i6 



INTRODUCTION. 



The bipolarity of the multipolar spindle now gradually manifests 

 itself, and the multipolar structure rapidly becomes a typical bipolar 

 spindle in which the chromosomes are arranged in the equatorial plate. 



FIG. 7. Heterotypic mitosis in pollen mother-cell (L. martagon}. Development of spindle continued. 



E, the weft of spindle fibers forms a multipolar complex. 



F, a multipolar complex in which bipolarity has begun to manifest itself; the weaker poles seem to be 



drawn in or together. 



G, bipolarity is established and chromosomes more regularly arranged in equator. 



H, mature spindle, showing only 3 of the 12 chromosomes; chromosomes fastened endwise to spindle. 



This transformation is probably brought about by certain of the larger 

 poles converging toward a common area or point, while others are 

 drawn in (Fig. 7, G). The mature spindle is either truncated at the 

 poles (sometimes broadly so) or pointed, and the chromosomes are 



