Spore-Culture. 113 
must be sterilized by heat; and after the nahrlésung has 
been introduced, the two ends of the tube are closed by 
cotton wool; this allows free access of air, but filters out 
any germs that may be floating in it. 
Or the culture may be made in a hanging-drop cell 
made of lead tubing, in the sides of which two holes have 
been cut (Fig. 13). I have found it most convenient to 
_ Fig. 13.—Hanging-drop culture cell, made of lead tubing, with two holes for the admis- 
sion of air; when in use with nahrlésung these openings are closed with cotton wool. 
wrap cotton wool round the cell, and to hold it in its place 
by a small elastic band. These cells require to be sterilized 
by washing with a weak solution of corrosive sublimate. 
