisture, they reproduce the vegetative form bacillus or spiril- 

 lum with all of its biological and morphological characters. They 

 are composed of condensed protoplasm which retains the vital char- 

 acters of the soft protoplasm of the mother cell from which it has 

 been separated ; and it is evident that whether reproduction occurs 

 by fission or by the formation of endogenous spores, the protoplasm 

 of the cells in a pure culture of any microorganism is simply a sepa- 

 rated portion of the protoplasm of the progenitors of these cells. 



Some of the bacilli grow out into long filaments before the forma- 

 tion of spores occurs ; and these filaments may be associated in bun- 

 dles or intertwined in irregular masses. At first the protoplasm of the 



