CHAPTER III 

 SPECIAL PROBLEMS OF MITOSIS 



Number, Form and Size of Chromosomes 



THE number of chromosomes in the somatic 

 nuclei of an organism is usually the same for 

 all the tissues and for all the individuals of the same 

 species. There are exceptions to both these state- 

 ments (i.e. organisms with different chromosome num- 

 bers in different tissues and species with different 

 chromosome numbers in different * varieties '), but 

 they need not be considered at present. The number 

 of chromosomes in a somatic nucleus is usually even 

 and is referred to as the somatic number. Where there 

 are size -differences between the chromosomes of a 

 somatic set it will usually be found possible to arrange 

 them in pairs (Fig. 5), the two members of each pair 

 being exactly alike in size, in position of spindle 

 attachment and (where they exist) of secondary con- 

 strictions. The complete set of chromosomes is thus 

 made up of two identical haploid sets. Organisms in 

 which this is so are called diploid organisms and the 

 somatic set may be called the diploid set. Sometimes 

 even in diploid organisms one pair of chromosomes 

 are unequal in size (Fig. 20) and sometimes the diploid 

 number is uneven in one sex, there being a chromo- 

 some which does not form a member of a pair in that 

 sex (Fig. 5/, g). In these cases the uneven pair or 

 the odd chromosome are sex -chromosomes. Their 

 behaviour will be considered later. In hybrids 

 between species whose chromosome sets differ it is 

 naturally not possible to arrange them in pairs since 

 the two haploid sets in the hybrid are not identical. 



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