66 



SOLID CUL Tl VA TION MEDIA. 



sphere inside and tliat outside tlie flask, but preventing all ingress of 

 germs from without. The preparation of these plugs is a matter of 



Fig. 25. — Aspergillus rcpens growing on bread paste — commencing growth, 

 great importance, for upon their effectiveness as filters depends the 

 purity of artificial cultures, not only in this special mode of cultiva- 

 tion, but in several of those to be mentioned subsequently. 



