QUALITATIVE DIFFERENTIATION 303 



different mutation properties and must therefore be inherently 

 different. We can now consider the cytological evidence. 



2. DIFFERENTIATION OF BEHAVIOUR IN THE CELL 



(i) Specificity. A quaUtative differentiation of the chromosomes 

 is shown by their specific pairing properties at all stages of meiosis. 

 It was first found by Sutton and Montgomery that particular 

 chromosomes paired at meiosis, and that these chromosomes were 

 similar in size and shape. This meant that the different chromo- 

 somes showed specific and constant affinities at meiosis. Then it 

 was found that in fusion-heterozygotes (McClung) and interchange- 

 heterozygotes (Belling) each part of a compound chromosome 

 similarly had specific properties. This rested on metaphase 

 observations. Later the development of the structures seen at 

 metaphase was traced back to the diplotene (Gairdner and D., 1931) 

 and pachytene (McClintock, 193 1) stages. The specificity was seen 

 to apply to the smallest visible elements, the chromomeres, when 

 zygotene pairing was studied in polyploids, for successive chromo- 

 meres, distinguishable in size, could be seen to pair independently 

 of their neighbours when they had a choice of association (Newton 

 and D., 1929). 



This specificity applies equally, as we have seen, to secondary 

 and somatic pairing. It is most strikingly demonstrated in the 

 somatic pairing in the permanent prophases of the Diptera, where 

 every chromomere associates with a corresponding one from the 

 partner chromosome (Ch. V). Specificity therefore corresponds 

 with the differentiation inferred genetically amongst genes by 

 Muller (1916). 



Three kinds of special differentiation in structure are visible in 

 the chromosomes beyond the simple differentiation into chromo- 

 meres. First, differentiations in size that are produced by particular 

 fixatives and are indications of specific reactions. Such are the 

 knobs seen in the pachytene chromosomes of Zea when fixed with 

 acetic acid (McClintock). Secondly, the special properties of the 

 centromere. These will be considered in detail in relation to its 

 mechanical properties (Ch. XII). Thirdly, the differentiation in 



