TEXT-BOOK OF BACTERIOLOGY. 169 



At first the colonies in the mass of the gelatin appear to the 

 naked eye as whitish dots. The microscope shows them as yellow- 

 ish, somewhat irregular lumps, without any particular character. 

 The surface colonies which have access to the oxygen of the air 

 slowly Uquefy the gelatin. As a rule, they then take a very re- 

 markable typical appearance. The colonies have the shape of a 

 kidney or crescent, and are peculiarly grained, looking like shagreen 

 leather. 



In the test-tube a liquefaction of gelatin is observable all along 

 the puncture, but it is generally far greater in the upper portion, 

 and extends but gradually to the lower part. The mass of bac- 

 terial growth sinks gradually to the bottom, and only a slight 

 turgidity of the upper part indicates that remnants of the culture 

 are still present there. A decided crust is never formed on the 

 surface. Even in large masses the culture remains quite colorless. 

 On obliquely-solidified agar Bacillus megaterium forms a dull white 

 or light-gray covering, which can be easily separated from the 

 medium below it. 



On potatoes it grows as a thick, greasj^, whitish-gray film, which 

 is usually very rich in spores and involution forms. 



Potato Bacillus. 



In the preparation of potatoes I recommended a particularly 

 careful cleansing and sterilization. I advised, too, that whenever 

 this precaution is neglected a pollution of the food medium is al- 

 ways observed, proceeding from one particular and constantly -re- 

 turning species of bacteria, whose germs are distinguished by a 

 high degree of resisting power, and which, on account of its special 

 relation to the potato, has been named the potato bacillus. There 

 are, in fact, several micro-organisms comprised under this collective 

 name and distinguishable from each other by slight differences; 

 yet we are not called upon here to explain these differences, and 

 shall only devote a few words of description to the commonest 

 species. 



It is found in the uppermost laj'ers of cultivated land, from 

 which it gets on to the surface of the potato. It is also found in 

 the faces of men and animals, in putrefjang fluids from various 

 sources, river-water, etc. 



Its separate cells are small rods, with rounded comers, often 

 united in twos, but seldom forming long threads. It has a lively 

 power of locomotion. 



On our usual culture media it thrives extremely well and gen- 



