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as is easily shown by inoculation into deep test-tubes of boiled 

 milk coloured with litmus. The red litmus is first in the depth, 

 later till near the surface quite discoloured, to turn red again by 

 shaking with air. The thickness of the red layer in the curdled 

 milk admits an accurate measure of the intensity of the growth and 

 of the reduction process. The thinner the red layer the more inten- 

 sive both functions must be. 



2. Factors of variability. 



Many, perhaps all lactic acid ferments display a high degree of 

 variability as well in physiological as in morphological properties. 

 Nevertheless this variability in different stocks, coming from different 

 isolations of the same species, is not always equal by far, which may 

 give rise to trouble in the study of the specific properties. The circum- 

 stances causing the variability are but partly known; decidedly 

 belongs to them an oxygen pressure, too far above or too far beneath 

 the optimum for the vital functions, which may, especially for the 

 bacterium of the long whey {Lactococcus hollandiae), be demonstrated 

 with exceeding clearness. 



This remarkable species is characterised by a vigorous slime 

 formation when cultivated in milk or whey, but loses this power 

 at temperatures above 20° C, as well at the ordinary pressure of 

 the atmospheric oxygen, as at complete exclusion of it, if the changed 

 influence is allowed to act during some time on the growing microbes. 

 This is shown by cultivating the whey in a closed bottle ; the upper layer, 

 just beneath the stopper, where a little air can find access, becomes quite 

 liquid and contains a hereditarily constant, common Lactococcus, 

 forming little acid and no slime. Also by cultivating the long whey 

 microbe in tubes of boiled milk with access of air, after one or two re- 

 inoculations, a Lactococcus is produced, which forms no slime at all. 

 If the material for the reinoculation is secured from the depth of the 

 cultures grown in closed flasks, at places where the access of air is 

 impossible, and the inoculation is repeated once or more in the same 

 way, a Lactococcus is likewise obtained which displays no trace of 

 slime production. 



At some depth beneath the surface, however, is a zone in which 

 unchanged, slime forming, hereditarily constant material is found. 



What in this case can be very easily ascertained, proves, at 

 accurate investigation, also to be true for the other species of lactic 

 acid ferments, namely, that they only then continue to display- 

 constant specific characters, when they are continuously cultivated at 



