72 GENETIC VARIATIONS 



together. There are indications (as Metz^ has set forth) that 

 the chromosomes have a transparent sheath, which keeps the 

 essential parts of the chromosomes — the genes — within the 

 sheath, from coming in contact. By radiations or heat this 

 sheath is dissolved or modified in spots, so that the chromo- 

 somes 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 an example (see Figure 12). 

 Suppose that a chromosome in this adhesive condition be- 

 comes looped, so that its two limbs cross and come in contact 

 (Figure 12, A). 



The parts in contact stick together, and the chromosome 

 breaks at the point of adhesion. The chromosome is thus 

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

 (Figure i'2, B). 



The three broken ends are still in contact, and there occur 

 at the tips growth processes, so that the broken ends, some or 

 all, may reunite. The two end pieces may reunite, leaving the 

 loop free. In that way are produced the "deletions"; short 

 chromosomes lacking all the middle part. In such cases the 

 free loop degenerates or is lost. 



In other cases the two end pieces may reunite with the ends 

 of the loop. Sometimes the union may be with the same ends 

 as before ; then there might be no evidence that any break had 

 occurred. But in other cases, since the four ends are all close 

 together, each end piece may unite with the other end of the 

 loop from that with which it was before united (Figure 12, 

 C). This produces the inversions. The chromosome now has 

 its two end parts as before, but in its middle part the order of 

 the genes is reversed. 



If the accidental crossing or contact is between two difTer- 

 ent chromosomes, then they adhere at the point of crossing. 



