THE ACHROMATIC FIGURE 



23 



single resting nucleus again, and there is no cell division. Thus nuclei 

 are produced with three or four times the normal number of chromosomes. 

 The mechanism by which the achromatic figure brings about the 

 separation of the daughter chromosomes and subsequently cell division 

 is still imperfectly understood. Two main theories are held : one that 

 the " fibres " of the astral rays and spindle figure are actually what 

 they appear to be, namely, fibres or threads, and the other that they are 

 merely lines of force or stress. The first and simplest form of the fibrillar 

 theory supposed that the mantle fibres are contractile, comparable to 

 muscle fibres, inserted at one end into the centrosome and at the other 

 into the chromosomes. The centrosome being held in place by the 

 astral rays, contraction of the mantle fibres pulls the chromosomes 



B 



Fig. II. 



Early and late anaphase in the formation of the first polar body in Echinus esctiUfitus. 



(Bryce, Q.J. M.S., 1903.) 



towards the centrosome. This simple theory, however, is met by in- 

 superable difficulties. One of these is that in telophase of most mitoses 

 the chromosomes come very close indeed up to the centrosome, thus 

 demanding an apparently impossible amount of contraction on the part 

 of the fibres, while such a contraction would of necessity be accompanied 

 by a relatively enormous thickening of the fibres, which, however, is not 

 observed. Again, in the formation of the polar bodies during the matura- 

 tion of the egg, the spindle fibres appear to exert a pushing rather than 

 —or at any rate in addition to — a pulling action in bringing about their 

 extrusion from the surface of the egg (Fig. ii). These and many other 

 considerations have led to the further hypothesis that the separation 

 of the daughter chromosomes is aided by an elongation of the fibres 

 which connect the separating daughter chromosomes, together with 

 those of the central spindle, which pushes the centrosomes apart. How- 



