532 CELL MECHANICS 



polar spindles became arrow-shaped at anaphase. It may be 

 inferred regularly from the behaviour of anaphase chromosomes in 

 pollen-grain mitoses (Geitler, 1935 ; D., 1929 c, 1936 h). The cell- 

 wall stops one group of chromosomes from moving further in mid- 

 anaphase, but the stretching of the spindle continues to push the 

 other group, which is still seen in the characteristic attitude of 

 movement. This anaphase change in the spindle is not surprising 

 in view of the modifying action of the centromeres on the spindle at 



Fig. 154. — Pollen grain divisions showing asymmetrical anaphases 

 caused by stretching of the spindle, one end of which abuts on 

 the wall. (Left) Podophyllum (D., 19366). (Right) Trades- 

 cantia, n = 11 -\- 2 ff. (D., 1929 c.) X 1,600. 



metaphase, for they have converted the centro some-spindle between 

 two points into the centromere-spindle between two plates (Fig. 154). 



The stretching of the spindle between the separating chromo- 

 somes at anaphase is clearly analogous to its stretching at meta- 

 phase in the absence of orientated centromeres. It seems that 

 cumulative stretching in the axis of repulsion is inherent in the 

 orientation of spindle particles under the influence of that repulsion. 

 If the orientation depends on the gradual concatenation of long 

 molecules, this property is inevitable, for the chains can grow only 

 in length. 



It has been supposed since the early hypotheses of van Beneden 



