204 



THE STUDY OF ANIMAL LIFE CHAP. 



the cell-substance or cytoplasm has in most cases a 

 definite structure, a microscopically minute organisation. 

 It is very different from white of egg, which is homo- 

 geneous and structureless. Sometimes it appears like a 

 network, sometimes like a fine emulsion ; and it may 

 exhibit a visibly different structure in different kinds of 

 organisms. Often it is possible, especially by the use of 

 staining and fixing re-agents, to distinguish a relatively 



FIG. 55. ADJACENT ANIMAL CELLS SHOWING THE NUCLEUS, THE PROTO- 

 PLASMIC NETWORK, AND THE BRIDGES VITALLY UNITING CELL AND 

 CELL ACROSS THE INTERVENING INTER-CELLULAR SUBSTANCE. 



(From the Evolution of Sex ; after Pfitzner.) 



more stable framework and a relatively more changeful 

 or labile material within the meshes. 



4. Organs and Organelles. In unicellular animals 

 there is often much structural complexity. There may 

 be locomotor cilia or flagella, offensive threads or tricho- 

 cysts, contractile fibrils, adhesive suckers, pulsating 

 vacuoles, and so forth, and it is convenient to have some 

 term for these, since they are not organs in the sense 



