METAPHASE PLATE 523 



mitted. The stretching of the spindle would then be the result of 

 an internal orientation and not the cause of it. 



(c) Mitotic Metaphase. At metaphase the chromosomes form a 

 plate equidistant between the two poles. WTien, owing to the pre- 

 sence of extra centrosomes, there are three or four poles, a three- 

 armed or cross-shaped plate is formed equidistant between them. 

 When, however, two of these have arisen by division at the beginning 

 of metaphase no plate is formed between them, showing that the 

 development of the plate to some extent depends on"the chromo- 

 somes (Kuhn, 1920 ; KoUer, unpublished). Spindles can, however, 



Fig. 149. — Comparison of mitotic and meiotic plates in Hosta 

 [n = 30). Large chromosomes being rigid in meiosis are 

 peripheral ; being flexible in mitosis lie at random on the plate. 

 (Akemine, I935) 



be formed in fertilised egg-fragments of Triton without any chromo- 

 somes (Fankhauser, 1934, a and h). This indicates that the lack of 

 spindle in the other case is due to competition for spindle-forming 

 components, a competition in which time of action is all-important. 

 How this comes about will be seen later. 



The formation of the metaphase plate at mitosis takes place in 

 three stages. The first is congression. The centromeres come to lie 

 in a plane equidistant between the two poles of the spindle. The 

 second is orientation. The centromeres come to lie so that the 

 chromatids on either side of them lie in the axis of the spindle ; the 

 centromeres themselves must therefore be polarised and orientated. 



That it is the centromere which is concerned in these movements 



