SPECIAL PROBLEMS OF MEIOSIS 85 



cum punctatum ^^ and Rhoeo discolor ^^ it is not pos- 

 sible to arrange the somatic chromosomes in pairs ; 

 that is to say that no one chromosome is completely 

 homologous to any other. Only as a result of long 

 genetical and cytological work has it been possible to 

 understand exactly what happens during the meiosis 

 of such organisms. The following account is neces- 

 sarily somewhat simplified but is substantially true. 

 If the chromosomes of an ordinary diploid organism 

 with a haploid number of 4 be represented as follows : 



abcdff ghij 



abcdef ghij 



klmno pqrst 



klmno pqrst 



then those of Oenothera muricata will be : 



abCiCd IkR^mn vuC^wx 



dcR ifi/ nmC^op xwR^yz 



feCzgh poK^qr ^y^-ouft 



hgRz^j rqC^st [^oiR-fia 



jiC^kl tsK^uv 



It will be seen that each chromosome consists of 

 three segments, two terminal ones (represented by 

 the small letters) and a median one (represented by 

 the ca})ital C's and R's). The median segment con- 

 tains the spindle attachment in all cases. None of 

 the median segments are homologous (i.e. they are 

 differential segments analogous to those present in 

 sex-chromosomes) ; the terminal segments in each 

 chromosome are, however, homologous to two other 

 terminal segments in different chromosomes. Thus 

 when pairing takes place at zygotene it does not 

 generally affect the median segments * but only the 

 terminal ones (Fig. 19a). At pachytene a con- 



* The median segments are sometimes partially homolo- 

 gous, in which case they may pair and form occasional 

 chiasmata (see Chap. VI). 



