THE MITOTIC CYCLE 



its elongation. In Amoeba on the other hand, the metaphase spindle is 

 only about one-fifth the diameter of the cell, so that very considerable 

 amounts of fresh material can be taken in. The extent of the elongation 

 is, in fact, about fivefold (see figures in Liesche^^^). 



Spindles and tactoids 



A discussion of the spindle in terms of a hydrated system of elongate 

 particles, inevitably raises the question of the tactoid hypothesis 

 (Bernal^'^'^). a tactoid is not only the name given to the lenticulate 

 aggregates in tobacco mosaic virus solutions, described by Bernal and 

 Fankuchen,^''^ but to any similar body, provided it has a corresponding 

 internal structure (not all lenticulate crystals do). This is a wide defini- 

 tion which certainly covers the spindle, and frequent statements can be 

 found that 'the spindle is a tactoid'. Such assertions, however, when the 

 definition of tactoids is so wide, are not particularly illuminating. The 



Table V. 



RELATION BETWEEN SPINDLE CURVATURE AT METAPHASE 

 AND SPINDLE ELONGATION IN ANAPHASE 



Curvature Elongation 



Material {measured as {anaphase lengthf 



breadth I length) metaphase length) 



Sea-urchin egg (Swann, unpublished) 0-49 f25 



Chick tissue culture cell (Hughes and 



SWANN-^*) 



Chortophaga Primary spermatocyte (Ris^*') 

 Chortophaga Secondary spermatocyte (Ris"*') 



need is rather to find out whether the spindle has any properties in 

 common with tactoids, and whether a general study of these bodies 

 can throw any light on the spindle. These are questions that have been 

 consistently shirked by cytologists. 



The only tactoids that have been much studied are those of tobacco 

 mosaic virus, which first led Bernal to put forward his hypothesis. 

 These are birefringent, lenticulate bodies that form spontaneously in 

 the lower layers of tobacco mosaic virus solutions. X-ray diffraction 

 studies show that they consist of particles about 150 A across, and some 

 ten times as long, separated by an amount of water that depends on the 

 strength of the solution and on the ions present. The particles are 

 arranged lengthwise in the tactoid, but conform to the lenticulate 

 shape. The tactoids have a general tendency to grow; small ones are 

 thin and needle-shaped, larger ones are longer and relatively thicker. 



Bernal and Fankuchen^'^ examined in some detail the reasons for 

 their characteristic shape. Because of the wide separation between the 

 constituent particles, they concluded that simple primary and secondary 



130 



