CONGRESSION 



525 



spindle in many animals, where all the chromosomes lie round the 

 edge of the spindle instead of being evenly distributed on the plate. 

 This difference has hitherto been represented as associated with 

 characteristic developmental properties, such as the enfolding of the 

 spindle by the prophase nucleus (cf. Belar, 1926). But the arrange- 

 ment on the metaphase plate must be due to repulsion from the 

 poles acting on the centromeres ; it follows that a distribution round 

 the edge of the plate will result from a stronger polar repulsion. 

 This view is borne out by the discovery that a more or less peripheral 



A n = l7 



Ti=20 



71 = 22 



Fig. 150. — Three pollen grain mitoses in Fritillaria pudica {^x = 39). 

 The increasingly hollow spindle is correlated with decreasingly 

 spiralised chromosomes and decreasingly efficient orientation 

 of centromeres. X 1,500 (D., 1935). 



distribution may occur in certain pollen-grain mitoses of plants 

 associated with other abnormaHties of spindle-relationship (D., 1936, 

 Fritillaria, Fig. 150; Upcott, unpub., Tulipa). Furthermore, both 

 in normal and exceptional circumstances every gradation is found 

 between the two extremes. 



(d) Meiotic Metaphase. At the first metaphase of meiosis the 

 behaviour of the chromosomes is like that at mitosis in some ways, 

 very unlike it in others. Congression, orientation and distribution 

 follow similar rules, but the agent of these changes is different. It 

 is not a polarised, potentially double centromere, but two separate 

 centromeres, which determine the movements of each bivalent. 

 These lie on either side of the equatorial plane, somewhat further 

 aoart than in the prophase nucleus as a rule. Congression may be 



