2 THE CELL 



The essential parts of the cell are, (i) the cystoplasm, whicn 

 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 

 oy a' cell-wall or cell- membrane, but this cannot be regarded as 

 one of- the essential elements since all cells do not possess such 

 rnem'jr&nes. 



(1) The Cytoplasm. This is a gelatinous or semi-fluid, 

 granular substance, transparent and generally colorless. Chem- 

 ically it consists of water and salts, together with various organic 

 substances, called proteids, which are complex combinations 

 of carbon, hydrogen, oxygen and nitrogen, and sometimes 

 phosphorus and sulphur. The proteids of the cytoplasm con- 

 tain 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 appear- 

 ance, 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 cyto- 

 plasm contains in its structure a meshwork 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 endoplasm, 

 while the clear outer area is called the exoplasm (Fig. i). 



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



