176 



ANIMAL AGGREGATIONS 



of experiments, is shown to be slightly less than 5 chances in a 100, 

 which is usually considered to suggest statistical significance. 



Petersen approached this problem from another angle. Experi- 

 ments were carried on to determine whether or not the division of an 

 animal in a Hmited amount of medium would accelerate the rate of 

 division of individuals isolated into the same volume. Both washed 

 and unwashed Paramecia were so tested. First, washed animals were 



TABLE XII 



Paramecia Washed Three Times and Transferred Singly, in Two's, and in 



Four's, to 40 Drops of Alkalinized, Bacterized Medium 



(From Petersen) 



isolated into 2 drops of bacterized medium. At the end of 12 hours 

 the animals were removed, and the slides in which fission had oc- 

 curred were noted. Five sets, consisting of 8 animals each, each set 

 being the fission products of i animal, were isolated singly into the 

 drops in which fission had recently occurred ; and comparable isola- 

 tions were made into those drops where animals had lived for the 

 same length of time but without division. There was no significant 

 difference in division rate at the beginning or at the end of the ex- 

 periments, on washed or unwashed individuals. 



Petersen says: "This is not what would be expected on the basis 

 of Robertson's work. His reports indicate that acceleration of divi- 



