MORPHOLOGY, REPRODUCTION, ETC. 15 



are a direct continuation of this peripheral zone of the bacterial 

 body, can not be decided. 



The manner in which bacteria move is naturally subject to some var- 

 iation depending upon the number and position of the fiagella possessed 

 by them. Whether bacteria exercise or not the power of motility de- 

 pends to a large extent upon their present or previous environment. 

 They are usually most motile in vigorous young cultures of from twenty- 

 four to forty-eight hours' growth in favorable media. In old cultures 

 motility may be diminished or even inhibited by acid formation or by 

 other deleterious products of the bacterial metabolism. 



At the optimum growth-temperature motility is most active, and a 

 diminution or increase of the temperature 

 to any considerable degree diminishes or 

 inhibits it. Thus actively motile organisms, 

 in the fluid drop, may be seen to diminish 

 distinctly in activity when left for any 

 prolonged time in a cold room, or when ^^^ 3.-Arhangement of 

 the preparation is chilled. Any influence, Bacterial Flagella. 



in other words, chemical or physical, which 



tends to injure or depress physiologically the bacteria in any way, at 

 the same time tends to inhibit their motility. 



Messea* has proposed a classification of bacteria which is based 

 upon the arrangement of their organs of motility, as follows: 



I. Gymnobacteria, possessing no flagella. 



II. Trichobacteria, with flagella. 



1. Monotricha, having a single flagellum at one pole. 



2. Lophotricha, having a tuft of flagella at one pole. 



3. Amphitricha, with flagella at both poles. 



4. Peritricha, with flagella completely surroimding the bac- 



terial body. 

 Bacterial Spores. — A large number of bacteria possesses the power of 

 developing into a sort of encysted or resting stage by a process commonly 

 spoken of as sporulation or spore formation. The formation of spores 

 by bacteria depends largely upon environmental conditions, and the 

 optimum environment for spore formation differs greatly for various 

 species. It is usually necessary that a temperature of over 20° C. 

 exist in order that spores may be formed. Unfavorable factors, Hke 

 acid formation, accumulation of bacterial products in old cultures, or 



' Messea, Cent. f. Bakt., I, Ref. ix, 1891. 



