THE MECHAXISM OF MITOSIS IO5 



in the equatorial plane ; and if, finally, the hinge-connection be re- 

 moved, each half of the ring closes to form a complete ring.^ 



Heidenhain has fully worked out a theory of mitosis based upon 

 the analogy of these pretty models. The astral rays of the cell 

 ("organic radii") are assumed to be in like manner of equal length 

 and in a state of equal tonic contraction or tension, the centrosome 

 forming the common insertion-point of the rays, and equilibrium of 

 the system being maintained by turgor of the cell. Upon disappear- 

 ance of the nuclear membrane and division of this insertion-point, the 

 tension of the rays causes divergence of the centrosomes and forma- 

 tion of the spindle between them, and by further contraction of the 

 rays both the divergence of the daughter-chromosomes and the division 

 of the cell-body are caused. A new condition of equilibrium is thus 

 established in each daughter-cell until again disturbed by division of 

 the centrosome.2 In some cases (leucocytes) the organic radii are 

 visible at all periods. More commonly they are lost to view by 

 breaking up into the cell-reticulum, without, however, losing their 

 essential relations. 



No one who witnesses the operation of Heidenhain's models can 

 fail to be impressed with its striking simulation of actual cell-division. 

 Closer study of the facts shows, however, that the contraction-hypothe- 

 sis must be considerably restricted, as has been done by the successive 

 modifications of Hermann ('91), Druner('95), and others. Hermann, 

 to whom the identification of the central spindle is due, pointed out 

 that there is no evidence of contractility in the central spindle-fibres, 

 which elongate instead of shorten during mitosis ; and he concluded 

 that these fibres are non-contractile supporting elements, which form 

 a basis on which the movements of the chromosomes take place. The 

 niantlc-fibrcs are the only contractile elements in the spindle, and it 

 is by them that the chromosomes are brought into position about the 

 central spindle and the daughter-chromosomes are dragged apart.^ 

 Drijner ('95) still further restricts the hypothesis, maintaining that the 

 progressive divergence of the spindle-poles is caused not by contrac- 

 tion of the astral rays ("polar fibres"), as assumed by Heidenhain 

 (following Van Beneden and Boveri), but by an active growth or 

 elongation of the central spindle, which goes on throughout the 

 whole period from the earliest prophases until the close of the ana- 

 phases. This view is supported by the fact that the central spindle- 



1 In a modification of the apparatus devised by Rhumbler ('97), the same effect is pro- 

 duced without the hinges. 



- Cf. p. 57. For critique of this hypothesis, see Fick ('97), Khumliler ('96, '97), and 

 Meves ('97, 4). 



3 Belajef? ('94) and Strasburger ('95) have accepted a similar view as applied to mitosis 

 in plant-cells. 



