196 ELEMENTS OF SCIENCE 



The division of a cell, or particle of protoplasm, is 

 indeed a necessary consequence of its complete nutrition. 



For new material can only be absorbed by its surface. 

 But as the cell grows, the proportion borne by its surface 

 to its mass, continually decreases ; therefore this surface 

 must soon be too small to take in nourishment enough ? 

 and the particle, or cell, must therefore either die or 

 divide. By dividing, its parts can continue the nutritive 

 process till their surface, in turn, becomes insufficient, 

 when they must divide again and so on. Thus the term 

 " feeding" has two senses. " To feed a horse," ordinarily 

 means to give it a certain quantity of hay, oats or what 

 not ; and such indeed is truly one kind of feeding. But 

 obviously, if the nourishment so taken could not get from 

 the stomach and intestine into the ultimate particles 

 and cells of the horse's body, the horse could not be 

 nourished and still less could it grow. It is this latter 

 process, called assimilation, which is the real and essential 

 process of feeding, to which the process ordinarily so 

 called is but introductory. 



Protoplasm has also the power of forming and ejecting 

 from its own substance, other substances which it has 

 made but which are of a different nature to its own. 

 This function, as before said, is termed secretion ; and we 

 know the liver secretes bile and that the cow's udder 

 secretes milk. 



Here again we have an external and an internal 

 process. The milk is drawn forth from a receptacle, the 

 udder, into which it finds its way, and so, in a superficial 

 sense, it may be called an organ of secretion. Neverthe- 

 less the true internal secretion takes place in the inner- 

 most substance of the cells or particles of protoplasm, of 

 the milk-gland, which particles really form that liquid. 



But every living creature consists at first entirely of 



