REPRODUCTION 137 



place midway between these aggregations, and a sep- 

 tum is formed in the interior of the cell at right angles 

 to its length. 



3 . The division deepens, the septum divides into two 

 lamellae, and finally two cells are formed. 



000 



CD CD 



OO 



FIG. 88. Fission of cocci. FIG. 89. Fission of bacteria. 



4. The daughter cells may remain united by the 

 gelatinous envelope for a variable time. Eventually 

 they separate and themselves subdivide. 



Cultures on artificial media, 

 after gro wing in the same me- 

 dium for some time i. e., when 

 the pabulum is exhausted show 

 "involution forms" (Fig. 90), 

 well exemplified in cultures of B. 

 pestis on agar two days old, B. 

 diphtheria? on potato four to 

 six days old. 



They are of two classes, viz. : 



(a) Involution forms charac- 

 terised by alterations of shape 

 (Fig. go). (Not necessarily . 



v & y ' v ^ FIG. 90. Involution forms. 



dead.) 



(b) Involution forms characterised by loss of staining 

 power. (Always dead.) 



Resting Stage. Spore Formation. Conditions in- 

 fluencing spore formation: In an old culture nothing 

 may be left but spores. It used to be supposed that 

 spores were always formed, so that the species might 

 not become extinct, when 



(a) The supply of nutrient was exhausted. 



