THE MITOTIC CYCLE 



lengthening of the spindle both occur in anaphase, though not in the 

 same proportions in different cells (Ris^^^). Hughes and Swann^^^ 

 have shown that in the chick fibroblast, the early rapid part of anaphase 

 movement is associated with polar contraction (Figure 44) . Arguments 

 for the 'traction fibre' theory of anaphase movement have been sum- 

 marized by Cornman;^^' the question is discussed further in the 



# 5 



Minutes 



Figure 44 Movement of spindle poles and chromosomes in anaphase of chick 

 osteoblast in culture, a Dimensions plotted from alternate phase-contrast and 

 polarized light photographs as indicated. In b the length of one set of chromo- 

 some fibres (from chromosomes to pole) is plotted. From Hughes and Swann"' 

 {By courtesy, J. exp. Biol.). 



following section, in which Swann considers the nature and behaviour 

 of the spindle. 



The contraction of spindle fibres during anaphase, however, raises 

 the question of why it is that the chromosomes move and the spindle 

 poles do not then approach (p 127). It is necessary to conclude that a 

 number of factors must then cooperate, such as the differential rigidity 

 of various regions of the cell. No simple theory of the spindle will 

 account for anaphase separation, nor, a fortiori^ for the behaviour of the 

 chromosomes in metaphase as well. During metaphase the chromo- 

 somes move from random positions into the 'equatorial plate'. The 

 spindle is already present before these movements begin, and is already 



116 



