﻿IX, B, 4 Barber: The Pipette Method 337 



were washed in salt solution before inoculation. In washing 

 small organisms, a fine pipette is brought to the edge of the drop- 

 let containing the cell and the liquid is removed, leaving the cell 

 in contact with the cover. Fresh liquid is substituted, and the 

 process repeated as often as desired. 



It is often difficult to remove bacilli of tuberculosis and some 

 bacterial spores from a hanging drop after they have settled to 

 the layer of surface tension at the bottom of the droplet. The 

 liquid passes into the pipette, but the cell remains stranded on 

 the cover. This difficulty may be avoided by taking up these 

 organisms before they have settled, or by placing the mouth of 

 the pipette directly over the cell. Bacilli of tuberculosis may be 

 made to cling to the side of the tip of the pipette and transported 

 in that way. The peculiar behavior of these organisms may be 

 due to the fat or mucus surrounding them, as most cells do 

 not give this difficulty. It is difficult to remove leucocytes from 

 a hanging drop of blood or any dilution rich in serum when they 

 have once settled to the bottom of the hanging drop. They may 

 often be isolated by bringing the mouth of the pipette directly 

 under them. 



NUTRIENT MEDIA 



All media of the ordinary sorts may be used in this method, 

 and where only small quantities are needed one has a wide range 

 of possibilities. Quantities of medium may be repeatedly taken 

 from the same test tube if one takes proper precautions against 

 contamination. A soft agar can be conveniently obtained by 

 melting the very top of a slant and allowing a portion of the 

 liquified agar to flow into the water of condensation. By mixing 

 different proportions of the liquified agar with the water of con- 

 densation, one may obtain various degrees of stiffness. Suffi- 

 cient liquid for small hanging drops may be taken from a small 

 blister or from the body of an insect or other animal ; or, where 

 the inoculation pipettes are used, from the contents of a living 

 plant cell or microscopical animal. If a small quantity of serum 

 free from corpuscles is needed, a deep hanging drop is made of 

 the whole blood, and, when the corpuscles have settled to the 

 bottom of the drop, serum may be drawn from its margin. A 

 bacterium-free liquid from the cultures of some microorganisms 

 may be obtained in the same way. 



ILLUMINATION 



The illumination of objects on the cover during isolation may 

 be improved by the use of a projection condenser which focuses 

 the light some distance above the top of the stage. Either day- 



