THE MITOTIC CYCLE 



differences in refractive index, but even the phase-contrast microscope 

 seldom shows fibres or striations in the Hving spindle. According to 

 Oettle^^'^ the phase microscope is capable of detecting refractive index 

 differences in small objects, of as httle as 0'004, so that it should be 

 possible to distinguish fibres differing in degree of hydration by as little 

 as 2 per cent from their surroundings. If there are fibres in the living 

 spindle, therefore, either they must be as hydrated as the protoplasm 

 round them or they must be sub-microscopic in size. 



Examined in polarized light, sea-urchin and chick spindles show no 

 sign of discrete fibres (Plate XIII (19, 20)) (Schmidtj^si 252 Hughes 

 and Swann;253 Swann^^* 255^ j^ both these examples, how- 

 ever, the chromosomes are numerous, so that spindle fibres might be 



f£ 8 V ¥ 8 12 



Distance afong spindle ar/j- microns from the equator 



W 



Figure 47 Coefficient of birefringence along the axis of the mitotic 

 spindle of a living egg oi Psammechinus miliaris. 



expected to be many and small. In the Chaetopterus tgg on the other 

 hand, the chromosomes are fewer. Here Inoue^^b has found, amidst 

 the general birefringence of the spindle, strands of stronger birefrin- 

 gence, some of them apparently running from the centrioles to the 

 chromosomes (Plate XIII (21) ). 



SwANN^^* has measured retardation curves for the living spindles 

 and asters of sea-urchin eggs, and calculated from them the coefficient 

 of birefringence at different points within the mitotic figure. In both 

 spindles and asters, he found a low coefficient of birefringence in the 

 centrosome, which rose to a maximum at about 5[x from the centre, and 

 then fell rapidly to a minimum at the equator of the spindle and at the 

 periphery of the asters (Figure 47). This rapid fall from the maximum 

 at 5(JL must be due either to a decrease in the proportion of orientated 

 material within the spindle and asters, or, if this proportion remains 



120 



