CHAPTER V 



GROWTH-CENTRES AND GROWTH-GRADIENTS 

 IN ACCRETIONARY GROWTH 



§ i. The Accretionary Method of Growth 



IN the organs we have so far been considering growth is 

 essentially of the compound-interest type. That is to say, 

 the increments of new material produced by growth are 

 alive, and themselves grow and produce new material in their 

 turn, so that growth is a multiplicative process. The rate of 

 growth may, and doubtless does, slow down with increasing 

 size and age, but this merely means that the multiplying factor 

 decreases as some regular function of physiological age. The 

 growth remains of compound-interest type, even if the actual 

 rate of compound interest is never constant but progressively 

 decreases. 



However, there are many other organs whose method of 

 formation is radically different, so that their growth is essen- 

 tially of the simple-interest type. In them, the increments 

 of new material produced by growth are turned into non-living 

 material as soon as formed and remain permanently (or until 

 cast off by ecdysis or other means) in the state in which they 

 were laid down. They do not contribute any further new 

 material, so that growth here is not a multiplicative but an 

 additive process. 



Here again the rate of growth may alter with age, but this 

 only means that the amount of new material added in unit 

 time steadily decreases ; and the growth remains of simple- 

 interest type even though the actual rate of simple interest 

 is continually altering. 



We may accordingly distinguish these two types of growth 

 as the multiplicative, intussusceptive or compound-interest 

 type on the one hand, the additive, accretionary or simple- 

 interest type on the other. 



The most familiar examples of organs growing by the accre- 

 tionary method are shells such as those of molluscs, brachiopods, 



