CULTURE MEDIA. 39 



transferred to a test tube by breaking off the sealed extremity with 

 sterile forceps and slipping it past the cotton plug, which must be 

 partly withdrawn for the purpose. Upon applying gentle heat to 

 the bulb its contents are forced out into the test tube (Fig. 15). 

 Blood serum which is collected without these special precautions 

 will require sterilization by heat, for which directions will be given 

 later. 



To obtain the clear serum from blood collected as above directed, 

 the jars containing it are set aside in a cool place in order that a firm 

 clot may form, care being taken not to shake them. After the clot 

 has formed they may be transported to the laboratory, where they 

 are placed in an ice box or in a cool cellar for from twenty-four to 

 forty-eight hours. By this time the serum has separated from the 

 clot, and it may be transferred to sterilized test tubes by means of a 

 suction pipette (Fig. 16), or may be distributed in little flasks as 

 above directed, 



Milk is largely used as a culture medium, and is especially useful 

 in studying the biological characters of various microorganisms, as 

 shown by their causing coagulation of the casein, or otherwise ; or 

 an acid or alkaline reaction of the liquid ; or peptonization of the 

 precipitated casein, etc. In the udder of healthy cows milk is quite 

 sterile, and by proper precautions it may be drawn into sterilized 

 flasks without any contamination and kept indefinitely without un- 

 dergoing coagulation or any other change. But in practice it 

 is easier to sterilize it in test tubes or small flasks by the use of 

 heat than to obtain it in a sterile condition from the udder of the 

 cow. 



Urine has been used to some extent as a culture medium, and 

 many bacteria multiply in it abundantly, although, on account of its 

 acid reaction, other species fail to grow in it. As contained in the 

 healthy bladder it is sterile, but the mucous membrane of the mea- 

 tus urinarius always contains numerous bacteria upon its surface, and 

 some of these are sure to be carried away with the current when 

 urine is passed. 



A culture fluid which the writer has found extremely useful, in 

 tropical countries where it is to be obtained, is the transparent fluid 

 contained in the interior of unripe cocoanuts called agua coco by 

 the Spaniards. In countries where the cocoanut is indigenous this 

 cocoanut water is largely used as a refreshing drink. It contains 

 about four per cent of glucose in solution, together with some vege- 

 table albumen and salts. Some microorganisms multiply in it with- 

 out appropriating the glucose, while others split this up, producing 

 an abundant evolution of carbon dioxide and giving to the fluid 

 a very acid reaction. The following are the results of an analysis 



