THE SARCODINA 161 



more common relationship. Food bodies like the elongated fila- 

 ments of certain green plants (Fig. 88 C) may be included in the 

 endoplasm without the surrounding mass of water. Active prey, 

 like another protozoan, is usually enclosed in a conspicuous vacu- 

 ole, formed either during ingestion by the inclusion of external 

 water or by the secretion of fluid into the vacuole after the capture 

 (Fig. 88 A and B). 



Digestion occurs within the food vacuole as is indicated by the 

 disintegration of the soft parts of the ingested food. The case of 

 green plants, which undergo color changes similar to those observed 

 in masses of such material when subjected to digestion in a test- 

 tube, is instructive. Since enzymes are necessary for digestion 

 wherever it can be studied on a large enough scale, it may be 

 inferred that enzymes are secreted into the digestive vacuoles 

 from the surrounding cytoplasm. It is claimed that, in some pro- 

 tozoa, bodies, like the secretory granules of the gland cells in a 

 vertebrate animal, surround the vacuole at a certain stage and dis- 

 appear as they are converted into the enzyme which enters the 

 vacuole. 



After digestion, the food vacuole undergoes a shrinkage in bulk 

 which is supposed to indicate the passage into the cytoplasm of the 

 products of digestion. This corresponds to the assimilation of 

 nutrient material by the cells of a higher animal. In such a man- 

 ner, nutrients, consisting certainly of proteins, and perhaps of 

 fats and sugars, but apparently not including starch, are digested 

 and become incorporated into the protoplasm of the protozoan cell. 

 The non-nutrient portions of the food, such as the siliceous skele- 

 tons of the green plants known as diatoms, and the shells of animals, 

 are egested when the food vacuole, with what remains of its original 

 contents, comes to the surface and breaks through the ectoplasm, 

 so that the amoeba flows away and leaves the contents of the vacu- 

 ole behind. In some cases the indigestible contents of several 

 vacuoles unite and are egested together. 



As with the vertebrates, the more important story is that of 

 the nutrients that become incorporated into the protoplasm upon 

 their passage outward from the food vacuoles. It is supposed that 

 in the single-celled amoeba, just as in each cell of a many-celled 

 organism like the human being, metabolic changes are constantly 

 taking place. Like other cells, the amoeba receives nutrients, 

 incorj)orates these into its protoplasm by a process of assimilation, 



