THE GENERAL OUTLINE OF MITOSIS 23 



internal compensating spiral ; a sliding of molecules 

 on one another takes its place. It does not appear 

 possible to decide at present which of the second 

 and third alternatives is actually found in the 

 chromosome. 



First Stage of Anaphase 



Metaphase is a period during which almost no 

 appreciable change takes place in the cell : it is 

 nearly always one of the shortest stages of mitosis. 

 At the end of metaphase the halves of the spindle 

 attachments (the latter having divided at prometa- 

 phase) appear to repel one another. At any rate the 

 proximal ends of the chromatids (those, that is to 

 say, which are attached to the spindle) begin to 

 diverge and to move up the sides of the spindle 

 towards the poles (Fig. 2d). From the mode of travel- 

 ling of the spindle attachments and in view of the 

 fact that the spindle itself does not undergo any 

 change of shape at this stage the hypothesis of an 

 active repulsion between the divided spindle attach- 

 ments is the only possible one. Certainly there is no 

 evidence for a ' traction of fibres ' at this stage — the 

 movement of the chromatids is autonomous and 

 depends on the spindle attachments. Why this 

 repulsion-force should not manifest itself earlier is 

 not clear ; perhaps the division of the spindle attach- 

 ments is not finally completed until the end of 

 metaphase. 



As a result of this movement of the chromatids the 

 attachment regions of the latter move up the spindle 

 towards the poles until in most cases they have 

 travelled about two -thirds of the distance from the 

 equator to the poles. Where the chromosomes are 

 short this means that the spHt halves are now com- 

 pletely separated ; where they are long the distal 

 ends (those farthest away from the spindle attach- 

 ments) wiU be still in contact (Fig. 2e). 



