INSECT'S STOMACH SNODGRASS 



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cell wall, which then closes over it (C). The food material is 

 thus ingested in its natural state. Once inside the body of the cell 

 it is there subjected to the processes of digestion. First the food 

 mass is surrounded by a fjlobule of water (fig. 3 A, a) ; then there 

 are discharged upon it from the cell protoplasm substances called 

 enzymes (&), which have the power of so altering the chemical 

 composition of at least parts of the food material that the latter 

 will be dissolved in the water globule; and finally the products of 

 digestion are absorbed into the body of the cell (c) and thus become 

 available as food to the cell tissue. Digestion of food material 

 inside a cell is intracellular digestion. The amoeba eliminates the 

 unused residue of its food by reversing the process of ingestion, 

 that is, it just moves on and leaves the refuse behind. 



A B ^^ 



FiouEE 3. — Diagrams of Intracellular digestion (A), and extracellular digestion (B). 



It is evident, now, that the method of feeding practiced by the 

 amoeba would have disadvantages for a many-celled animal com- 

 posed of cells adherent in masses and having little or no power of 

 individual activity. But, since intracellular digestion is possible, 

 there can be also extracellular digestion (fig. 3 B), provided only 

 that the cell w'all is permeable to a water solution {a) of enzymes 

 (b) formed within the cell. If digestion takes place outside tlie 

 cell, however, the products of digestion must be ingested by the 

 cell (c) through the cell wall, and this process demands that the 

 cell wall be permeable also to the fairly complex molecules of the 

 digested food substances, and still be impervious to the colloidal 

 molecules of the cell protoplasm itself. The development of extra- 

 cellular digestion with subsequent ingestion, therefore, has involved 

 a high degree of perfection in semipermeable membranes. Extra- 

 cellular digestion, it will be seen, includes three essentials: (1) It 

 must change insoluble parts of the natural food material into a form 

 that is soluble in water; (2) the substances rendered soluble must also 

 be in a molecular form that will pass through the cell walls; (3) the 



