NATURE OF CELLS 



17 



are made up of an aggregation of cells, — nucleated masses 

 of protoplasm. In the many-celled animals, the cells are 

 of various shapes and have various functions to perform. 

 Some cells are cyUndrical, soure spherical, some flat and 

 scaleUke, some Hke a cube, some Like a pyramid, and some 



Fig. 6. — Stages in the division of an amoeba. After Schultze. 



greatly elongated and spindle-shaped. Some cells perform 

 the function of motion, e.g. those of the muscles; others 

 carry impulses, e.g. the nerve cells; others secrete digestive 

 fluids, e.g. those of the salivary glands. To sum up, then, 

 a living animal cell may he defined as a mass of living jjro- 

 toplasm with a mtcleus and usually without a definite wall. 



hekeick's zool. — 2 



