142 



J. A. DAWSON 



During the experiment, twin formation has occurred among 

 animals from each of these sources. In all, eight different series 

 have been obtained. The manner of formation and history of 

 each series follows. 



Series II. The ancestor of series II was a twin isolated as 

 soon as its occurrence was noted, from a thirty-day-old mass 

 culture in a Petri dish of single animals from culture T 15 (see 

 Selection Experiment) on December 16, 1917. The progeny of 

 this animal comprising culture S2T were isolated as soon as fission 

 occurred in each case up to the fifth generation. Following is a 

 record of the culture : 



One of the twins from the fifth generation divided to give eight 

 twins, all of which were isolated to form subculture S2T1.- Of 

 the eight, three died, and the remaining animals in the fourth 

 generation produced eight twins and four single animals (66.7 

 per cent remaining twins). The fifth generation gave fourteen 

 twins, twelve single animals (53.8 per cent). The most vigorous 

 animals were carried along by daily isolations, and, from de- 

 scendants of these, several subcultures of four lines each were 

 started. 



The subsequent history of these subcultures is summarized as 

 follows : 



^ In these cultures the series number is given first. A 'twin' culture is indi- 

 cated by the letter 'T' and a culture of single animals by the letter 'S.' 



