NATURE OF PLANTS 



49 



activity of the cell and determines the kind of cell that shall be 

 formed and the work that it shall perform. As the nucleus 

 enlarges the chromatin increases in amount and often forms a 

 ribbon-like structure (Fig. 31, B) which finally contracts and 

 divides, thus forming numerous small masses of chromatin, called 

 chromosomes (Fig. 31, C). At this stage in the division delicate 

 colorless strands begin to appear and ultimately grow into a 

 spindle as shown in Fig. 31, D. There are two sets of these 

 strands or fibrillar ; an inner and an outer series. The outer 

 fibrillae, in some way not undersood, arrange the chromosomes in 

 the center or equator of the spindle (Fig. 31, D) where each 

 chromosome divides by a longitudinal division into two equal 

 parts. The fibrillae now separate the two halves of each chromo- 

 some and pull them to the opposite poles of the spindle so that 

 each pole receives a half of every chromosome (Fig. 32, E). 



Fig. 32. Later stages in the division of the cell : E, the fibrillae pulling 

 the separated halves of the chromosomes to the opposite poles of the 

 spindle. The inner series of fibrillae are now seen at the equator between 

 the two groups of chromosomes. F, the chromosomes arranged at the 

 poles and the inner fibrillae increasing in size and number. G, the fibrillae 

 have increased in number until they nearly reach the opposite walls of the 

 mother cell. Their thickening at the equator is the first indication of the 

 wall separating the two new cells. H, position of the new wall clearly 

 indicated. — I. D. Cardiff. 



The chromosomes now become rearranged at the poles and two 

 new nuclei are gradually formed like the original nucleus (Fig. 

 32, F, G). In the meantime the inner fibrillae of the spindle have 

 shortened and become thicker at the equator (Fig. 32, G). This 



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