28 



THE ACTINOMYCETES, Vol. I 



Petri dishes and incubated for 2 weeks at 

 optimum temperatures. One ml of 40 per 

 cent formaldehyde is then added dropwise 

 to the surface of the agar outside and be- 

 tween the growth zones. The plates are 

 kept at room temperature with covers in 

 place until all of the formaldehyde has been 

 absorbed. After 2 to 3 hours, the plates are 

 sealed with rubber sealers and stored at 4° C. 



Cultures so treated have retained most of 

 their original characteristics for longer than 

 2 years. The plates can also be stored at 

 room temperature for several w^eeks. Mor- 

 phological features, color of spores or myce- 

 lium, and growth habit remained essentially 

 unchanged in most cultures. In a few cases, 

 changes in pigmentation have been observed 

 when the formaldehyde was added. 



