30 MITOSIS: THE CONSTANCY OF THE CHROMOSOMES 



spindle itself, more especially because with dehydration in hyper- 

 tonic solutions the stretching of the spindle is exaggerated. Prob- 

 ably it is a property correlated with the special distribution of its 

 water content. Where, in meiosis in Stenobothrus, one pair of 

 chromosomes in separating offer, on account of their greater length, 

 more resistance to separation than the rest, the whole spindle is 

 tilted evenly. This argues not merely pressure from the middle 

 part of the spindle but an independent coherence of the middle 

 part and of the polar parts. The effect of the stretching of the 

 spindle can be seen in pollen grain mitoses where one pole has been 

 lying close to the wall. The spindle then continues to push the 

 inner set inwards, while the movement of the outer set has been 

 stopped by the wall in mid-anaphase (cf. Ch. XII). Where the 

 chromosomes are very long, as in mitosis in Tradescantia and 

 Aggregata, their separation is helped by their shortening still further 

 at late anaphase. 



(vi) Telophase. When the chromosomes reach the pole they 

 contract longitudinally and form a compact mass which passes 

 through stages in some respects comparable with the reverse 

 processes of prophase. These appear to consist essentially in the 

 imbibition of water. In some organisms the chromosomes form 

 separate vesicles at first (Belar, 1928 b), and where the resting 

 stage is short these may persist, separately identifiable, to the next 

 division (v. infra). 



(vii) Duration of Mitosis. The duration of successive stages of 

 mitosis has been recorded from observations of living cells by Belar 

 (1926 b, 1929 b, Tradescantia) and others. The following observations 

 indicate the range of variation found, but do not include either the 

 quickest or the slowest examples. The third case is an observation 

 of meiosis, which will be specially considered later. 



Apart from observations of the living cell the relative frequencies 

 of different stages found in fixed material show great variation in 

 their timing. Certain stages in the prophase of meiosis may be 

 indefinitely prolonged, and in glandular tissue mitotic prophases 

 have been found that are as permanent as the cells containing them 

 (Ch. V). Telophase, on the other hand, is a uniformly short 

 stage. 



