SPECIAL PROBLEMS OF METOSIS 75 



before) only about 4 bivalents are formed, the 

 remaining 50 chromosomes being left as diploid 

 univalents. ^^ Finally, in diploid liybrids between 

 the Cabbage and Radisli no bivalents may occur, all 

 the chromosomes forming univalents.^ ^ ^^ 



Univalent Chromosomes at Meiosis ^^^' ^^' ^^ 



A univalent chromosome at meiosis may arise in 

 two ways — it is either a chromosome which has never 

 undergone pairing at zygotene, or else it is one which 

 has paired to form a bivalent whose two component 

 chromosomes have separated again at diplotene 

 owing to the fact that no chiasma was formed 

 between them. Most univalents are probably of the 

 former type, but only a close study of all stages of 

 meiosis can reveal the precise mode of origin in 

 each particular case. The behaviour of univalents 

 at the metaphase of the first meiotic division is 

 interesting, although variable. Since they only 

 possess one undivided spindle attachment (instead of 

 two as in the case of a bivalent, or one which is in 

 process of division as in a mitotic chromosome) 

 they do not necessarily become associated with the 

 spindle in the equatorial plane, but attach them- 

 selves anywhere between its two poles. As the 

 bivalents separate into their component halves and 

 pass to the poles the univalents are left in the central 

 part of the spindle (the ' stem-body '). The first 

 meiotic division may be completed without r.ny 

 separation of the univalents into their chromatids 

 in which case they may (1) be distributed at random 

 between the two nuclei, (2) form small supplementary 

 nuclei enclosed in their own nuclear membranes, 

 (3) mechanically prevent complete separation of the 

 two main groups of chromosomes after the first 

 division so that these come to be included in a single 

 diploid interkinesis nucleus. Alternatively, (4) the 

 spindle attachments of the univalents may finally 

 undergo division at the end of the anaphase of the 



