THE MITOTIC CYCLE 



apparently do not. Every spindle is the product of a number of definite 

 orientating centres without which it cannot form. Tactoids, on the 

 other hand, have no centres. 



The shape of many spindles, as was discussed earlier, can be explained 

 by the action of the two centrioles ; but in many cells, particularly those 

 of the higher plants, centrioles are apparently absent, and in these cases 

 tactoid forces may be involved. It is by no means certain, however, that 

 centrioles are really absent in plant cells, while in a number of cells 

 where centriolar activity is undeniably absent, the spindle poles remain 

 permanently diffuse. It is not certain, therefore, that the char- 

 acteristic shape of the spindle is necessarily due to the same effects as 

 the shape of the tactoid. 



Since spindles only form from a number of orientating centres, they 

 show a different pattern of early growth from tactoids. Whereas small 

 tactoids are needle-shaped, spindles form in a number of ways, none of 

 which resembles the growth of a tactoid except perhaps the small 

 acuminate spindles that form round individual chromosomes in species 

 such as Acroschismus (Hughes-Schrader^^^). Since tactoids do not 

 possess orientating centres, they never, of course, produce asters. 



Once again, as the result of orientating centres, the internal structure of 

 the spindle is complicated by regions of greater and lesser birefringence. 

 Except for the rather doubtful possibility that tactoids may contain 

 within them negative tactoids, there is no reason to suppose that the 

 internal structure of the tactoid varies from point to point, and con- 

 siderable reason to suppose that it does not. 



Even the similarity between the two bodies as regards birefringence 

 is superficial. The polarized light photographs given by Bernal and 

 Fankuchen show that tactoids are uniformly bright, whereas spindles 

 are always less birefringent at the equator. The reasons for this have 

 been discussed by Swann^^* and briefly also in the present section. It 

 must be due either to a thinning out of sub-microscopic fibrils or to a 

 change in molecular and micellar arrangement. Neither of these effects 

 is to be found in the tactoid. 



Finally, there is the question of the lengthening of the spindle in 

 anaphase. Whereas the tactoid grows markedly thicker as it elongates, 

 the spindle does not. Measured from the photographs of Bernal and 

 Fankuchen, the smaller tactoids appear to have a ratio of length to 

 breadth of 4:1, which decreases to about 2:1 in the larger tac- 

 toids. From the evidence of Ris,^^^ Hughes and Swtann^^^ and 

 Swann^^* 2^^ it appears that the metaphase spindle has a length to 

 breadth ratio of between 1*5:1 and 2:1, while in late anaphase 

 this ratio rises to between 2 : i and 3:1. 



If the differences between spindles and tobacco mosaic virus tactoids 

 have been unduly laboured, it is only because they have not been 



132 



