TEXT-BOOK OP BACTERIOLOGY. 7 



traction of the. protoplasmic contents (iodine, for example), then the 

 membrane becomes' clearly visible. It is either stiff or elastic, and 

 thereby determines the behavior of the whole cell; that is to say, 

 whether it can show contortions and bendings, or whether it must 

 remain in an unaltered stiff condition. 



Of great importance is the fact that the membrane, in its outer 

 layers, possesses a pronounced inclination to bulge. 



B5'^ absorbing water it passes into a jelly-like state, and then 

 surrounds the cell with a gelatinous covering, often very thick. 



This is well seen in a preparation of Friedlander's so-called cap- 

 sule-cocci as they are found in pneumonia. 



This capsule is, in fact, nothing more than such a gelatinous 

 sheath, which becomes visible as distinguished from the proper 

 bacteria cell by its much smaller capacity for absorbing color. 



Still more remarkable is the conduct of the membrane when the 

 bacteria divide. It prevents the immediate separation of the newly- 

 formed members; keeps up their connection, and thereby causes 

 the origin of bacterial groups from their simplest to their most 

 highly-developed forms. 



When two cocci still cleave together after their division — diplo- 

 cocci; or joining one to the other in rows — streptococci; arranging 

 themselves in clearly-defined groups — the staphylococci; when the 

 rod bacteria remain chained together in long threads, and when 

 on the surface of nutritive fluids containing bacteria the separate 

 cells unite into dense masses or films of mould; all this is but a 

 consequence of membrane-bulging. 



The union of cells of the same species have been called zooglea, 

 and from their behavior it has been attempted to gain criteria for 

 the distinction of certain species. 



The zooglea develop best in liquid media. This can be seen by 

 examining a pure culture of the Bacillus subtilis (the hay bacillus) 

 in beef bouillon in one of Erlenmeyer's little flasks. On the surface 

 can be seen the even, strong, slightly-furrowed grayish-white cov- 

 ering. 



If the plug of cotton-wool be removed from the mouth of the 

 flask and some of the scum taken out with a platinum needle, we 

 observe that it still coheres, and even in distilled water will dissolve 

 but little. The same thing takes place when bacteria are changed 

 from an originally solid field of nourishment to a fluid one. 



But the formation of firmly-united groups is by no means con- 

 fined to liquid substrata. This may be seen on the surface of a 

 slice of potato on which is growing a culture of the so-called potato 

 bacillus. It appears as a curiously-creased grayish-yellow covering. 



