100 MEIOSIS IN DIPLOIDS AND POLYPLOIDS 



at the distal ends and short chromosomes with subterminal centro- 

 meres have a higher chiasma frequency than longer ones with 

 median centromeres. Both these results agree with the time-limit 

 explanation of incomplete pairing, since small chromosomes and 

 ends must be freer and quicker in movement than long chromosomes 

 and middles. 



In some species the pairing is interrupted later than in others, 

 and all degrees of localisation occur on this account in Fritillaria 

 (D., 1936 e). Probably elsewhere a time limit determines incomplete 



Stages in pairing 



Zygotene 

 in Fritillaria 



Pachytene 



in 

 F. Meleagris 



F. Elwesii 



F. imperialis 



Fig. 30. — Stages in zygotene pairing in Fritillaria, showing how 

 its interruption at various stages by the time-Hmit leads to 

 locahsation in some species. (D., 1935 b.) 



pairing, but owing to a less regular beginning of pairing near the 

 centromere the localisation of pairing and of chiasmata is less 

 readily recognised (e.g., Lilium regale, Mather, 1935). 



(iv) Relational Coiling. At pachytene the chromosomes were 

 coiled round one another. Coiling of three kinds is found at early 

 diplotene (D., 1935 a, 1936 h) : (i) The loops on either side of a 

 chiasma lie in the same plane and two chromatids therefore cross 

 over one another in either a right- or left-hand spiral direction. 

 Later, successive loops come to lie at right angles to one another, 

 and thus abolish this coiling, (ii) In so doing they alter the 

 relationships of the chromatids, which are also coiled round one 

 another between chiasmata, probably in the opposite direction to 



