176 PROBLEMS OF RELATIVE GROWTH 



have elements in common with the axial gradients of Child — 

 viz. a dominant region (here the growth-centre) which exerts 

 a graded effect on neighbouring regions, so that the whole 

 system is a field-system. 



It is already clear that in Child's scheme, place will have 

 to be found for qualitative differences among gradients. The 

 primary morphogenetic gradient in, e.g., a worm must differ 

 qualitatively from the activity-gradient concerned with the 

 addition of new segments throughout life. The primary 

 animal-vegetative axial gradient in an Amphibian egg must 

 differ qualitatively from the latter field-system of which the 

 organizer (dorsal lip) is the ' dominant region '. And specific 

 growth-gradients will constitute another main type. 



This section has been admittedly very speculative, but as 

 tentative conclusion we may suggest that gradient-systems, 

 all perhaps of essentially similar nature, are concerned with 

 primary differentiation, the time-relations of early develop- 

 ment, certain physiological properties of parts of the adult 

 organism, certain regenerative capacities, and with growth- 

 intensity. These gradient-systems will all obviously in the 

 long run be ' metabolic ', but may be specialized in qualita- 

 tively different ways according to the type of activity which 

 is graded within them. 



The primary gradient of the egg and early embryo may 

 be expected to persist throughout life and to have a minor 

 effect on the graded distribution of growth-promoting sub- 

 stances — i.e. some growth-gradients will be secondary effects 

 of the primary axial gradient. But in addition we may expect 

 that gradients concerned specifically with growth-intensity 

 may come into existence supplementary to and largely inde- 

 pendently of the primary axial gradient, but will then exert 

 their indirect effect upon such proportions of the primary 

 gradient as still persist. 1 



§ 4. Heterogony and Hormones 2 

 We must also consider the relation of hormones to hetero- 

 gony. Here we must at the outset remind ourselves of an 

 important point — that the action of a hormone always demands 



1 The important paper of Smirnov and Zhelochovtsev (1931) has 

 appeared too late to receive the discussion it merits. It contains a 

 detailed mathematical analysis of relative growth in the leaves of the 

 Nasturtium (Tropaeolum major) under conditions of normal and 

 reduced illumination and brings the results into relation with a general 

 conception of a ' gradient-field ' of growth. See also Werner (p. 258) . 



2 See also the work of Robb (p. 257). 



