144 J. A. DAWSON 



This twin divided to produce twins and single animals as follows : 



1st generation. 2 twins (100 per cent). 



2d generation. 3 twins, 2 single animals (60 per cent). 



3d generation. 4 twins, 8 single animals (33.3 per cent). 



4th generation. 7 twins, 18 single animals (28 per cent). 



(Dec. 22) 5th generation. 12 twins, 40 single animals (23 per 

 cent). 



Eight of these twins were isolated on depression slides to study 

 their later history in small mass culture, the remainder being 

 isolated as controls and carried on slides, singly, with daily changes 

 to fresh medium. 



It was found in the cases where the progeny of the twins were 

 allowed to multiply on the slide that division went on fairly 

 rapidly. In four days the average number of animals on each 

 slide was twenty-six, the proportion of twins to single animals 

 being 20 per cent. At this time quite a few pairs were observed 

 to be sticking together, and on nearly every slide were one or 

 more- cannibals. Two days later, cannibals were observed on 

 every slide. On January 2nd only two twins were seen, and on 

 the next day these had disappeared. The average number of 

 single animals on a slide was now twenty-five. Thus in twelve 

 days under these conditions no twins survived, while those which 

 had been carried on in individual slides were still dividing to 

 give twins. Further experiments on this series were not attempted. 

 This is, however, the only case in which definite evidence as to 

 the method of origin of twin forms has been obtained, and as the 

 transformation of this pair of single animals into a twin was 

 somewhat unexpected, it is regretted that more observations 

 were not made during the process. 



Series V. The ancestor of series V was formed in a six-day- 

 old stock culture of single animals coming from a cannibal ances- 

 tor which had been allowed to multiply on a depression slide. 

 Isolation was made January 9, 1917. 



The history of the first four generations follows : 

 1st generation. 2 twins 

 2d generation. 4 twins 

 3d generation. 7 twins, 2 single 

 4th generation. 11 twins, 12 single 



