THE SPINDLE 521 



spindles flat ; since the cells are globular the spindles must have 

 opposed special resistance to pressure. Finally the spindle is seen 

 to be more refractive to light than the rest of the cytoplasm. 



The same tests show the relative rigidity of different cell 

 organs. The nuclear ground substance at late prophase is less 

 rigid than the spindle ; the chromosomes at late prophase and 

 metaphase are more rigid. 



The special rigidity of the spindle is of a peculiar kind. The 

 shrinkage when the spindle loses water is greater crosswise than 

 axially. It may thus be caused to spHt axially either in numerous 

 small fissures or in a few broad clefts. These, especially when 

 stained, give the spindle a striated appearance, and they have been 

 described as the " spindle-fibres." What is their significance ? 

 Evidently the water content of the spindle is not evenly distributed 

 but lies in axial channels between the more rigid portions of the 

 spindle. This is shown equally by the special axial expansion 

 of the spindle at a later stage of mitosis. There are therefore 

 in the spindle axial structures of high rigidity which may be de- 

 scribed as fibres, although the " fibres " that are seen are merely 

 the channels between them. Their structure is not inherent in 

 their materials like that of the fibres of connective tissue ; it 

 must be determined by special conditions external to them, as 

 is the structure of fibrillse or ciHa in the Protozoa or the arrange- 

 ment of iron filings in a magnetic field. This has been shown most 

 clearly by Chambers (1925) ; a needle passed through the spindle 

 at metaphase does not disturb the chromosomes to which connective 

 fibres might be supposed to be attached. 



Another possible source of misunderstanding about spindle 

 fibres must be removed. The threads connecting the centromeres 

 to the bodies of the chromosomes are often stained at the first 

 metaphase of meiosis although the centromeres themselves are not. 

 These threads have often been illustrated and described as spindle 

 fibres. We now see that these " fibres " are an integral part of the 

 chromosomes and, although they depend for their arrangement 

 on the structure of the spindle, they are not themselves a part of 

 that structure. 



Experiments showing the absence of connective fibres do not solve 



