282 THE RATE OF GROWTH [ch. 



on the other. This is as much as to say that growth, while its own 

 energy tends to increase, leads also, after a while, to the establish- 

 ment of resistances which check its own further increase. 



Our knowledge of the whole complex phenomenon of growth is 

 so scanty that it may seem rash to advance even this tentative 

 suggestion. But yet there are one or two known facts which seem 

 to bear upon the question, and to indicate at least the manner in 

 which a varying resistance to expansion may affect the velocity 

 of growth. For instance, it has been shewn by Frazee* that 

 electrical stimulation of tadpoles, with small current density and 

 low voltage, increases the rate of regenerative growth. As just 

 such an electrification would tend to lower the surface-tension, and 

 accordingly decrease the external resistance, the experiment would 

 seem to support, in some slight degree, the suggestion which I have 

 made. 



To another important aspect of regeneration we can do no more 

 than allude. The Planarian worms rival Hydra itself in their powers 

 of regeneration; and in both cases even small bits of the animal 

 are likely to include endoderm cells capable of intracellular digestion, 

 whereby the fragment is enabled to live and to grow. Now if a 

 Planarian worm be cut in separate pieces and these be suffered to 

 grow and regenerate, they do so in a definite and orderly way ; that 

 part of a shce or fragment which had been nearer tq the original 

 head will develop a head, and a tail will be regenerated at the 

 opposite end of the same fragment, the end which Had been tailward 

 in the beginning; the amputated fragments possess sides and ends, 

 a front end and a hind end, like the entire worm; in short, they 

 retain their polarity. This remarkable discovery is due to Child, 

 who has amplified and extended it in various instructive ways. 

 The existence of two poles, positive and negative, implies a 

 "gradient" between them. It means that one part leads and 

 another follows; that one part is dominant, or prepotent over the 

 rest, whether in regenerative growth or embryonic development. 



We may summarise, as follows, the main results of the foregoing 

 discussion : 



(1) Except in certain minute organisms, whose form (hke that 



* Journ. Ezper. ZooL vii, p. 457, 1909. 



