CONSTITUTION OF THE CHROMOSOMES 1 63 



end is transferred to m; a similar piece oi m to p (figure 



34). 



4. In a small proportion of the cells the chromosome 



breaks in two places, so that a piece from the middle por- 

 tion of p is transferred to m, and vice versa. 



5. Very rarely the chromosomes break in three places, 

 so that there is exchange of two separate pieces. More than 

 three breaks are extremely infrequent. 



The consequences in inheritance of the fact that a single 

 chromosome contains many genes, and that exchange of 

 genes occurs between the two members of a pair of chromo- 

 somes, are taken up in later chapters. 



NOTES AND REFERENCES ON CHAPTER VI 



The account of the constitution of the chromosomes given in Chapter 

 VI is based on evidence given in papers by the students of Drosophila, 

 particularly T. H. Morgan, C. B. Bridges, A. H. Sturtevant, and 

 H. J. Muller. References to many of the original papers are given in 

 the publications cited in Note i of Chapter I. 



