120 BIOLOGY AND TECHNIQUE 



Filtering Media through Cotton. — The filtration of media after 

 clearing, either by the addition of eggs or by the coagulation of the pro- 

 teids originally contained in it, is best done through absorbent cotton. 

 A small spiral, improvised of copper wire, is placed as a support in the 

 bottom of a large glass funnel. A square piece of absorbent cotton is 



Fig. 22. — Berkefeld Filter. 



then spUt horizonta y. giving two squares of equal size. Ragged edges 

 and incisures should be avoided. These two layers of cotton are then 

 placed in the funnel, one piece above the other in such a way that the 

 direction of the fibers of the two layers is at right angles one to the other. 

 They are then gently depressed into the filter with the closed fist. The 



