CELLS IN DIVISION 



valence forces could not be responsible and they suggested that the 

 structure is held together by long range forces generated by the ionic 

 atmosphere round each particle. The shape appears to be due to a 

 compromise between the tendency of the highly asymmetrical virus 

 particles to produce linear aggregates, and the rounding-up tendency 

 of the surface tension at the edges of the tactoid. 



The familiar 'positive' tactoids are not the only form of aggregate 

 to be found in these solutions. There are also lenticulate cavities or 

 'negative' tactoids within more or less uniformly orientated masses of 

 virus solution, and various other aggregates, regular and irregular. 



Since both spindles and tactoids are similar in shape, and consist of 

 asymmetrical elements that give rise to birefringence, Bernal suggested 

 that the parallel between the two structures might be a close one. He 

 went on to suggest that since they both have a tendency to elongate, the 

 study of tactoids might throw light on the anaphase separation of 

 chromosomes. He also made a few tentative suggestions about the 

 possibility of the chromosomes generating negative tactoids, which 

 grew in size at the expense of the spindle, and in so doing, moved the 

 chromosomes to the poles, and disorientated the whole mitotic structure. 

 These ideas, however, have seemed too speculative to be of much value, 

 and as a hypothesis of anaphase movement, the tactoid hypothesis 

 has met with considerable criticism (Schrader^^* 274^_ On the other 

 hand, the spindle is still often referred to as a tactoid, and it is a question 

 of some importance to decide whether such a comparison has any 

 justification. The three points of similarity between spindles and tac- 

 toids have already been mentioned. It remains to discuss the points of 

 difference, which are more numerous. 



Tactoids consist of large asymmetrical particles which are all more 

 or less identical, and give rise to extremely well defined X-ray patterns. 

 The nature of the particles within the spindle is uncertain, and X-ray 

 diagrams have yet to be obtained. In such protoplasmic structures as 

 have been examined by this means, however, the X-ray patterns are 

 very diffuse, and there is no evidence for precisely arranged particles of 

 constant size. 



The forces holding the virus particles together are long range ionic 

 ones. As a result, tactoids are extremely labile, and can be disintegrated 

 merely by shaking. For this reason they cannot, of course, be fixed. 

 Spindles and protoplasmic structure in general, on the other hand, are 

 certainly held together by secondary valence forces, and probably by 

 primary valence forces as well (Frey-Wyssling^'^^). In consequence, 

 they are far more robust than tactoids; spindles, for instance, can be 

 pushed round the cell, or even squeezed out of it (Foot and Stro- 

 bell^'^^), and they can be preserved by fixation. 



Whereas tactoids grow and fuse with great ease, spindles and asters 



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