42 BACTERIOLOGY. 



which retard and eventually stop growth. Assuming- the 

 continuance of favorable conditions, a single cell, dividing 

 in 30 minutes, would be represented at the end of 12 hours 

 by more than sixteen million descendants. In 24 hours the 

 number would rise to more than two hundred and eighty 

 billions. 



The above figures will serve to impress the fact that 

 bacteria multiply with extreme rapidity. Moreover, they 

 will help to understand what marked chang-es may result in 

 a comparatively short period of time when certain bacteria 

 develop in milk, meats or in the living- animal body. Small 

 as bacteria are, owing- to their rapid multiplication, they 

 may give rise to poisonous substances in sufficient amount 

 to cause in a few hours profound poisoning, or even death. 



FIG. 8. Cell division (diagrammatic), a Bacillus, showing deflection of outer 

 membrane; b Micrococcus. 



The process of division can best be observed in large 

 bacilli. Owing to the absence of a definite nucleus, and of 

 any cell structure, the change that takes place during 

 multiplication is very simple. When the bacillus has 

 attained the fully developed stage, a slight transverse 

 constriction appears at the middle of the rod. The ring- 

 like process of the cell-wall gradually extends toward the 

 center of the cell until eventually the protoplasm becomes 

 divided into two halves. Usually the first indication that 

 cell division has taken place is the appearance of the clear, 

 delicate transverse line. The division of bacilli and of 

 spirals is always transverse, never longitudinal. 



The wall that divides the cell into two is to be consid- 

 ered as an ingrowth of the cell membrane. Inasmuch as in 



