132 MEIOSIS IN DIPLOIDS AND POLYPLOIDS 



(d) Indifferent : one centromere lies indifferently with respect to 

 the others. 



(ii) Each of the chromosomes may disjoin from those with 

 which it is directly associated (provided it is not directly associated 

 with more than two), or it may pass to the same pole with one or 

 more of them. 



(iii) Where an even number of chromosomes are associated the 

 multivalents may divide evenly or unevenly ; thus four chromo- 

 somes may divide into two and two or into three and one. In this 

 way great differences may occur between the numbers of chromo- 

 somes in the two daughter-nuclei, Prunus cerasus (^x) gave excep- 

 tionally a distribution of 19 + 13 (Plate V). Similarly, tetraploid 

 Datura (Belling and Blakeslee, 1924 b) and Primula sinensis (D., 

 1931) give germ-cells with unequal numbers of chromosomes 

 (cf. Levan, 1933 «)• 



(iv) Where an odd number of chromosomes are associated whole 

 chromosomes may separate to the poles, or one of the component 

 chromosomes may be left on the equator, and, after an interval, 

 divide, the halves passing to opposite poles after the whole chromo- 

 somes. They then pass at random to one pole or the other in the 

 second division or are lost. In triploids, therefore, germ-cells are 

 formed with an approximately random distribution of the chromo- 

 somes in the third set. This has been achieved by the random 

 distribution, either of univalents (or odd members of trivalents) at 

 the first division, or of daughter univalents (or odd members of 

 trivalents) at the second division (v. Ch. VIII). The detailed 

 behaviour of unpaired chromosomes will be considered later. 



Where associated chromosomes pass to the same pole, all trace 

 of their association is lost at the second division if an interphase 

 occurs (e.g., in Hyacinthus, -^x ; D., 1929 h). The second division 

 is therefore normal apart from such a difference as there may be 

 between the numbers of chromosomes in the two daughter-nuclei. 



5. THE CONDITIONS OF MEIOSIS 

 We are now in a position to ask ourselves what conditions 

 distinguish meiosis from an ordinary mitotic division. Do new 

 forces come into play to produce a pairing of homologous chromo- 



