GENETIC VARIATIONS 329 



With relation to these changes, there are several ques- 

 tions that require answers. First, how does it happen that in 

 a cell, two chromosome breaks occur instead of one? Sec- 

 ond, how do the broken ends find each other and reunite — 

 often resulting in a new combination? And third, how does 

 it happen that in such reunited chromosomes the order of 

 the genes in the middle piece is commonly, reversed? 



There is a simple explanation of these strange relations, 

 one that throws light on the way the breakage is brought 

 about. The numerous long thread-like chromosomes in the 

 cells undergoing radiation lie close together and are more 

 or less curved, coiled and tangled. They may cross each 

 other and come in contact, but under ordinary conditions 

 when this happens they do not stick together. 



But under the action of radiation, the chromosomes 

 change physically in such a way that when they come in 

 contact by crossing at a certain point they stick together. 

 There are indications that the chromosomes have a trans- 

 parent sheath, which keeps the essential parts of the chro- 

 mosomes — the genes — within the sheath from coming in 

 contact. By radiations this sheath is seemingly dissolved 

 or modified in spots, so that the chromosomes within it 

 may actually come in contact. When this occurs they stick 

 together, and through strains set up in the intracellular 

 processes they may break. The rest of the process will be 

 best understood from a hypothetical example. Suppose that 

 a chromosome in this adhesive condition becomes looped, 

 so that its two limbs cross and come in contact (figure 

 67, A). The parts in contact stick together, and the chro- 

 mosome breaks at the point of adhesion. The chromosome 

 is thus left in three pieces, a central loop and two end pieces 

 (figure 67, B). 



The three broken ends are still in contact, and there oc- 

 cur at the tips growth processes, so that the broken ends, 

 some or all, may reunite. The two end pieces may reunite, 



