Ill] CONCLUSION AND SUMMARY 153 



the rapid formation of a comparatively firm skin; and that the 

 dwindUng of velocities, or the negative acceleration, which follows, 

 is the resultant or composite effect of waning forces of growth on 

 the one hand, and increasing superficial resistance 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 establishment 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 afiect 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. 



Delagef has lately made use of the principle of specific rate of growth, 

 in considering the question of heredity itself. We know that the chromatin 

 of the fertilised egg comes from the male and female parent alike, in equal or 

 nearly equal shares; we know that the initial chromatin, so contributed, 

 multiplies many thousand-fold, to supply the chromatin for every cell of the 

 offspring's body; and it has, therefore, a high "coefficient of growth." If we 

 admit, with Van Beneden and others, that the initial contributions of male and 

 female chromatin continue to be transmitted to the succeeding generations 

 of cells, we may then conceive these chromatins to retain each its own coefficient 

 of growth ; and if these differed ever so little, a gradual preponderance of one 

 or other would make itself felt in time, and might conceivably explain the 

 preponderating influence of one parent or the other upon the characters of 

 the offspring. Indeed O. Hertwig is said (according to Delage's interpretation) 

 to have actually shewn that we can artificially modify the rate of growth of 

 one or other chromatin, and so increase or diminish the influence of the maternal 

 or paternal heredity. This theory of Delage's has its fascination, but it calls 

 for somewhat large assumptions; and in particular, it seems (Kke so many 

 other theories relating to the chromosomes) to rest far too much upon material 

 elements, rather than on the imponderable dynamic factors of the cell. 



* J. Exp. Zool. VII, p. 457, 1909. 

 f Biologica, ni, p. 161, June,. 1913. 



