No. 500] LIFE CYCLE OF PARAMECIUM 



daily bipartitions of each line. The following curve 

 represents graphically the average rate of division of the 

 four lines of the culture, and this again averaged for ten- 

 day periods. 



The culture was carried on at the Thompson Biological 

 Laboratory . of Williams College, Williamstown. Mass., 

 during May and June, 1907; at the Marine Biological 

 Laboratory, Woods Holl, Mass., during July and August, 

 1907 ; and at the Sheffield Biological Laboratory of Yale 

 University, New Haven, Conn., from September, 1907, to 

 the present time, May, 1908. 



The culture medium during the earlier months of the 

 work consisted of hay or grass infusion. But, except 

 during periods in which the culture was employed as a 

 control for certain experiments, the infusion was made 

 with hay or grass from various localities and different 

 proportions of hay and water were used almost daily. 

 Water from various sources was employed. In every 

 case the temperature of the infusion was raised to the 

 boiling point, and then allowed to attain the temperature 

 of the laboratory before being used. In some cases the 

 infusion was made fresh daily, in other cases it was 

 allowed to stand twenty-four hours before being used. 



Beginning' in February, 190S, a much more varied cul- 

 ture medium was employed. It was found that Para- 

 mecium can exist in nearly any infusion which may be 

 made from materials collected in ponds and swamps, and 

 accordingly, in the hope of supplying as far as possible all 

 the elements which may be encountered in the "normal" 

 habitat of the organism, water was taken from ponds, 

 laboratory aquaria, etc., together with its animal and 

 plant life. In other words, no definite method was em- 

 ployed in selecting the material, but it was simply col- 

 lected at random from many sources, thoroughly boiled, 

 and then used. This culture medium affords a striking 

 contrast to that employed by Calkins, which he described 

 thus, 4 'The hay infusion was made every day, the same 

 amount of hay and water being taken each time and 



