ELEMENTARY VITAL PHENOMENA 



167 



stances in general, except that sooner or later after their exit 

 from the living cell the substances assume a solid form. The 

 solidifying of the excretions at the surface does not prevent the 

 repetition of the process, and thus eventually all substances of the 

 kind become excreted and solidified upon the outside. Thus 

 originate the cell-membranes of tissue-cells, the cellulose coats of 

 plant-cells, the chitinous coats of insects, and the calcareous shells 

 of Foraminifera. 



This process and at the same time the mode of growth of these 

 superficial structures can be illustrated by an experiment which 



IG. 57. A, Amoeba. A pale contractile vacuole lies in the endoplasm beside the dark nucleus 

 , Paramcecium. At each pole is a star-shaped contractile vacuole ; the upper is in the act 

 of contracting, while the lower is beginning to fill itself from several small drops of liquid 

 that are flowing together. 



was suggested by Traube, and was much discussed in his time. 

 If a drop of a thick solution of gelatine be allowed carefully to fall 

 into a solution of tannin, there appears about the drop a so-called 

 precipitation-membrane of gelatine tannate, since at the surfaces 

 of contact of the gelatine and the tannin the two substances 

 undergo a chemical combination. This precipitation-membrane 

 shows the peculiar phenomena of growth both in surface and in 

 thickness, and on account of its similarity to a living cell Traube's 

 drop of gelatine in the tannin solution has been termed an " arti- 

 ficial cell." Since the gelatine solution attracts water to itself, 

 tannin in solution comes constantly through the membrane to the 



