20 BIOLOGY AND TECHNIQUE 



VARIATIONS OF BACTERIAL FORMS 



Variations from the basic forms considered in the preceding sec- 

 tion may occur, but are not common among bacteria under normal 

 conditions. Thus the formation of club shapes by a thickening of the 

 bacillary body at one or both ends has been frequently observed 

 among bacteria of the diphtheria group, and in the glanders bacillus, 

 and an" irregular beading is not infrequently observed in tubercle 

 bacilli under normal conditions. Such pictures can not, in these 

 cases, be regarded as degeneration or involution forms, since they are 



FIG. 6. DEGENERATION FORMS OF BACILLUS DIPHTHERIA. (After Zettnow.) 



visible in young, actively growing cultures under ordinary condi- 

 tions. It is a well-known fact, furthermore, that the sizes and con- 

 tours 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 instance, 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 with- 

 out transplantation, there may occur variations which often depart 

 considerably from the ground type, known as degeneration or invo- 

 lution forms. Thus, in the case of the diphtheria bacillus, old cul- 



