THE BEHAVIOR OF CHROMATIN 



287 



tant (Fig. 68). The fibers of the net become dissociated 

 from one another in such a way that long threads are 

 formed. Sometimes one continuous thread appears; at 

 others a number of separate threads are formed. They 

 gradually elongate and become slender and uniform in 

 diameter. Then a phase begins during which the thread 

 shortens and thickens. If there is a single thread it now 

 breaks into a certain definite number of separate seg- 



chr w n.m 



Fig. 68. — Selected Stages in Cell Division of Cells of the Root 

 Tip of the Spiderwort. 1. The chromatin has organized itself into 

 a long slender thread or spireme and the cytoplasm begins to show 

 indications of the organization of the spindle. 2. The spireme has 

 broken into fragments, the chromosomes, each of which is split 

 lengthwise and arranged at the middle of the spindle where the 

 two exactly equivalent halves are being separated. 3. The halves 

 are completely separate and moving toward their respective poles 

 of the spindle. 4. The chromosomes have arrived at the poles and 

 in a close mass but have not yet been surrounded 



by a nuclear membrane. 5. The nuclear membrane and nuclear 

 sap have formed and the chromosomes are beginning to undergo 

 a process of vacuolation which will- eventually reduce each one to a 

 state more or less resembling a very much broken rubber sponge, 

 p.c, polar cap of cytoplasm out of which the spindle fibers are organ- 

 ized; n.m., nuclear membrane; s, spireme; n', nucleolus; sp, spindle 

 fibers; p, pole of spindle; chr, chromosomes. 



ments called chromosomes. This number is fixed and 

 constant for the cells of any particular plant or animal. 



