INSECT'S STOMACH SNODGRASS 387 



rapid. With iniuiy larvae, the cclluhir wall of the stomach is now 

 cast off entire and, a whole new cell layer is generated from the 

 replacement cells. Thus the insect enters its next feeding stage with 

 a fresh secreting layer in its stomach, wdiich proceeds to digest and 

 absorb the cells of the old discarded layer ! Here is an example of 

 economy characteristic of living things, but quite foreign to the 

 physical world. At the final change of the larvae to the adult the 

 stomach wall will be again renewed, after which it generally lasts 

 until death, though certain beetles are said to continue periodically 

 the shedding and renewal of the stomach epithelium as long as they 

 live, regardless of the fact that they do not moult again. 



In retrospect several points of interest may be emphasized. 

 The first is that the vital processes of living things depend upon 

 fundamental properties of inanimate matter. Second, the property 

 of protoplasmic matter that we term life is a chemical mechanism, 

 called metabolism, for the continual liberation of energy. Third, the 

 structural organizations of animals are mechanical devices for giving 

 the vital process optimum working conditions, and for utilizing the 

 energy released by the body cells. Fourth, food is material that the 

 body cells can use for energy production and for growth ; digestion 

 is the rendering of raw lood stuffs of nature into a form soluble in 

 water and suitable for cell food ; the stomach is an outside part of tlie 

 body turned in to make a pocket in which digestion takes place. 

 Fifth, the general sequence of developmental processes by which an 

 individual animal grows from the egg to the adult is determined by 

 the course of the evolutionary history of the race. Sixth, the details 

 or actual mode of develoj^iment does not of necessity repeat literally 

 the ancestral history of the species, since the young animal, whether 

 the embryo in the Qgg or the free-living larva, must adapt its 

 inheritance to its own v»ay of living. The truth of this is particularly 

 evident in the developmental history of the insect's stomach. 



72774—35 20 



