182 CLASSIFICATION OF THE MICRO-ORGANISMS. 



formation of micrococci into bacilli or vice versa. The positive 

 observations which furnish support to Zopf's views proceed 

 almost entirely from investigators who have not had a long 

 and special instruction and experience in bacteriological re- 

 search. These, however, are the investigators who are most 

 liable to fall into errors which lead to a mistaken assumption 

 of an alteration in form, and which are only avoidable by long 

 practice and particularly careful observations. In the first 

 place, it may easily happen that any roundish body which is 

 like a coccus maybe mistaken for a true coccus. Such a mis- 

 take is possible, for example, with spores and with involution 

 forms (see p. 175) ; further, young bacilli just formed by fission 

 show often only a very slight excess of the long diameter (see 

 Sources of P- 173) ; bacilli standing upright and seen in transverse 

 error in the section resemble cocci ; by the use of staining materials a 

 proof. method to which we owe the most beautiful facts, but which 



in unskilled hands is a dangerous means and often leads to 

 mistakes a coccus form may be produced by staining very 

 old bacilli which no longer take up the colouring matter in 

 their whole extent, or by heating too strongly, or by employ- 

 ing differentiating materials for too long a time, so that again 

 only a partial staining results. In investigating cultivations 

 also, the possibility of contamination from without is never 

 absolutely excluded, and abnormal forms must always in the 

 first place be closely tested on the suspicion that they may be 

 derived from contamination with other fungi; it is only 

 when we repeatedly obtain similar results after employing 

 all precautions for maintaining pure cultivations that we can 

 conclude that all the forms observed have arisen from one 

 and the same species. 

 Eefutation of These numerous sources of error certainly justify us in 



some state- exercising a certain reserve with regard to the statements as 



ments made . . 



in support of to an extensive alteration in form of the bacteria. Many of 



Zopf's these statements have already been shown to be undoubtedly 



hypothesis. erroneous . thus Kurth has, in the case of the bacterium 

 Zopfii which is according to Zopf a striking example of the 

 great mutability of form of the fission fungi, seen, according 

 to his own description, spores, but not cocci, originating from 

 the bacilli ; for these " cocci " did not multiply by fission, but 

 grew directly to rods in fresh nutritive solutions, and were less 

 sensitive to external influences than the rods. In like manner 

 the bodies which Hauser of late saw arising from bacilli, and 

 which he erroneously termed cocci, do not produce new cocci 

 by fission; while the spirulina of the same author represent 

 simple bending and irregular intertwining of threads such 

 as occurs in every long flexible thread. To mention another 

 example, we may refer to dc Bary's bacillus megaterium. 



