74 AN INTRODUCTION TO MODERN GENETICS 



be regarded as analogous to the somatic pairing which causes homo- 

 logous chromosomes to he near each other in mitotic metaphase in 

 organisms such as Diptera. Catcheside^ has recently discussed the 

 mechanical conditions involved. The main importance of the pheno- 

 menon is the clue it provides as to the constitution of secondary poly- 

 ploids (p. 261). 



Fig. 30. Secondary Pair- MA A 



ing. — Polar views of meta- ^^ 



phase. A, in an apple, a 



secondary polyploid with a *^^ 



haploid number of 17 made ^ 



up of two sets of 7 (the Tfl>^# •••PF 



usual basic number in C£ 



Rosaceae) with three addi- /^ 3 



tional chromosomes (after 



Darlington). 6, in Brassica oleracea, a secondary polyploid with haploid number 



9 made up of a basic set of 6 plus 3 extra chromosomes. Each dot represents two 



closely associated homologous chromosomes. 



(After Catcheside.) 



1 Catcheside 1937. 



.2 ••^* • •• 



