2 THE CELL 



The essential parts of the cell are, (i) the cytoplasm, 

 which is, a special name given to the protoplasm forming the 

 cell-body and (2) the nucleus, which is a small round or 

 oval body embedded in the cytoplasm. A great many cells 

 are surrounded by- a cell-wall or cell-membrane, but this 

 c;aaiftot be. regarded ;as one of the essential elements since all 

 cells do not possess such membranes. 



-,*(i)' The Cytoplasm. This is a gelatinous or semi-fluid, 

 granular substance, transparent and generally colorless. 

 Chemically it consists of water and salts, together with vari- 

 ous organic substances, called proteids, which are complex 

 combinations of carbon, hydrogen, oxygen and nitrogen, and 

 sometimes phosphorus and sulphur. The proteids of the cy- 

 toplasm contain little phosphorus, while those of the nucleus 

 are rich in it. 



The cytoplasm does not always present the same structural 

 appearance since its constituents vary in their condition and 

 arrangement. In some cells it has a clear homogeneous ap- 

 pearance, while in others it contains fine spherical particles 

 which give it a granular structure. When these granules are 

 large and clear, and are surrounded by denser areas they give 

 to the cytoplasm an alveolar outline. But most frequently 

 the cytoplasm contains in its structure a mesh work of threads 

 or fibrils which give it a reticular appearance. This network 

 of fibrils is called the spongioplasm which encloses a less firm 

 portion, the hyaloplasm ( Fig. i ) . 



However,, in all these varieties, the cytoplasm has both an 

 active and a passive structure. In young granular cells the 

 active substance is represented by small spherical particles, 

 called microsomes (Fig. i). These are not always evenly dis- 

 tributed throughout the cytoplasm, but are grouped in an area 

 near the nucleus, while the area next the cell-wall is almost 

 free from granules. The dense inner area is called the endo- 

 plasm, while the clear outer area is called the exoplasm 



(Fig. i). 



(2) The nucleus, which is the second essential part of a 



