THE CELL AND ITS STRUCTURE. 



SO large that the middle parts would get no oxygen 

 and no food, on account of being so far from th& 

 surface. Division begins ia the nucleus. This goes 

 through a remarkable cycle of changes, to which has- 

 been given the name of karyokinesis, which means, 

 literally, tlie actiuity of the nucleus. As has jast been 

 stated, the substance of the nucleus is seen, when 

 highly magnified, to consist of a network of fibres 

 that stain dark in a material that stains more lightly ; 

 these are therefore called respectively chromatin and 

 achromatin. When the nucleus is about to divide. 



Fig e — Saccessive stages of the division of the nucleus, showing ^spmdle- 

 shaped mass of achromatin. and dark masses of chromatin. 



the chromatin forms a wreath at the outside of it, 

 and then the loops of the wreath sharpen into points, 

 so that it becomes a star, while the achromatin forms 

 a long spindle-shaped mass which looks as if it were 

 threaded through the wreath. The next change is 

 that the chromatin star splits in half, and the halves 

 travel towards the ends of the cell. Then the twO' 

 new stars go back to the wreath form, and mean- 

 while the protoplasm of the cell is dividing across. 

 When the halves are severed the new nuclei assume 

 the usual shape. 



