FISSION RATE OF STYLONYCHIA PUSTULATA 455 



ing method, which is a modification of that described by Jennings 

 ('13): 



As culture medium, re of 1 per cent HorHck's malted milk 

 ■was employed, a fresh supply being made daily. This is the 

 medium adopted by Miss Peebles ('12). One gram of the malted 

 milk powder was dissolved in a 100 cc. graduate in a few cc. 

 of boiling spring water; this was then diluted to 100 cc. with 

 more of the boiling spring water. Six and one-quarter cc. of 

 this 1 per cent solution were next diluted to 100 cc. with boiling 

 spring water and this tg per cent solution was filtered and 

 cooled. 



The animals were cultivated on ground glass slides having 

 each two circular depressions capable of holding four or five 

 drops of liquid apiece. These were kept in moist chambers. 

 Three drops of culture medium were used in each depression 

 and no cover-glasses were employed. The 'fast' lines were kept 

 in the left concavities and the corresponding slow lines in the 

 right concavities of the same slides, so that conditions were 

 uniform for the two sets. 



Uniformity of bacterial content in the culture medium was 

 secured by washing the animals in fresh culture medium before 

 transferring to a new slide. The animals were allowed to swim 

 about for a time in the fresh medium, in order to wash them- 

 selves largely free of bacteria; they were then transferred to the 

 definitive shde, in new fluid. The pipette used in transferring the 

 individuals was invariably sterilized in boiling water after each 

 transfer, thus absolutely preventing the unintentional introduc- 

 tion of any individual which might cling to the pipette; there 

 was thus no possibility of admixture of the 'fast' and 'slow' 

 sets. The slides were labeled in lead pencil; the number of 

 fissions and selections at each examination were likewise recorded 

 on them, to be later transferred to permanent records. The 

 individual lines were designated in accordance with the plan 

 set forth by Jennings ('13). 



Each concavity contained characteristically two parent indi- 

 viduals, the products of a single fission. The slides were ex- 

 amined daily or oftener. When one of the two individuals was 



