I GROWTH 13 



digest either starch or fat two very important constituents 

 of the food of the higher animals. Mineral matters must, 

 however, be taken with the food in the form of a weak 

 watery solution, since the water in which the animalcule 

 lives is never absolutely pure. 



The Amoeba being thus permeated, as it were, with a 

 nutrient solution, a very important process takes place. The 

 elements of the solution, hitherto arranged in the form of 

 peptones, mineral salts, and water, become rearranged in 

 such a way as to form new particles of living protoplasm, 

 which are deposited among the pre-existing particles. In a 

 word, the food is assimilated or converted into the actual 

 living substance of the Amoeba. 



One effect of this formation of new protoplasm is obvious : 

 if nothing happens to counteract it, the Amoeba must grow, 

 the increase in size being brought about in much the same 

 way as that of a heap of stones would be by continually 

 thrusting new pebbles into the interior of the heap. This 

 mode of growth by the interposition of new particles among 

 old ones is called growth by intussusception, and is very 

 characteristic of the growth of protoplasm. It is necessary 

 to distinguish it, because there is another mode of growth 

 which is characteristic of minerals and occurs also in some 

 organized structures. A crystal of alum, for instance, 

 suspended in a strong solution of the same substance, grows ; 

 but the increase is due to the deposition of successive layers 

 on the surface of the original crystal, in much the same way 

 as a candle might be made to grow by repeatedly dipping it 

 into melted grease. This can be proved by colouring the 

 crystal with logwood or some other dye before suspending 

 it, when a gradually-increasing colourless layer will be 

 deposited round the coloured crystal : if growth took place 

 by intussusception we should have a gradual weakening 



