106 CELL-DIVISION 



fibres are always contorted during the metaphases, as if pushing 

 against a resistance ; and it harmonizes with the facts observed in 

 the mitoses of infusorian nuclei, where no asters are present. This 

 view has been accepted, with slight modifications, by Flemming, 

 Boveri, Meves, Kostanecki, and also by Heidenhain. A nearly 

 decisive argument in its favour is given by such cases as the polar 

 bodies, or the mitosis of salamander spermatocytes as described by 

 Meves ('96, '97, 3), where the spindle-poles are pushed out to the 

 periphery of the cell, the polar astral rays meanwhile nearly or quite 

 disappearing (Fig. 130). This not only strongly indicates the push 

 of the central spindle, but also shows that the assumption of a pull 

 by the polar rays is superfluous. But beyond this both Driiner and 

 Meves have brought arguments against contractility in the other 

 astral rays, endeavouring to show that these, like the spindle-fibres, 

 are actively elongating elements, and that (Meves, '97, 3) the actual 

 grouping of the rays during the anaphases is such as to suggest that 

 even the division of the cell-body may be thus caused. A pushing 

 function of the astral rays is also indicated by infolding of the nuclear 

 membrane caused by the development of the aster as described by 

 Platner, Watase, Braus, Griffin, and others. 1 The contraction-hypothe- 

 sis is thus restricted by Driiner and Meves to the mantle-fibres 

 alone, though many others, among them Flemming and Kostanecki, 

 still accept the contractility of the astral rays. 



(U) Other Facts and Theories. Even in the restricted form indi- 

 cated above the contraction-hypothesis encounters serious difficulties, 

 one of which is the fact urged by me in an earlier paper ('95), and 

 subsequently by Richard Hertwig ('98), that in the eggs of echino- 

 derms and many other dividing cells the daughter-chromosome 

 plates, extending through the whole substance of the spindle, 

 wander to the extreme ends of the spindle a process which 

 demands a contraction of the fibres almost to the vanishing point, 

 while in point of fact not even a shortening and thickening of 

 the fibres can be seen (Fig. 52). Moreover, in these cases, no 

 distinction can be seen between central spindle-fibres and mantle- 

 fibres, and we can only save the contraction-hypothesis by the 

 improbable assumption that fibres indistinguishably mingled, and 

 having the same mode of origin, structure, and staining-reaction, have 

 exactly opposite functions. The inadequacy of the general theory 

 is sufficiently apparent from the fact that in amitosis cells many 



1 Cf. p. 68. It should be pointed out that the originator of the pushing theory was 

 Watase ('93), who ingeniously developed an hypothesis exactly the opposite of Van Bene- 

 den's, assuming both astral rays and spindle-fibres to be actively elongating fibres, dove-tailing 

 in the spindle-region, and pushing the chromosomes apart. This hypothesis is, I believe, in- 

 consistent with the phenomena observed in multiple asters and elsewhere, yet it probably 

 contains a nucleus of truth that forms the basis of Druner's conception of the central spindle. 



