88 Lectures on Bacteria. [ ix. 



Cases of this kind have been observed also in other Bacteria ; 

 these are the cells with which we made acquaintance before on 

 page 10 under Nageli's name of involution-forms. Whether they 

 are really retrograde states, as this name would express, or 

 diseased forms, I shall not undertake to say in the case of 

 the Micrococcus aceti. They certainly do not appear at all in 

 some cultures, or only one by one, while in others they are ex- 

 traordinarily numerous, and in the latter case I could never find 

 that they gave the 'impression of being incapable of further 

 development.' Positive statements, however, are at present no 

 more possible respecting their significance in the history of 

 development than they are respecting the conditions of their 

 presence or absence. 



A Micrococcus has been found by E. Chr. Hansen, and named 

 by him M. Pasteurianus, which behaves in every respect in 

 the same way as M. aceti, except that its cells 

 throughout the successive generations show the 

 blue reaction of starch with iodine (see page 5), 

 while the ordinary M. aceti is coloured yellow 

 by that reagent. This fact shows at once that 

 M. aceti although certainly the usual is not the 

 only vinegar-forming species. In fact the power 

 of producing acetic acid has been observed in 

 some other Bacteria, which are, however, comparatively unim- 

 portant to us for our present purpose. 



Micrococcus aceti has the power not only of producing but 

 also of destroying vinegar. When it has oxidised all the alcohol 

 of a fluid into acetic acid, it may continue to develope, as 

 Pasteur showed, and by a further process of oxidation convert 

 the acetic acid into carbonic acid and water, the final products 

 of all decomposition. 



Fig. 10. Micrococcus aceti (mother of vinegar) ; roundish cells, single 

 and united in rows, also rows of elongated rod-like and fusiform or swollen 

 flask-shaped members ; the latter from a culture at a temperature of 40 C. 

 Magn. 600 times. 



