28 



INTRODUCTION. 



fastened to the fibers either at the free ends or at the place of bending. 

 Now, in order that the two segments of such a chromosome may come 

 into contact side by side, as is frequently the case, the parallel parts of 

 the loop need only be brought closely together. This may happen 



E 



FIG. ii. Second, or homotypic mitosis, in pollen mother-cells (Lili-utn). 



A, multipolar stage of spindle ; chromatin spirem not completely segmented into chromosomes. 



B, bipolarity established; chromosomes more regularly arranged. 



C, mature spindle ; chromosomes more regularly disposed in equator, placed radially or tangentially 



on spindle. 

 D, metakinesis. E, the anaphase. 



before the spirem is completely segmented, for in many chromosomes 

 the two segments are very closely applied and twisted upon each other 

 before the spindle is mature. In like manner parallel portions of the 

 spirem may come in contact either before or after segmentation. In 

 many other cases, both in the mature spindle and during its develop- 



