iv] NATURE OF THE SPINDLE 51 



differing from that composing the rest of the cell, while in 

 others considerable differentiation takes place, giving rise 

 to dense, deeply stained threads which seem to support the 

 idea of a special archoplasm. This intermediate view is 

 supported by the fact that during the later stages of mitosis, 

 when the astral rays and spindle-fibres still persist, the 

 centrioles may divide and begin to form new asters around 

 them in preparation for the next division, which are quite 

 distinct from the persistent rays of the division that is still 

 in progress. Such a formation of new rays is not consistent 

 with the view that the rays and fibres are mere re-arrange- 

 ments of a fibrous structure already existing, but at the 

 same time it is hard to reconcile with the archoplasm 

 hypothesis in its original form, for if all or nearly all of the 

 archoplasm were used up in the old system of rays, it is 

 difficult to understand from what source the new rays are 

 derived. If, however, the rays and fibres are formed as a 

 temporary modification of the cytoplasm by the action of 

 some force acting in the centrosomes, the difficulty would 

 seenrto be overcome. 



The two views so far considered agree in regarding the 

 spindle and astral rays as consisting of fibres, probably of a 

 viscid or semifluid consistency, and in supposing that when 

 the spindle is fully formed, its fibres, or at least the outer 

 "mantle-fibres," are attached to the chromosomes, and 

 draw the halves apart towards the two poles by their con- 

 tractility when the chromosomes divide. Sharply contrasted 

 with this view is the hypothesis of which HARTOG is the chief 

 exponent, which regards the whole structure as due to the 

 production of lines of force radiating from the centrosomes. 

 HARTOG speaks of the rays and spindle-fibres as "chains 

 of force," that is to say, chains of particles drawn into a 

 linear arrangement by an attractive force just as iron-filings 



42 



