144 RUTH L, PHILLIPS 



Their method is undoubtedly reliable, for their tests were 

 thorough and showed that no bacteria would be carried over by 

 the animal after the last washing, thus excluding the contamina- 

 tion of pure cultures of bacteria from this source. This method 

 was modified by using small watch-crystals, 1 inch in diameter, 

 in place of depression slides. From five to seven of these can be 

 easily baked in a single Petri dish. Five was the number used 

 for carrying the cultures from day to day, whereas seven were 

 employed for washing each animal. These watch-crystals were 

 kept from slipping about by asbestos mats cut so as to fit exactly 

 the bottom of a Petri dish. Each mat was perforated with as 

 many holes as there were watch-cyrstals to be placed in the 

 moist chamber. In this way the individual cultures were secured 

 from slipping, and the entire apparatus could be baked repeatedly. 

 The asbestos, furthermore, served as a sponge to keep the air 

 within the Petri dishes moist. 



The use of this apparatus made it necessary to wash each 

 animal seven times instead of five, but this slight disadvantage 

 w^as so outweighed by the advantages of the method that it was 

 employed throughout the course of the work. A further change 

 in the method of Hargitt and Fray consisted in the use of sterile 

 0.1 per cent standard hay infusion for washing the Paramecia 

 instead of sterile water. The infusion is easily kept in stock, and 

 its use as a washing fluid lessens the danger of injury during the 

 sterilizing process by avoiding a change of medium at this time. 



The pipettes used in handling the Paramecia were made by 

 drawing out soft glass tubing to capillary fineness. The tip of 

 each was annealed so that an animal coming in contact with it 

 w^ould not be injured. The upper end of each pipette was 

 plugged with cotton to prevent contamination with the bulbs 

 which were not sterilized. A large number of these pipettes was 

 kept on hand, and a fresh one used for the transfer of each ani- 

 mal from one w^atch-glass to another. Mason jars were found 

 more convenient holders than the ordinary pipette boxes, which 

 were too long. These jars with their contents were carefully 

 baked before using and, in addition to this, each pipette was 

 carefully flamed before it was introduced into a culture fluid. 



