INGESTION AND DEFECATION 69 



smaller than the amoeba, for instance a unicellular 

 green plant (alga) or a colony of bacteria, the proto- 

 plasm flows round it or draws it into the body, so as 

 completely to enclose the prey. With the prey a drop 

 of water may be ingested, forming a food vacuole. The 

 food is then acted upon by enzymes formed by the 

 amoeba's protoplasm, and the digestible parts digested, 

 i.e. broken down into soluble organic substances, 

 probably proteins, carbohydrates and fats, which can 

 be assimilated by the amoeba, i.e. built up again into 

 new " amoeba-protoplasm," while the indigestible parts 

 are ejected from the body as faces. This process of 

 defalcation is sometimes loosely called " excretion," 

 but it must be carefully distinguished from the excretion 

 of urea and carbon dioxide which are formed by the 

 chemical breaking down of the proteins and carbo- 

 hydrates of the body The ingestion of the prey when 

 the pseudopodium has once come into contact with it, 

 and also the ejection of faeces, can be explained by the 

 purely physical force of surface tension, for it has been 

 shown that a drop of chloroform (representing the 

 amceba) placed in contact with a minute fibre of glass 

 covered with a varnish of shellac will ingest the fibre, 

 remove the coating, and then eject the fibre (see Fig. 2). 

 A drop of chloroform will also coil up a flexible fibre 

 of shellac within itself just as an amceba will coil up 

 a flexible algal thread (Fig. 3). 



By the formation of new protoplasm the amceba 

 naturally adds to its bulk and thus grows in size, unless 

 of course the breaking down processes (katabolism) 

 which result in excretion of the katabolites balance or 

 exceed the building up or assimilative processes. 



Amceba is also sensitive to external stimuli, i.e. its 

 motion is directed in relation to such stimuli. For 



