THE CHROMOSOMES 



6 9 



reasons for believing that their differences are much 

 more profound than mere inspection would suggest. 

 At fertilization a set from each parent goes to the forma- 

 tion of the zygote, and (excepting the odd chromosome, 

 to be considered later in connection with sex) the two 

 sets correspond in the sense that each type of chromo- 

 some has its mate. Thus the cells of the individual are 

 duplex or double, containing a contribution from each 

 parent. Shortly before the reduction division, the cor- 

 responding chromosomes, derived from the parents of 

 the reproducing individual, are seen to become coupled, 

 the pairs uniting side by side or twisting more or less 

 around one another. This phenomenon is called synap- 

 sis. There is reason for thinking that when they sepa- 

 rate, they do not always retain their original integrity, 

 that there may be some interchange of materials. This 

 matter has been investigated by Dr. T. H. Morgan and 

 his associates at Columbia University, with very re- 

 markable results. In the study of inheritance in flies 

 of the genus Drosophila, it was found that certain char- 

 acters were not inherited in accordance with the theory 

 of random sampling, but came out in groups. At first 



One set of 

 chromo- 

 somes from 

 each parent 



Synapsis 



Linkage, 

 and its 



results 



FIG. 15. 



Ba ^** a!5a=r . Drawing by C. E. Allen 



Cells of lily (Lilium canader.se). i, synapsis; 2, resting stage. Greatly 

 magnified. 



