LIFE -HISTORY OF ROTIFER 227 



found that the greatest degree of uniformity could be obtained 

 with a solution of malted milk. A stock solution of Ye of 1 per 

 cent strength was made from Horlick's Malted INIilk in powder 

 form as follows : ^ gram of the powdered milk was weighed and 

 put in a glass beaker; approximately 5 cc. of boiling water was 

 added, and the powder stirred until in suspension and free from 

 lumps, when the remainder of the 100 cc. of boiling spring water 

 was added. This solution was than put into a liter graduate and 

 filled to 800 cc. with filtered cold spring water. A still greater 

 degree of uniformity in culture medium could have been obtained 

 with distilled water, but Proales did not thrive well in cultures 

 made in this way. 



In a study of individual performance, or in pedigreed cultures, 

 a single organism was isolated in 5 drops of culture fluid in each 

 of the depressions of a hollow-ground slide; these slides were 

 kept on glass plates in large covered Stender dishes partly filled 

 with water to prevent evaporation of culture fluid. Reserve 

 mass cultures were always kept in small watch-glasses with 

 about 5 cc. of malted milk, changed once each week. 



The early deposition of eggs, together with the rapidity with 

 which these eggs hatched and produced other egg-laying indi- 

 viduals in their turn, necessitated the examination of all slides 

 once in every twenty-four hour period, and if individual data were 

 to be obtained for the eggs it was necessary to remove each to a 

 separate depression. Various modifications of the general 

 method were made to meet the requirements of the different 

 changes in environment, but these can best be discussed in the 

 section to which they particularly belong. 



THE NORMAL LIFE-HISTORY 



The normal life-cycle of Proales is complete within eight 

 days, although great variation occurs even when the organisms 

 are kept under constant conditions of temperature and food. 

 Early in the history of the race (second week of February, 1919) 

 among 100 isolated individuals cultivated in xg^ per cent malted 

 milk at laboratory temperature, 69 lived for 5 days, 27 for 4 days, 

 and 4 for 3 days, giving an average length of life of 4.65 days. 



