46 



THE CHROMOSOMES 



haploid set is called aneuploidy : it is obvious that 

 there is no hard and fast distinction between polysomy 

 and aneuploidy (see Table IV). 



Owing to the fact that in bisexual organisms the 

 sex -determining mechanism depends in general on 

 the segregation of a single pair of chromosomes at 

 meiosis pol3^1oid species and varieties are usually 

 found only in groups where reproduction does not 

 depend on bisexuality (most Angiosperms, partheno- 

 genetic and hermaphrodite animals such as the 

 Pulmonata , Oligochaeta, &c. ) . If polyploidy occurred 

 in bisexual species it would completely upset the 

 sex -determining mechanism. ^^^ Since the vast ma- 

 jority of Angiosperms are monoecious while the bulk 

 of the Metazoa are dioecious polyploidy is far com- 

 moner in the higher plants than in animals. 



