58 GENETICS 



the cytological standpoint into two groups, viz., 

 chromosomal aberrations and factor mutations. Chro- 

 mosomal aberrations are accidents or irregularities 

 occurring in the nuclear make-up of the germ- 

 cells. These aberrations may be of various kinds and 

 probably take place during meiosis when the germ- 

 cells are going through the preparatory process of 

 reduction of the chromosomes which precedes the for- 

 mation of the fertilized egg. 



For example in the unpairing of homologous chro- 

 mosomes after synapsis it is conceivable that the pro- 

 cess may not be clean-cut and complete but that a piece 

 of one chromosome may adhere to its mate thus 

 changing its size and composition. Or again, a frag- 

 ment of a chromosome, during the complicated elimi- 

 nation performances accompanying the marriage cere- 

 mony of germ-cells, may be shuffled out and lost, thus 

 creating a deficient chromosome. Such accidents to the 

 germ-cells would be reflected in all the subsequent 

 mitotic divisions of the somatic cells derived there- 

 from and a mutation would be the result. At any rate 

 an examination of the nuclear structure of mutants 

 frequently reveals chromosomal irregularities so that 

 an unmistakable relation between the two phenomena 

 undoubtedly exists. 



Another irregularity that occurs is an unequal 

 migration of the chromosomes to the poles of a germ- 

 cell during the reduction division, which, of course, 

 results in a cell progeny of mature gametes having 

 a number of chromosomes unlike the number in the 

 normal gametes. This appears to be the reason for 



