PROTOPLASM 



309 



fluid, and (2) a network of a more viscid and more highly 

 refractive and sometimes granular substance. This network 

 may be either the sectional view of a sponge-like structure or 

 of an emulsiform structure or of true fibres interlaced. The 

 granules, which are called microsomes, vary greatly in size and 

 number and are frequently absent or too small to be seen. They 

 are often less conspicuous in the peripheral layers of the cyto- 

 plasm, which is therefore distinguished as ectoplasm. 



(Attraction-sphere enclosing two centrosomes.) 



Plastids lying 

 in the cyto- 

 plasm 



Vacuole 



Passive bodies 

 (metaplasmor 

 paraplasm) 

 suspended in 

 the cyto- 

 plasmic mesh- 

 work 



FIG. 179. Diagram of a cell. (From Hegner's Zoology, after Wilson, published 

 by the Macmillan Co.) 



Beside these constant constituent elements of the cytoplasm 

 there are also a large number of structures, which occur only 

 in certain cells or at certain times. Among these may be men- 

 tioned here the chromoplasts and amyloplasts, the vacuoles 

 filled with cell sap, the ingested food particles, the reserve elabor- 

 ated food substances, such as starch, oil, aleurone, and the se- 



