barber: heredity in certain micro-organisms. 15 



successive selections, the series selected showed the same 

 elongated form as the non-selected check. In other words, 

 the continual selection of cells approaching the parent type 

 brought the new race no nearer to that type. 



A long series of experiments was conducted to ascertain 

 the nature of the sports giving rise to permanent new races, 

 their relative proportion to normal cells, and the conditions 

 under which they arise. 



The type of cell which most frequently produced perma- 

 nent new races is characterized by one or more long, narrow 

 prolongations attached to the mother-cell. In hanging drop 

 cultures such cells seemed to appear most frequently in beef 

 peptone broth to which one per cent, of glucose had been 

 added. They appeared in both acid and alkaline broth, 

 though in one long series, at least, they seemed most abun- 

 dant in the alkaline. They were more often found in shallow 

 hanging drops than in deeper ones, and in obtaining them I 

 had best success by sowing fresh cells in long shallow drops, 

 many of which may be made on one cover-glass. Their more 

 frequent appearance in shallow drops may be in part only 

 apparent, since such drops may be more readily searched. 

 They were found on solid as well as liquid cultures. 



In a considerable number of experiments the proportion of 

 these cells relative to normal ones was estimated. Taking 

 two of these experiments for illustration, in acid beef peptone 

 broth containing one per cent, glucose they were found after 

 four days' growth at room temperature in proportions varying 

 from 1 in 5000 in some hanging drops to 1 in 46,000 in oth- 

 ers. In roll tubes of glucose gelatin, after six days at room 

 temperature, the proportion varied from 1 in 1000 to 1 in 

 10,000. On the average, they appeared in numbers less than 

 the proportion 1 in 5000. Sometimes long series of hanging 

 drop cultures, including hundreds of thousands of cells, gave 

 no variations of this character. In the above estimates I re- 

 fer to comparatively young cultures. On old cultures on 

 solid media the proportion of irregular cells may be some- 

 what larger. 



Most of these variations were found at room temperature. 

 They were found in young as well as old cultures, and in two 



