74 GENETIC VARIATIONS 



and both may break there (Figure 12, D). Then the ends, in 

 becoming reunited, may in some of the cases readily exchange 

 partners. Thus are produced the well known translocations; 

 two chromosomes that have exchanged parts (Figure 12, E). 



Thus all these remarkable phenomena, so inexplicable 

 otherwise, find a simple explanation based on conditions that 

 exist within the cells. No other explanation than one along the 

 lines suggested appears possible. 



The changes in the chromosomes produced in these ways 

 are of great interest in themselves. They result in producing 

 new combinations of genie materials, and the study of their 

 effect on heredity is just at present one of the favorite occupa- 

 tions of geneticists. But for our present purpose their interest 

 lies in the light which they throw on the cause of the chromo- 

 some breaks and of their associated gene mutations. They 

 make it probable that the breaking of the chromosomes is 

 produced, not by the striking of an electron against the 

 chromosome at a certain spot, but by the sticking together of 

 the chromosomes. What the radiation does, what the heat 

 does, is to modify or dissolve the chromosomal sheath. When, 

 in this condition, parts of the chromosomes come in contact, 

 all the rest follows. The role of the radiation or the heat is to 

 make a general physical or physiological change in the cell 

 contents, resulting in ready adhesion and breaking of the 

 chromosomes. The breaking is not due to the striking of an 

 electron at the point of breaking; this idea is quite inconsis- 

 tent with the usual occurrence of two breaks in place of one, 

 as well as with the fact that heat produces the same effect. The 

 greater the energy of the radiation, the greater its effect in 

 making the chromosomes adhesive and breakable; therefore 

 the greater the number of breaks and injuries produced. 



Important in this connection is the fact that breakage and 

 the accompanying changes are not limited to chromosomes 



