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MORGAN— THE CONSTITUTION OF 



[April 23, 



requirements of Mendel's postulates. There were two further steps 

 necessary to bring the two lines of inquiry into complete accord ; 

 namely, (i) correspondence between the number of the chromo- 

 somes and the groups of inherited characters, and (2) the inter- 

 change between the members of the same pair of chromosome. 



Fig. 2. Diagram to illustrate segregation of chromosomes. The four 

 pairs of chromosomes in the upper circle (a), conjugate in (b) (synopsis 

 stage), prepare for separation in (c) and undergo segregation so that each 

 germ cell (d, d') comes to contain one member of each pair. 



The number of chromosomes is small in comparison with the 

 large number of different characters that an animal or a plant pos- 

 sesses. We should expect therefore if in any animal or plant 

 a sufficient number of character-differences were known that the 

 characters would be found to be inherited in groups, and that the 

 number of such groups should be the number of chromosome pairs 

 that such an animal or plant possesses. In very few cases have 

 enough characters been found to make such a comparison of any 

 value. 



