308 GERM-CELL CYCLE IN ANIMALS 



other, we say that they lie in a second chromosome. 

 The characters of the third group have not as yet 

 been so fully studied, except to show that they are 

 linked. We place them in the third chromosome 

 without any pretensions as to which of the pairs of 

 chromosomes are numbered II and III. 



" The arrangement of these characters in groups 

 is based on a general fact in regard to their behavior 

 in heredity, viz., A member of any group shows linkage 

 with all other members of that group, but shows inde- 

 pendent assortment with any member of any other 

 group" If the factors which determine these groups 

 of characters are situated in the chromosomes, as the 

 hypothesis demands, we should expect each group 

 to act as a unit in heredity. Occasionally, however, 

 the characters of a group appear to act independently, 

 and there must thus be an interchange of factors at 

 the time of synapsis. As already stated (p. 254), an 

 interchange of substances between chromosome pairs 

 during synapsis is possible and even probable. Mor- 

 gan explains the degree of crossing over of characters 

 in the following way : The factors which determine 

 the characters are arranged in the chromosomes in 

 a linear series ; those factors that are near together 

 will have less chance of being separated than those 

 that lie farther apart. The relative distances be- 

 tween these factors can be judged by the frequency 

 of interchange as determined by breeding experi- 

 ments. It has thus been possible to locate certain 

 factors in the chromosomes more or less accurately 

 and to predict with some degree of certainty the re- 



