392 CELL HEREDITY 



It now appears tliat the analysis ot gene action requires two kinds of 

 information: the identification of all the components of the system, and 

 the recognition of which element is blocking phenotvpic expression at a 

 particular time. Since repressor action mimics mutation, it is essential 

 to find a way to distinguish between them. On our present view, re- 

 pression should be much more readily reversible than mutation, and in 

 microbial systems at least, an operational distinction should be possible. 

 In higher organisms, however, or indeed even in Paramecium, where 

 growing conditions are kept very uniform, the reversibility of cytoplas- 

 mically determined repression may be extremely difficult to demonstrate. 

 This difficulty may limit the usefulness of such organisms for these 

 aspects of genetic analysis. 



The distinction between repression and mutation may also be a key 

 problem in the analysis of differentiation. In the nuclear transfer ex- 

 periments, for example, changes in some genetic constituents are detected 

 by transfer of nuclei from late stages of larval development back into 

 very early stages. These changes exhibit surprising stability in subse- 

 quent subcultures carried out by transfer of nuclei back into very young 

 blastulae after allowing 12 to 14 divisions at each step. In these experi- 

 ments, the genetic changes exhibit a permanence which seems to exclude 

 repression of the sort we have been considering. Nonetheless, differen- 

 tiation as a general process involves the restriction of potentialities, and 

 in this sense, it may involve the "turning off of some genetic elements. 

 Because of the regularity of timing and the 100 per cent effectiveness 

 of the turning-off process, investigators have been loath to invoke the 

 concept of mutation. Repression, which is a directed, highly specific, 

 and highly efficient process, provides a more acceptable conceptual basis 

 for an orderly restriction of potentialities than does mutation. Where 

 does the permanence come from? Perhaps the conversion of the condi- 

 tional permanence of cytoplasmically induced repression to a more 

 stabilized permanence, resembling that of mutation, is at the root of 

 differentiation. 



REPLICATION AND THE ORIGIN OF LIFE 



The scientific analysis of evolution has been one of the fruitful areas of 

 inquiry stemming from the development of genetics. Conventionally, 

 the study of evolution was restricted to considerations of organisms as 

 they are known from natural history and from paleontology. In recent 

 years, a particularly exciting new field of research has appeared, inquir- 

 ing into the origin of life itself. Deriving much of their impetus from 



