METHOD OF VAN ERMENGEM. 191 



minute. The cover-slip is now dried, then xylol is 

 poured over the surface, the excess being removed with 

 filter-paper. The cover-slip is now mounted in xylol 

 balsam. 



THE METHOD OF VAN ERMENGEM. Another 

 method of demonstrating the presence of flagella is that 

 suggested by Van Ermengem. It is somewhat more 

 complicated than either of the preceding methods. The 

 steps in the process are as follows : 



In the centre of a perfectly cleaned cover-slip place 

 a drop of a very dilute suspension, in physiological salt- 

 solution, of a 10- to 18-hour old agar-agar culture of 

 the organism to be studied. The suspension of the 

 organisms in the salt-solution should be very dilute in 

 order to favor the isolation of single cells on the slip 

 and also to obviate the occurrence of excessive precip- 

 itation. The slips are then to be dried in the air and 

 fixed over a gas-flame in the usual manner. 



The mordant used consists of: 



Osmic acid (2 per cent, solution) 1 part. 



Tanniii (10-25 per cent, solution) 2 parts. 



Place a drop or two of the mordant on the cover-slip 

 to be stained, and allow it to act for one-half hour at 

 room-temperature, or for five minutes at 50 to 60 C. 

 Wash carefully in water and in alcohol, and then im- 

 merse for a few seconds in the " sensitizing bath," viz., 

 a 0.25-0.5 per cent, solution of silver nitrate. Without 

 washing, bring the slip into a watch-crystalful of the 

 " reducing and reinforcing bath," viz. : 



Gallic acid 5 grammes. 



Tannin 3 " 



Fused potassium acetate ,. 10 " 



Distilled water . . 350 " 



