48 THE OYSTER. 



They are simply minute, definitely limited masses of 

 living matter, with the power to grow when furnished 

 with food ; and after their size has thus increased, they 

 have the power to multiply by splitting up into smaller 

 and more numerous cells, which in their turn grow 

 and multiply in the same way. 



They at first exhibit no traces whatever of the uses 

 to which they are to be put, but as they grow older 

 they gradually become specialized in various direc- 

 tions and are built up into the tissues and organs of 

 the body, losing at the same time their sharp dis- 

 tinctness and fusing with each other. 



Just as certain cells become gland cells, others 

 muscle cells, and so on, certain cells of the adult 

 body become set apart as reproductive cells ; eggs in 

 the female and male cells in the male. 



The egg cells grow until they become very much 

 larger .than any of the ordinary cells of the body ; at 

 the same time their outlines become sharply defined, 

 and they become dark-colored and granular. The 

 granular appearance is due to the fact that as they 

 approach maturity they become filled with food, which 

 is stored away in them as a provision for the time 

 when they are to be cast off from the body of the 

 oyster, to lead an independent existence. 



The male cells are very much smaller than the eggs, 

 they contain little food, and when they are mature 

 each of them is furnished with a long cilium or vibra- 

 ting hair, by means of which the cell is able to swim 



