146 ORIGIN AND NATURE OF LIFE 



which unicellular organisms often form their 

 external envelopes, and of the manner in 

 which cells provide themselves with limiting 

 membranes or cell-walls. 



The chief colloid of the milk, on account 

 of its affinities, accumulates on the surface, 

 the accumulation gives increased concentra- 

 tion, the pressure of the increased concen- 

 tration causes the multi-molecules to build 

 together, the larger molecules fall out of 

 solution as particles, and these join to form 

 a close network or film. 



Nor is it only film formation and delimita- 

 tion of the constituent cells of an organism 

 from one another that is served by this 

 property of colloids of concentrating at an 

 interface. Such interfaces occur on the 

 surfaces of the minute microscopic granules 

 which are found in all living cells. Here 

 also there concentrate not only ordinary 

 colloids present in solution within the cell, 

 but bodies formed by the cell called ferments 

 or enzymes, themselves also colloids. 



This concentration within the cell on sur- 

 faces favours union, and it is in this way, by 

 continued aggregation, that starch granules, 

 protein granules, and drops of fat or oil, 

 grow within living cells. 



It is a wonderful adaptation of nature that 



