548 D. D. WHITNEY 



numbers in a small amomit of the culture water. Three or four 

 drops of this material was added to about 8 cc. of old filtered 

 culture water in which a general mixed culture of Protozoa and 

 rotifers were living and placed in watch-glasses. These glasses 

 were placed in sunlight at a temperature of about 25°C. and the 

 Dunaliella remained active as long as they were in the sunlight. 



It, however, usually happened that when these watch-glasses 

 were left in the sunlight for any appreciable length of time, the 

 temperature would rise to 31 to 40°C., which would be fatal to 

 the rotifers. To obviate this difficulty, a smaller battery jar 

 was set and fastened into a larger battery jar which had padding 

 in the bottom of it. This allowed the top of the smaller jar to 

 protrude above the top of the larger jar. A space was thus 

 made between the two jars into which running water was con- 

 ducted, and by regulating the flow the temperature could be kept 

 at almost any point desired while this water-jacket jar was in 

 direct sunlight. The rotifers in the Dunaliella culture were 

 placed in the inner jar in watch-glasses and the jar was covered 

 with a glass plate. By this method a constant temperature of 

 about 25 to 26°C. was maintained, at which degrees the Dun- 

 aliella were the most active in swimming about in the culture 

 water. The rotifers eat them only when they are active, chiefly 

 because when they are motionless they form a scum at the sur- 

 face of the culture water or become fastened to the sides of the 

 dish and in either place they are inaccessible to the rotifers. 



At the beginning of the experiments a female rotifer was taken 

 from a jar containing a general culture of rotifers which had 

 developed from fertilized eggs a few months previously. This 

 female was fed upon pure cultures of the colorless flagellate, 

 Polytoma, and became the progenitor of a parthenogenetic strain 

 of rotifers which produced almost entirely female offspring 

 through twenty-five generations. The method of causing a 

 strain to produce only female offspring has been described in a 

 former paper, but as the method of making the food cultures 

 has been improved it will be briefly described again. Pure 

 cultures of Polytoma were made in the following manner: 800 

 grams of fresh horse manure were put into 1200 cc. of water and 



