THE LIFE OF A CELL. 3 



digest ; they grow and multiply ; they move and feel. 

 Their perceptions, indeed, are no doubt confused and 

 undifferentiated, and perhaps devoid of consciousness. 

 The soft protoplasm of which they consist is dimly 

 affected by external stimuli, as, for instance, by the 

 waves of light or of sound. These forms, however, are all 

 minute, and, indeed, almost invisible to the naked eye. 

 The larger animals are built up of a number of cells. 



Let us, then, consider the possible modes in which an 

 organ of sense, say an eye, may have originated. 



In the simpler forms, the whole surface is more or less 

 sensitive. Suppose, however, some solid and opaque 

 particles of pigment deposited in certain cells of the skin 



Fig. 1.— Diagram of skin, c, Cuticle ; h, cellular or hypodermic layer. 



(Fig. 1). Their opacity would arrest and absorb the 

 light, thus increasing its effect, while their solidity would 

 enhance the effect of the external stimulus. A further 



I 



Fig. 2.— Diagram of skin, c, Cuticle ; h, cellular or hypodermic layer. 



step might be a depression in the skin at this point, 

 which would serve somewhat to protect these differen- 

 tiated and more sensitive cells, while the deeper this 

 depression the greater would be the protection. 



The epithelial cells frequently secrete more or less 

 matter, which may form a more or less solid ball. 

 This might be set in vibration by the sound-waves, 

 and would thus increase the effect on the epithelial 



