26 INTRODUCTION. 



Losing their identity in a multitude of descendants that are subse- 

 quently destroyed in the performance of some useful function. 

 Finally, some of the embryonic cells form structures of only tran- 

 Bient utility — e. //., the thymus and Wolffian body — which partially 

 disappear as the individual becomes older. It follows, then, that we 

 must regard the tissues of the body as more or less plastic and capa- 

 ble of some degree of adaptation to the conditions presented by the 

 whole organism. 



