338 GENERAL DISCUSSION 



SHAPIRO: I do not agree that chromosome breakages are a secondary 

 effect. The chromosome breakages, and also jjoint mutations, are a 

 primary and local process. 



At the present time the presence is presumed of two forms of primary 

 variation of the chromosomes, real and potential. The potential varia- 

 tions in certain circnmstances can be reversible or transformed into 

 real mutations. 



This process of reversal of potential lesions is a secondaiy process 

 that depends upon the metabolism of the cells. Thus, it is necessary 

 to distinguish the injuries to the chromosomes, which are primary 

 processes, and their realization, a process both secondary and depend- 

 ent uj^on the metabolism of the cell. The inadequate definition of the 

 restrictive nature of these two processes is reflected as it seems to us in 

 those sentences which Bacq formulated for discussion : III the nuclear 

 variations were regai'ded as primary, in IV the nuclear variations were 

 regarded as secondary. 



The existence of potential variations and the dependence of their 

 fate upon the cellular metabolism complicates the study of the primary 

 process. But this does not mean that the process of damaging the 

 chromosomes is not primary. 



BACQ : When someone studies under a microscope the rearrangement of 

 chromosomes, he is studying cells wliich have been irradiated up to 

 the point of entry into the jjrophase. You cannot see the re-arrangement 

 of clu'omosomes before the cell enters into metaphase or anaphase. A 

 cytologist cannot see what transpires between the instant of irradiation 

 and the metaphase stage. He therefore erects a hypothesis in regard 

 to phenomena which have taken place prior to the stages actually 

 observed by him. 



However, the metabolic processes are continued throughout that 

 period of time when, properly sj^eaking, the cytologist cannot see any- 

 thing. 



gray: The remarks which have just been made by Bacq indicate that 

 metabolism has possil:)ly already completed its work. This cannot be 

 excluded. 



In some cases the amount of observable variation in the chromosomes 

 depends upon the metabolic processes which occur between irradia- 

 tion and observation. But not all stages of chromosome damage, and in 

 particular not all types of damage, are under the influence of meta- 

 bolism. This can be proved. Therefore, it still remains an open question, 

 whether certain chemical reactions are capable of bringing about 

 directly the destruction of the chromosomes. 



