LIVING SUBSTANCE 



83 



mfusorian cells, and others, not rarely show in their body- 

 contents small Algm, Bacteria, and Infusoria, which they have 

 taken up from the outside (Fig. 25, i), and which sometimes are 

 scarcely to be distinguished from other solid constituents of the 

 protoplasm. These food-organisms become gradually digested and 

 disappear. 



There appear also frequently in the cell-body as products of 

 digestion, both in the cells that ingest solid, and those that ingest 

 only liquid food, definite granules, usually roundish, and varying 

 greatly in nature (Figs. 7 and 22, 1), which Altmann has grouped 

 m part under the name granula, and which, as has already been 



FiG.25.-/, Frog's leucocytes, or white blood-cells, each containing a bacterium. (After Metschnikoff.) 

 //, A plant-ceU containing starch-grains, HI, Starch-grains isolated — a, from the potato ; b, 

 from corn ; c, from the pea. 



seen, he regards as elementary organisms, the ultimate living 

 elements of the cell. The composition and significance of most of 

 these metabolic products of living substance which in the form of 

 granules help to constitute the protoplasmic body, is not yet 

 known. But some are characterised very exactly and are easily 

 recognised, such as the concentrically stratified starch-granules in 

 plant-cells (Fig. 25, // and III), the fat-droplets in the cells of 

 the lacteal glands, the glycogen-gramdes in liver-cells, the •pigment- 

 granules in the pigment-cells of the skin of many coloured animals 

 (Fig. 17, d), the aleurone-grains, consisting of proteid, in the cells 

 of sprouting plant-seeds, the crystals of calcium oxalate in plant- 

 cells, of calciicm guanin in pigment-cells, and many others, special 

 mention of which would lead us too far. 



G 2 



