20 BIOLOGY AND TECHNIQUE 



fact, furthermore, that the sizes and contours of bacteria may vary to 

 some extent according to the medium on which they are grown. This 

 may, to a degree, be due to osmotic relations. On fluid media, for in- 

 stance, many bacteria may appear larger and of a less dense consistency 

 than do members of the same species cultivated upon solid media. 



Degeneration Forms. When bacteria are grown under conditions 

 which are not entirely favorable for their development, or when they 

 are grown for a prolonged period upon artificial culture media without 

 transplantation, there may occur variations which often depart consider- 

 ably from the ground type, known as degeneration or involution forms. 



FIG. 7. DEGENERATION FORMS OF BACILLUS PESTIS. (After Zettnow.) 



Thus, in the case of the diphtheria bacillus, old cultures may contain 

 long, irregularly beaded forms with broad expansions at the ends. 

 In the case of B. pestis the fact that large numbers of oval, vacuolated 

 bodies in old cultures are formed regularly has become of differential 

 value. 1 These degeneration forms are shown most characteristically 

 when the bacteria are cultivated on agar containing 3 to 5 per cent NaCl. 

 Among the cocci, marked evidences of involution are often seen in 

 cultures of the meningococcus in the form of large, swollen poorly- 

 staining spheres, and in the case of the pneumococcus in the so-called 

 shadow forms which have the appearance of empty capsules. There are 



1 Hankin and Leumann, Cent. f. Bakt., I, xxii, 1897. 



