CONSTITUTION OF THE CHROMOSOMES I41 



X and the father's X have exchanged parts, In some cases, 

 while together in the cells of the daughters; the exchange 

 has occurred in about i to i 3^ per cent of them, not in the 

 others. The newly combined chromosomes behave in later 

 generations just as did the original combinations. The new 

 combinations (red-yellow and white-gray) follow in later 

 generations particular single X-chromosomes. 



In this formation of new combinations, it is clear (i) 

 that two characteristics that were connected with different 

 X-chromosomes (as red eyes and yellow body) later be- 

 come combined with a single X-chromosome; and (2) that 

 characteristics that were first connected with a single X- 

 chromosome (as white eye and yellow body) have later 

 become separated, so as to be connected with two different 

 X-chromosomes. It is clear therefore that the white and the 

 yellow are connected with different parts of the chromo- 

 some, and that these different parts can be separated. 



This same process of forming new combinations of parts 

 of previously existing X-chromosomes has been found to 

 occur (as we shall see later) with any of the other hundred 

 or more diverse kinds of X-chromosomes that are known to 

 occur in Drosophila. The process of exchanging parts of 

 the chromosomes Is commonly spoken of as "crossing- 

 over" of a part from one chromosome to another. So re- 

 markable a process has of course attracted the attention of 

 many Investigators, and it has been studied in hundreds of 

 thousands of cases. Details concerning it we take up later. 



All this answers the question, proposed earlier, as to 

 whether the chromosome is a simple unit, or is composed 

 of a number of different parts with diverse functions. The 

 chromosome does indeed consist of diverse parts that may 

 become separated. And It is clear that these diverse parts 

 have different functions, different effects on development. 

 One part of the X-chromosome of the original father. In 

 the experiment we have described, causes the eye to be 



