THE COMPLEMENT 75 



the general course of anaphase separation as we shall see in dealing 

 with meiosis. The fact that the interlocking is exceptional means 

 that when the chromosome divides it does so along a cleavage 

 surface which is helical in relation to the chromosome thread and, 

 as a rule, exactly compensates for the relic spiral in which this 

 thread is lying. The two halves will then separate freely at 

 anaphase. Such a constancy of cleavage direction implies a 

 constancy in the orientation of the particles making up the thread 

 and a specificity in the direction of cleavage, or perhaps more pro- 

 perly the direction in which the new particles are laid down which 

 go to make the sister chromatids (Ch. XII). 



5. THE COMPARATIVE MORPHOLOGY OF THE CHROMOSOME 



COMPLEMENT 



(i) Specific. In the previous sections comparison has been made 

 of sister-cells, sister-individuals and sister-races. It may now be 

 extended to related species and genera. The more distant the 

 relatives compared, the less certain are the grounds of inference, 

 but it will be seen that the relationships of species and genera can 

 often be expressed conveniently in terms of the changes inferred 

 from the study of smaller groups. It will also be seen later that 

 more detailed study at meiosis bears out these conjectures in many 

 instances. 



In the first place our knowledge of polyploidy within the species 

 suggests that when groups of species have chromosome numbers in 

 a series of multiples of a common number and the chromosomes 

 are of similar form, these species are probably derived from forms 

 with this " basic number " by polyploidy (Winge, 1917). The 

 application of this inference will be considered later in detail 

 (Ch. VI). 



Structural changes may be inferred with some certainty from a 

 detailed comparison of chromosome form within genera and larger 

 groups. Thus the chromosome complements within Fntillaria and 

 the Acrididae may be represented diagrammatically as follows 

 (F, a long chromosome with median centromere ; /, a short chromo- 

 some with terminal or sub-terminal centromere). 



