THE LIVING WORLD 195 



of the entire organism whereof such protoplasm forms a 

 part, is augmented and so growth is brought about. 



The small particles of protoplasm which constitute 

 " cells " are far indeed from being structureless. Besides 

 the nucleus already mentioned there is a delicate net- 

 work of threads of a substance called chromatin within 

 it, and another network permeating the fluid of the cell 

 substance which invests the nucleus, often with further 

 FIG. 28. 



C 



CELL FROM A SALAMANDER. 



, nucleus; n', nucleolus embedded in the network of chromatiu 

 threads ; k, network of the cell external to the nucleus ; a, 

 attraction-sphere or archoplasra containing minute bodies called 

 centrosomes ; cl, membrane enclosing the cell externally ; n/, 

 membrane surrounding the nucleus ; c, centrosomes. 



[Drawn by Mr.]. E. S. Moore.'] 



complications. These networks generally perform (or 

 undergo) a most complex series of changes every time a 

 cell spontaneously divides. In certain cases, however, 

 it appears that the nucleus divides into two in a more 

 simple fashion, the rest of the cell contents subsequently 

 dividing each half enclosing one part of the previously 

 divided nucleus. It is by a continued process of cell 

 division that the complex structures of the most com- 

 plex organisms is brought about. 



