INTRODUCTION. 25 



can be liberated when required and directed toward the accomplish- 

 ment of some definite purpose. The functions of both groups 

 require an active intracellular metabolism, resulting in the forma- 

 tion of particular chemical substances. In this they are alike. 

 But in the first group the production of those substances is, in 

 itself, the functional purpose of the process, while in the second 

 group those substances are merely a means for holding energy in 

 the latent condition. If we may so express ourselves, the first 

 group utilizes energy for the elaboration of material, the second 

 group elaborates material for the utilization of energy. 



In the adult, under normal conditions, each kind .of cell, if it 

 reproduce at all, gives rise to cells only of its own kind. But when 

 the conditions are morbid, a sort of reversion may take place, the 

 progeny of a given cell then showing less evidence of specialization 

 than the parent cell. Such reverted cells, or their descendants, may 

 never develop into more specialized cells, or they may regain the 

 original degree of specialization possessed by the first cell, or, fin- 

 ally, they may become specialized along some divergent line of devel- 

 opment, giving rise to a tissue that is nearly or remotely akin to 

 that from which they started, according to the degree of reversion 

 which has taken place. The reversion appears never to extend 

 further back than the degree of specialization that is marked by 

 the formation of the three embryonic layers in the history of devel- 

 opment ; for example, epithelium which springs from either the 

 entoderm or ectoderm does not revert to a primitive condition from 

 which it can develop into bone or some other form of connective 

 tissue normally derived from the mesoderm. Examples of rever- 

 sion will be met with in the chapters on Inflammation, Tumors, and 

 Metaplasia. 



