21 



hanging drop is apt to "become markedly more concentrated 

 during the period of an experiment, which would lead to 

 erratic results. The covers were sealed in position with 

 vaseline. It was not found necessary to take the precaution 

 recommended by Clark of first allowing the expanding air to 

 escape through a small opening in the vaseline seal; possi- 

 bly because the temperature of the thermostat here use! was 

 only a little above the temperature at which the preparat- 

 ions were made. 



For ease in handling the cultures, the slides were 

 placed in sheet metal trays which could be piled one upon 

 another in the thermostat so as to form a rack. These trays 

 were 15 cm. wide and 20 cm. long with vertical flanges at 

 the ends, these flanges extending upward. about 1.3 cm. and 

 downward about C.4 cm. They were so bent that the lower 

 flanges of the upper tray fitted outside the upper flanges 

 of the lower one, when one tray was placed upon another, and 

 many trays could thus be arranged in a compact and rigid 

 pile without any disturbance to the slides. The bottom of 

 each tray was perforated with circular openings about 1.5 cm 

 in diameter and 0.5 cm. apart, to facilitate circulation of 

 air. Each tray carried 14 slides. The slides were always 

 transferred to and from the thermostat by means of these 

 trays, a whole series of cultures being thus moved together. 



The cultures were kept during germination in an el- 

 ectrically heated and automatically regulated thermostat, in 



