52 



FORMATION OF NEW CELLS 



minute particles (chromatin) which are associated with the Hnin 

 threads. It has been supposed that this substance controls to a 

 considerable extent the activity of the cell and determines the 

 kind of cell that shall be formed and the work that it shall per- 

 form. 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 



Fig. 31. Cell division in root of corn: A, cell with nucleus enlarging pre- 

 liminary to division — /, linin; ch, chromatin. The central dark body is the 

 nucleolus. B, later stage, the chromatin has increased and appears as a ribbon- 

 like skein. C, formation of the chromosomes, cr. D, formation of the spindle 

 and the arrangement of the chromosomes in the center of the spindle. — 

 I. D. Cardiff. 



division delicate colorless strands begin to appear and ultimately 

 grow into a spindle as shown in Fig. 31 , 1?. There are two sets of 

 these strands or fibrillae; an inner and an outer series. The outer 

 fibrillse, in some way not understood, 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 fibrillse 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). 

 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 fibrillar of the spindle have 

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



