SPECIAL PROBLEMS OF MITOSIS 33 



both in normal cells (in which ring chromosomes have 

 apparently arisen spontaneously) and in X-rayed 

 material. ^3^' ^^^' ^^^ Here the two ends of the chromo- 

 some are fused together so that a closed ring results. 

 In Drosophila melanogaster there is a stock (the 

 * closed-X ' stock) in which the X-chromosome has 

 its proximal and distal ends fused so as to form a 

 ring. ^2^ In these cases the two rings which separate 

 from one another at anaphase may become inter- 

 locked like two links of a chain. This may lead to 

 both chromatids going to the same pole instead of 

 to opposite poles. 



During prophase and metaphase the chromatids 

 of which the chromosome is composed are held to- 

 gether in a paired condition throughout their length 

 (Fig. 2). It appears (and observations on meiosis 

 confirm this) that they are held together, not merely 

 in a mechanical way (such as would result from a 

 common investing sheath — like two sausages in a 

 skin) but by a force of mutual attraction between the 

 homologous genes of which the chromatids are com- 

 posed. Speculation as to the nature of this force is 

 outside the scope of this book ; but at the moment 

 it appears to be unparalleled in biological systems. 

 Now is this force exhausted when two chromatids 

 are in contact, in the paired condition, or does it 

 extend to other homologous chromatids (in other 

 words is the force exerted merely between pairs of 

 genes, or between threes and fours, &c.) ? In diploid 

 nuclei at prophase and metaphase each chromatid is 

 represented four times and if there was some ' resi- 

 dual ' attraction we should expect homologous 

 chromosomes to lie side by side in close approxima- 

 tion. Usually this is not the case, i.e. there is little 

 or no residual attraction, but it is exactly what does 

 happen in the two-winged flies (Diptera) including 

 Drosophila (Fig. 20) where the homologous chromo- 

 somes lie side by side (but not actually touching) 

 during prophase and metaphase. This state of 



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