202 



THE CELL AND PROTOPLASM 



follow the lines of orientation of the 

 equally-spaced virus particles. 



The theor}^ of factoids makes it possible 

 to go a long way in the explanation of the 

 mechanism of cell division, for it provides 

 secondary forces affecting the poles of the 

 factoids which are due not to any imme- 

 diate reaction between these poles, but 

 to the whole field of orientation of the tac- 

 toid itself. These forces overcome the dif- 

 ficulty implicit in the earlier crude elec- 

 trical or diffusion theory of spindle for- 

 mation because they admit of considerable 

 geometrical variation; they do not act in 

 straight lines but along the flow lines of 

 the long particles and consequently may 

 follow curved or even sinuous tracks. Not 

 only do long particles suspended in an un- 

 oriented medium tend to form factoids, but 

 any enclosure of this medium in the tactoid 



itself takes on a tactoid shape which may 

 be termed a negative tactoid (Fig. 2, B). 

 The poles of the negative tactoid have a 

 tendency to move apart as the orientation 

 of the tactoid becomes more pronounced, 

 and also tend to move bodily into the 

 equatorial plane of the original tactoid. 

 On the basis of a few very simple hypoth- 

 eses, it is therefore possible to give a simple 

 account of the external mechanisms of cell 

 division : 



1. That tlie cell (probabl}' the nucleus) 

 contains protein material which under 

 suitable ionic and chemical conditions ag- 

 gregates into long particles of dimensions 

 of the order of 1000 A x 100 A. 



2. That structures exist in the cell 

 toward which these particles tend to set 

 themselves at right angles, i.e., cell walls, 

 centrosomes, and centromeres. 



A 



Fig. 2. Tactoids of dificroit sizes. A, ])ositive tactoids; P., negative taetoids. 



