38 



PROTOPLASM AND THE CELL 



While small, the bacteria nevertheless have fairly definite 

 forms which may be grouped for convenience under three main 

 types: i. the bacillus or rod; 2. the coccus or ball; and 3. the 

 spiral or corkscrew. They are frequently united in chains or 

 filaments, in plate form (sarcina), or embedded in a gelatinous 

 matrix which they secrete (zoogloea) (Fig. 16). 



The bacteria are usually regarded as single-celled organisms 

 although the complete cell structure is rarely present. The 

 majority have no cell nucleus but contain from one to many 

 granules of chromatin distributed throughout the cell, these 



O 



B 



D 



FIG. 16. Comparative size of human blood corpuscle, typhoid bacillus, influ- 

 enza bacillus, giant bacillus from the intestine of a cockroach, and a common 

 water spirillum. 



granules performing the functions of nuclei (Fig. 16, D). The 

 cells are enclosed in firm cell membranes probably composed 

 of cellulose or an allied substance which are unbroken except 

 in a small number of forms provided with flagella. 



Reproduction. All bacteria multiply by transverse division 

 of the cell (Fig. 17). Division is followed by rapid growth, and 

 cycles of growth and division follow one another in quick suc- 

 cession (hay bacillus 30 minutes, cholera vibrio 20 minutes). 

 "It has been estimated that if bacterial multiplication went on 

 unchecked, and the division of each cell took place as often as 



