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J. A. DAWSON 



since stock animals from culture AT which had been carried from 

 previous cultures of the same race continued to thrive in the Petri- 

 dish cultures and representatives of the race are still living, 



Fig. 7 The ordinates represent the percentage of animals remaining twins. 

 The abscissas represent the generation number. All cultures remained 100 per 

 cent twins for the first three generations. 



although no twins are present. It is noteworthy that in stock 

 cultures the twins, which were bred late in the history of culture 

 AT, remained twins longer than was the case when their occur- 

 rence was first noted. 



