67 



microscopy at a magnification of 600X. The concentration 

 of spores in these extracts (0-6 spores/field) was low enough 

 to facilitate counting and individual study of spores. The 

 percentage of mature and immature MB spores was based on 

 the first 200 spores observed. 



Spore Measurements 



Spore measurements were made using an A, E. I. Cook 

 Image-Splitting micrometer calibrated for the microscope 

 used. The spores were immobilized for measurement by trap- 

 ping them between a layer of agar and the coverslip. This 

 was accomplished by pipetting a small pool of warm, liquid 

 Noble Agar (Difco Laboratories, Detroit, Mich.) (1.5%) on 

 the slide with a dropping pipette and bulb, and withdrawing 

 a drop of the agar just before gelling occurred. This pro- 

 duced an agar bed which was rather flattened on top. A 

 very small drop of spore suspension was then placed on a 

 coverslip, inverted and placed on the agar. 



Spore Yield 



Spore yield was determined by weighing groups of 25 

 pupae in advanced disease on an electric balance, homogeniz- 

 ing these in distilled water, adjusting the volume appropri- 

 ately, and counting spores with a hemacytometer (phase- 

 contrast microscopy, 300X) . 



