LIVING SUBSTANCE 23 



contrast to cytoplasm), owing to the fact that it 

 has about the same refractive index as the cyto- 

 plasm, is usually almost or quite invisible in living 

 cells, and must be fixed and stained before it can be 

 easily seen. Its outline is always sharply distinct 

 from the adjacent cytoplasm, and often a limiting 

 membrane seems to be present, though the presence 

 of the latter in all living cells is not definitely estab- 

 lished. 



Within the nucleus the protoplasm is further dif- 

 ferentiated into two substances, one that stains 

 very readily with most dyes, and for that reason is 

 called chromatin, and another (the linin) that stains 

 with great difficulty, and looks like a sort of network 

 or scaffolding supporting the chromatin. Both 

 chromatin and linin are surrounded by a watery 

 transparent fluid sometimes termed hyaloplasm. 

 In many cells, especially egg-cells of animals, a 

 nucleolus is prominent, a rounded aggregation 

 of chromatin material which is found to be chem- 

 ically different from the true chromatin, but the 

 nature and function of which has never been clearly 

 understood. 



The nucleus owes its acid nature to the chromatin, 

 which is largely made up of some form of nucleic 

 acid, a complex substance having a characteristic 

 percentage of phosphorus. When fixed and stained, 

 the chromatin usually appears in the form of gran- 

 ules of either large or minute dimensions, sometimes 

 arranged in the form of a skein or network, with 

 " knots " at the intersections. Only in cells that 



