90 



C. F. ROBINOW 



Fig. 5. Electron micrographs of shadowcast parasporal protein crystals 

 showing variation in size. 



a. B. cereus var. sotto (containing cell wall debris). 



b. B. cereus var. thuringiensis. 

 . c. B. cereus var. alesti. 



Table I shows the influence of media on the size and composition of the 

 spores of these varieties. The agar of Howie and Cruickshank (1940) is 

 much better supphed with minerals, is higher in phosphorus but lower in 

 nitrogen than the nutrient agar (Difco). The higher nitrogen content of 

 nutrient agar is reflected in the greater content of N/spore and in the slightly 

 greater average weight/spore. The nutrient agar however produced spores 

 of a smaller average volume and of a lower RNA content. When grown on 

 nutrient agar, our common B. cereus control contained 13%; var. sotto (-{-) 



