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LECTURES ON BIOLOGY 



When we describe the cells as the building-stones of the 

 organic world we are doing justice only to one side of their 

 nature, for they are not only building-stones, but at the same 



FIG. 22. CELL-FORMS. 



(1) Flagellate cell of a sponge; (2) ganglion-cell from the spinal cord of man; 



(3) epithelium of a flagellate cell from the stomach cavity of a sea-anemone; 



(4) ciliated cell of a mussel; (5) cell from the intestinal epithelium of man; 

 (6) striated muscle-cells from cardiac wall of frog. 



time builders. The growth of each higher multicellular organism 

 depends upon the power of each individual cell to enlarge and 

 multiply, to produce out of itself new building material. It 



