CHAPTER III 



MITOSIS : THE VARIATION OF THE CHROMOSOMES 



The Subordination of the Chromosomes to the Genotype — Numerical Variation 

 of the Chromosomes — Polyploidy — Structural Variation of the Chromosomes — 

 Fragmentation, Translocation and Other Changes — Comparative Morphology of 

 the Chromosome Complement — The Sources of Variation. 



II n'y a reellement dans la nature que des individus. 



Lamarck, Discours, 1804. 



I. THE MITOTIC CONSTANTS 



The chromosome complement of an individual is constant not 

 only in number but in certain other respects in which it is usually 

 characteristic of its race or clone. These mitotic constants are 

 three in number : — 



1. The Linear Constant. Each chromosome consists of a linear 

 arrangement of particles which is constant in order and normally 

 has two ends, i.e., it is not circular or branched. Two kinds of 

 particles whose constant position is especially expressed at mitosis 

 are the centromere and the nucleolar organisers which determine 

 centric and nucleolar constrictions. Centromeres cannot arise de 

 novo and chromosomes cannot divide without them. Whether 

 they can exist without chromosomes is not known. 



2. The Volume Constant. The bulk of the chromosome at 

 metaphase is usually constant. It may, however, be affected by 

 developmental conditions. Chromosomes are sometimes smaller in 

 rapidly dividing nuclei. 



3. The Spiralisation Constant. The degree of coiHng of each of 

 the chromosomes in the complement is equal and constant. This is 

 shown by the equal diameter of their chromatids and by the con- 

 stant length of each chromosome. Spiralisation does not proceed 

 further than to reduce each part of a chromatid between con- 

 strictions to a sphere if this is of less than the standard diameter. 



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