GROWTH 13 



meates it. It has been proved by experiment that proteids 

 are the only class of food which Amoeba can make use of : 

 it is unable to 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 re-arranged 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 neces- 

 sary 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 to 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 colour- 



