THE CELL. 37 



STRUCTURE OF THE CELL. 



The parts of a cell are the cell-wall, protoplasm, controspheres, 

 nucleus, and vacuoles. 



(1) The cell- wall. This is the thin membrane inclosing the 

 protoplasm. With animals, the cell-wall consists of protein; with 

 plants, it is composed of cellulose. Protein is a compound possess- 

 ing the same elements as starch, but with nitrogen added. Cellu- 

 lose, C 6 H 10 O 6 , is an isomeric form of starch, having the same com- 

 position, but differing from it in being homogeneous in structure, 

 not granular, and in being less easily dissolved and less readily con- 

 verted into dextrin. 



(2) Protoplasm, This is the living substance of the cell, the or- 

 ganic basis of life. It contains carbon, hydrogen, oxygen, nitrogen, 

 sulphur, and sometimes phosphorus, iron, and other elements. No 

 chemical formula can be given for its composition, but it consists 

 of unstable, constantly changing molecules. When dead, its chem- 

 ical nature is changed, and it consists of protein, f carbo-hydrates, 

 water, and mineral salts. Of these, protein alone possesses nitrogen. 

 Living protoplasm is irritable, unstable, deoxidizing ; has the power 

 to eliminate carbon di-oxide, and can reproduce itself, and, by as- 

 similation, manufacture the innumerable products characteristic of 

 plants and animals. The protoplasm of adjacent cells is sometimes 

 connected by delicate threads, which pass through their walls. 

 Protoplasm is a viscid, transparent substance resembling egg-al- 

 bumen. It is never completely fluid. It is not homogeneous, but 

 somewhat complicated in structure. It consists of two parts, the 

 cytoplasm and the nucleoplasm. The cytoplasm constitutes the 

 bulk of the protoplasm in the cell. It consists of an outer, dense 

 film, the ectoplasm, and an inner, semi-liquid portion, the ento* 

 plasm, containing a fibrous sponge-work holding in its threads the 

 microsomes, centrosplieres, and nucleus. Microsomes are minute 

 spherical masses supposed to contain nutrition for the growing cell, 

 but their real functions are not well understood. The neucleoplasm 

 enters into the structure of the nucleus and nucleolus. 



(3) The centrospheres. --These are minute bodies associated 

 with the nucleus and scarcely larger than the microsomes. Each 

 centrosphere consists of an outer, hyaline film of cytoplasm, within 



