THE MITOTIC CYCLE 



chick, that the rate of separation fell off steadily (Figure 44) . However, 

 Ris^^' found an initial sigmoid inflection with the spermatocytes of 

 Chortophaga, and Hughes and Preston^^^ found a similar inflection with 

 Amphibian cells. It is hard to know what significance to attach to these 

 curves. Hughes and Swann^^^ pointed out that the general form of 

 their curves suggested a contractile mechanism, but it is questionable 

 how far chromosomes can be treated as bodies being pulled through a 

 viscous medium. The fact that both large and small chromosomes move 

 at the same rate, for instance, suggests that ordinary dynamic con- 

 siderations may not be relevant. It is clear, however, that it would be 

 difficult to account for any of the curves on the basis of a repulsive force 

 between chromosomes. If Swann's hypothesis about the cause of 

 chromosome movement is correct, the velocity, though depending to 

 some extent on the response of the contractile elements, will be largely 



Table IV. EXTENT OF CHROMOSOMAL FIBRE CONTRACTION 



determined by the rate of release of the active substance. If this is so, 

 it is of doubtful value to search for any mechanical significance in such 

 curves. 



It is well known that chromosome separation is due not only to a 

 contraction of the chromosomal fibres, but in some cases, to a general 

 elongation of the spindle as well. Ris^^^ 26? recognizes that in most cells, 

 anaphase movement is due to a combination of the two. The idea that 

 chromosomes may be pushed as well as pulled apart is an old one. It 

 forms part of several theories of mitosis, in particular that of Belar, 

 who postulated in his earlier papers a definite body, the 'Stemmkorper' 

 which forces the daughter groups apart. Ris, however, maintains that 

 growth is nowhere localized. 



If a spindle is to elongate it must either take in new material, or, if 

 the volume remains constant, it must become thinner. Only the observa- 

 tions of Belar^*^ give any indication of thinning, and there is no doubt 

 that the spindle normally increases in volume as it lengthens. Curves of 

 elongation, taken from the data of Ris,^^' Hughes and Swann,^^^ and 

 Swann,^^^ all agree in showing that it remains more or less constant in 

 size at metaphase, whereas it grows rapidly during anaphase. The 



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