I04 



CELL-DI VISION 



Pig- 51- — Heidenhain's model of mitosis (mainly from 

 Heidenhain). 



A. Dotted lines show position of the rays upon sever- 

 ing connection between the small rings. R. Position upon 

 insertion of " nucleus." C. D. Models with flexible hinged 

 hoops, showing division. 



intervals a series of rub- 

 ber bands (astral rays). 

 At the other ends these 

 bands are attached to a 

 pair of small rings (cen- 

 trosomes) fastened to- 

 gether. In the position 

 of equilibrium, when the 

 rays are stretched at 

 equal tension, the rays 

 form a symmetrical aster 

 with the centrosomes at 

 the centre of the circle 

 (Fig. 51, yi). If the con- 

 nection between the cen- 

 trosomes be severed, they 

 are immediately dragged 

 apart to a new position of 

 equilibrium with the rays 

 grouped in two asters, as 

 in the actual cell (dotted 

 lines in Fig. 51, ^). If 

 a round pasteboard box 

 of suitable size (nucleus) 

 be inserted between two 

 of the rays, it assumes 

 an eccentric position, the 

 cell-axis being formed by 

 a line passing through its 

 centre and that of the 

 pair of small rings {cf. 

 the epithelial cell, p. 57), 

 and upon division of the 

 aster it takes up a position 

 between the two asters. ■ 

 In a second form of the 

 models the circle is 

 formed of two half rings 

 of flexible steel, joined 

 by hinges; the diver- 

 gence of the small rings 

 is here accompanied by 

 an elongation and partial 

 constriction of the model 



