( J7 ) 



birds, I refer to the academical thesis of Mr. Sonies, which is now 

 going through the press and will soon be published, also in "Petrus 

 Cannier"'. I will only mention that the small polar cartilage, discovered 

 by Noordenbos in mammals and which also appears in Selachians, 

 was found by Sonies also in birds. 



Microbiology. "On Lactic acid fermentation in milk". By 



Professor Dr. M. W. Beijerinck. 



(Communicated in the meeting of April ïiti, 1907 J. 



In milk left to itself, which in consequence of spontaneous infection, 

 contains the more generally distributed germs, with certain regula- 

 rity some special floras are observed, whose composition is chiefly 

 controlled by two factors-, temperature and oxygen pressure.' [f the 

 latter is very slight, that is, it' the microbes of the milk are reduced 

 to more or less anaerobic conditions, the floras become simple of 

 composition and produce certain fermentations. The three principal 

 of these are the Aërobacter-, the Butyric acid- ami the Lactic acid 

 fermentations, of which the two first are always characterised by the 

 evolution of hydrogen and carbonic acid, whilst in the lactic acid 

 fermentations, which may occur under different forms, beside the 

 lactic acid, no gas at all, or carbonic acid only is formed. Sometimes 

 this fermentation is accompanied by a vigorous slime formation, 

 which slime consists of the swollen cell walls of the inferred lactic 

 acid ferments. 



For domestic purposes the lactic acid fermentation should be con- 

 sidered as useful; both the others as noxious. 



The fermentation experiment the dairy industry applies to judge 

 of the purity of milk has for its object to determine the commonness 

 or the rarity of the germs of Aërobacter and of the butyric acid 

 ferment. To this end a high standing glass is filled with milk, placed in 

 a water bath of 40° ('. and it is observed whether any fermentation 

 gas is evolved, and if so, after how much time. In good milk this 

 production of gas does not occur because then the lactic acid ferments 

 develop so quickly that the other microbes are expelled. Artificially 

 the Aërobacter fermentation is easily obtained by infecting non- 

 aciditied milk with faeces, soil or canal water and cultivating at 

 about 37° to 40° C. After 6 to 12 hours production of gas is observed 

 originating from Aërobacter coli or more rarely from A. aërogenes. 

 The nature of the thereby obtained varieties changes with the 

 temperature. 



At temperatures beneath 40° the Aërobacter fermentation, after 



2 



Proceedings Royal Acad. Amsterdam. Vol. X. 



