UNIVALENTS 



411 



When the paired chromosomes begin to separate at anaphase the 

 unpaired chromosomes follow one of two courses : (i) those lying 

 far away from the equator are included with the group of daughter 

 bivalents passing to the nearest pole ; (ii) those lying near the 

 equator move on to the plate, orientate themselves axially, and 

 divide after a short interval into their two chromatids, which then 

 pass to opposite poles as in mitosis. The first gives post-division of 

 the univalents, the second pre-division. 



In Tritictim-Mgilops hybrids Kihara (1931) in a detailed account 

 has shown the probable history of the univalents to be chiefly of 

 the second type, which is as follows : (i) The bivalents arrange 

 themselves on the equatorial plate ; the univalents remain distributed 

 at random, (ii) The univalents arrange themselves on the edge of 

 the equatorial plate, (iii) The bivalents divide ; the univalents 

 remain on the equator, (iv) The univalents divide and their halves 

 follow the daughter bivalents to the poles. The variations 

 commonly observed in univalent behaviour are probably due to 

 various degrees of delay in the movement of the univalents relative 

 to those of the bivalents. 



In the extreme case they altogether fail to divide at the first 

 division, some being included in the daughter nuclei and others 

 lost in the cytoplasm. 



Where multiple configurations are formed it often happens that 

 one of the chromosomes is left on the plate at anaphase (e.g., in 

 Hyacinthus, Ch. IV). The same result may follow in a simple 

 bivalent, dividing while lying on an accessory plate to one side of 

 the main plate, owing to overcrowding (e.g., Fritillaria, Ch. XII). 

 The chromosome going to the further pole is stopped on the 

 equator. Such a false univalent divides at late anaphase like a 

 chromosome that has not been paired at all. But it may show 

 two peculiarities in Tulipa (Upcott, unpub.). In the first place 

 the two chromatids, owing to one or both of them having crossed 

 over at chiasmata with the chromatids of a partner, do not lie 

 close together, and may differ in structure and in length. In the 

 second place, owing presumably to abnormal strain, the chromatids 

 may be distributed unequally on either side of the centromere, two 

 sister chromatids remaining attached to the same daughter centro- 



