MUTATION IN DIDINIUM NASUTUM 



S. 0. MAST 



Zoological Laboratory of The Johns Hopkins University 



The origin of heritable variations or mutations consti- 

 tutes one of the most fundamental- problems of biology. 

 It has long since been recognized that evolution depends 

 upon such variations, and they have consequently been 

 extensively studied by a considerable number of inves- 

 tigators, e. g., Darwin, DeVries, Batson, Kammerer, 

 Tower, Stockard, MacDougal, Jennings, Morgan, et al. 

 These studies have resulted in the accumulation of a mass 

 of facts of great importance, but the nature of the origin 

 of the variations in question is still shrouded in mystery. 



In a series of experiments on the effect of conjugation 

 and encystment in Didinium, extending from April, 1910, 

 to May, 1914, there suddenly appeared, in the latter part 

 of July, 1912, a marked difference in the rate of fission in 

 the progeny of a single individual. This difference ap- 

 pears to have been permanent, as the results presented 

 herewith indicate. And it seems to show, in opposition to 

 the conclusions reached by a considerable number of in- 

 vestigators, that variations in organisms reproducing 

 asexually are at times heritable. 



The difference in rate of fission mentioned was discov- 

 ered in a group of five pure lines, all of which had been 

 carried from the beginning of the experiment. These 

 lines all originated from the same individual, and before 

 the mutation occurred they had produced, without con- 

 jugating, an average of 721 generations ; and without en- 

 cysting, 197 generations. Throughout this entire period 

 there was remarkably little difference in the rate of fission 

 in the five lines. Tlie total number of fissions produced 

 by these lines during the 40 days. immediately preceding 

 the appearance of the mutation was respectively 164, 171, 

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