174 PROBLEMS OF RELATIVE GROWTH 



different results if extended over a longer period of regenera- 

 tion. But he adduces new and important results from grafting 

 experiments on earthworms, showing that a difference in 

 ' differentiation potential', as when a young head is grafted on 

 an old body, gives much better results than the converse 

 experiment or than even autoplastic grafting of young heads 

 on young bodies. From this he deduces the existence of a 

 ' differentiation gradient ' of importance in morphogenesis and 

 regeneration. 



Sinnott (1930) has elicited a curious fact in regard to the 

 variability of cell-size in tissues of the petiole of maple-leaves 

 (Acer). He finds that there exists a gradient between surface 

 and exterior, variability being least in the size of epidermal 

 cells, and increasing cell-layer by cell-layer towards the centre 

 of the petiole. 



We have also the well-known work of Lund (e.g. 1923, 

 1928) who finds in various organisms a system of gradients 

 in bio-electric potential persisting throughout life, and has 

 shown that organic polarity and morphogenesis can be con- 

 trolled by electrical means. A correlation between electrical 

 and metabolic gradients has been shown by Purdy and Sheard 

 (p. 260). And on the chemical side, the recent paper of Wata- 

 nabe (1931) may be consulted ; he finds a gradient in amount 

 of oxidizable substance in earthworms, running parallel with 

 the gradients postulated by Child for this organism. 



It would doubtless be possible to multiply examples ; but 

 this deliberately heterogeneous list will serve to emphasize 

 the wide range of gradient phenomena which may exist in 

 the adult animal body. 



It next falls to discuss the relations between the growth-gradi- 

 ents with which we have been concerned, and other gradient- 

 systems, notably the axial gradients, as I shall term them. 



Here we are on speculative ground ; but there are certain 

 indications which make us suspect some real connexion be- 

 tween the two. In the first place, there is the co-existence 

 of a growth-gradient and an axial gradient in Planaria (see 

 Abeloos, 1928). It would be extremely interesting to follow 

 this up in greater detail and especially to see whether changes 

 in steepness of the axial gradient were quantitatively associ- 

 ated with changes in the growth-gradient. In this case, it is 

 worth recalling, size is the chief index of physiological age, 

 and growth-partition is almost entirely a matter of size, 

 whether in fed specimens which are increasing, or in starved 

 specimens which are diminishing in size. 



