BACTERIA 



35 



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 chroma tin distributed throughout the cell; these 

 granules correspond to the nuclei of tissue cells (Fig. 16, D). 

 The cells are enclosed in firm cell membranes, probably com- 

 posed of cellulose or an allied substance, which are unbroken ex- 

 cept in a small number of forms provided with flagella. 



O 



B 





m 



FIG. 16. Comparative size of A, human blood corpuscle, B, typhoid bacillus, 

 C, influenza bacillus, D, giant bacillus from the intestine of a cockroach, and E, 

 a common water spirillum. 



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 

 once an hour, the descendants of each individual would in two 

 days number 281,500,000,000, and that in three days the prog- 

 eny of a single cell would balance 148,356 hundredweight!" 

 (Jordan, General Bacteriology, p. 61.) Such increase does not 

 take place in nature, however, because of various external in- 



