INSECT'S STOMACH — SNODGRASS S65 



structure such that the protophisuiic molecules cannot escape 

 through it. The term " cell ", therefore, might be supposed to mean 

 one of the tiny capsules in which the substance of life is imprisoned, 

 but in the biological sense it refers to the protoplasmic mass itself 

 and its retaining wall. The mixture of things that constitutes proto- 

 plasm has a physical state in solution of the kind that chemists term 

 colloidal. The component molecules are of relatively large size and 

 have a complex structure. Diagrams of these molecules found in 

 works on chemistry are often very intricate, but it is probable that 

 the living molecules are still more intricate, because the diagrams are 

 based necessarily on a study of dead protoplasm — the very methods 

 of analysis being fatal to live matter, and it seems certain that live 

 protoplasm must be different from dead protoplasm. 



The cell wall, or cell membrane, as the cell covering is called, is a 

 most important thing for the life of the cell, and it is in itself a 

 highly remarkable tissue, though its structure is not yet fully known. 

 Each cell must have a covering capable of retaining its own proto- 

 plasmic substance, but it must discharge its waste products and 

 receive its food materials through this same wall. The cell wall, 

 therefore, must be of such a nature that colloids will not pass 

 through it, while at the same time it must be freely permeable in 

 both directions to water and to solutions of chemical substances 

 composed of relatively small or partially disintegrated molecules. 

 A thin tissue having this property of being impervious to some 

 things and freely penetrable by others is called a semipermedble 

 inenibrane. The cell wall resembles a semipermeable membrane, but 

 it has certain properties that no other natural or artificially pre- 

 pared membrane possesses. 



The food material that an animal takes into its stomach must con- 

 tain substances from which cell food can be prepared. Most ani- 

 mals consume much material that is useless from a nutritional stand- 

 point, but they have so adapted themselves to the form of food- 

 stuffs in nature that their organization does not function properly 

 without the nonnutrient parts of their ordinary diet. Hence, animals 

 have instincts for eating certain things, and few of them can be 

 made to eat things that their instincts refuse, even though the offer- 

 ing may contain all the nutritive elements needed to supply the 

 wants of their colls. This generalization perhaps does not apply to 

 the young of the human species, or if it does, dietitians do not believe 

 in instinct, and parents have another word for it. 



When it comes to feeding by the cells themselves, there is no ques- 

 tion of instinct, taste, selectivity, perversity, or any other quality that 

 might decide what an animal will or will not take voluntarily into 

 its stomach. The only determining factor here is the physical per- 



