SPECIAL CHARACTERS 31 



they resist the agencies quickly fatal to the adult 

 or vegetative forms. Bacteria in their ordinary de- 

 velopment are said to be vegetating, and we must 

 differentiate between the vegetative stage and the 

 spore-forming stage. 



Protozoa (sing., Protozoon). Protozoa are single-cell 

 animals of protean shape. They vary in size from that 

 of the smallest bacterium to nearly one-quarter of an 

 inch in length. They are made up of a fairly well- 



FIG. 9. Unstained spores in distended ends of bacilli. (Park.) 



formed wall which may have an appreciable thickness 

 or be merely an immeasurable line. Their cytoplasm, 

 unlike that of bacteria, is usually far in excess of the 

 nucleus. It is sometimes homogeneous, at other times 

 full of granules, septa, or a dividing meshwork. The 

 nucleus is a complex body varying from a simple, 

 bladder-like mass to a dense and intricately wound 

 skein. The vital activity of the protozoan cell seems 

 to lie in a small body, usually in the protoplasm, but 

 originating from the nucleus, called the centrosome. 



