CELLS IN DIVISION 



constant, to changes in the molecular and micellar arrangement. The 

 decrease, however, is of the order of tenfold or twentyfold, so that the 

 molecular and micellar changes would have to be considerable. On the 

 other hand the fall in the coefficient of birefringence is very approxi- 

 mately an inverse square, which, for simple geometrical reasons, is the 

 relationship to be expected if the spindle and asters consist of a number 

 of distinct unbranching fibrils radiating from the centrosomes. There 

 would seem, therefore, to be two possible structures for the achromatic 

 figure. Either it is orientated throughout, with a varying degree of 

 orientation from point to point, or it consists of discrete fibrils radiating 

 from the centres like the spokes of a wheel. These two structures are 

 illustrated diagrammatically in Figure 48. 



So far only a single study of the spindle has been made with the 

 electron microscope (Rozsa and Wyckoff^^'''), but the results are ex- 

 tremely interesting. Using the ordinary precipitating fixatives, these 







a b 



Figurt 48 Diagrams showing possible types of fine structure in the spindle and 



asters built up from asymmetrical particles of a material present in constant 



proportions in which the orientated fraction a is constant throughout, and b is 



variable. In b fibrils are surrounded by an unorientated medium. 



authors found that the spindle was irregularly fibrous, bearing in fact 

 a considerable resemblance to its appearance in the light microscope 

 after comparable treatment. With neutral formalin on the other hand, 

 they found that it was entirely homogeneous, with no trace of fibres 

 or orientation of any sort. It is, of course, possible that formalin was not 

 preserving the orientated structure of the spindle, but this is extremely 

 unlikely, since formalin is generally regarded as the best fixative for 

 structures within the cell. Far more probably the living structure was 

 very well preserved. For, as we have seen, there appear to be no signi- 

 ficant variations within the spindle, so that we should not necessarily 

 expect a perfectly fixed spindle to show any structure under the electron 

 microscope, unless the resolution was sufficient to show the actual 

 molecules or micelles. In this particular work the resolution was not 

 apparently very high, though appreciably greater than with the light 

 microscope. 



From these various lines of evidence, it is possible to draw a number 

 of conclusions about the structure of the living spindle. First of all, it 



121 



