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Morphogenesis and form  ·  Article

Gradient modification by depressing agents and developmental acclimation

Child argued that an axis of the body is a gradient in metabolic rate, and that the gradient shows in how regions respond to depressing agents. Regions of high rate acclimate more readily and are least susceptible in the long run.3 The two ways of testing therefore give opposite susceptibility orders from one underlying gradient.4 Developmental defects in embryos follow the same gradient.1

Earliest held
1915, Child, C. M.
Most discussed in
Physiological Foundations of Behavior, 1924
In the library
169 passages in 9 works
Rewritten
2026-10-10
18501875190019251950197520002025Child, C. M., 19151Child, C. M., 19152Child, C. M., 19153Child, C. M., 19154Loeb, J., 19165Child, C. M., 19216Child, C. M., 19247Manicka S, Levin M, 20228Shreesha L, Levin M, 20239
01

Defects along the axis

Child (1915) described embryos in which, under certain conditions, the anterior region develops more or less normally while the posterior region is held back. He cited his 1912 study of Planaria dorotocephala for the axial gradient involved. He explained the pattern by acclimation. Posterior regions have a lower metabolic rate, do not acclimate as readily as anterior ones, and so are retarded or inhibited to a greater extent.1 The resulting monsters are more or less normal in front and increasingly abnormal toward the rear. Where acclimation plays no part, the anterior regions may be most affected and the posterior least, which gives another type of monster.2

02

Two forms of the method

Child (1915) distinguished a direct form of the susceptibility method from an indirect, or acclimation, form. In the acclimation form the degree of acclimation varies with metabolic rate. High-rate regions become acclimated more readily and are least susceptible in the long run, while low-rate regions undergo less acclimation and are inhibited more.3 He held that the gradients seen with the two forms run in opposite directions but express the same metabolic gradient.4 Loeb (1916), writing on pieces of stem that form a new polyp, passed on Child's suggestion that an axial gradient exists in the stem.5

03

Range and the permeability question

In 1921 Child wrote that modifications of the same general type, with respect to a particular axial gradient, can be produced in widely different organisms, among them flatworms, echinoderms, fishes and frogs.6 In 1924 he reported that regions more susceptible to higher concentrations of external agents generally take up vital dyes faster. He denied that this reduces susceptibility to a matter of surface permeability, as a physical condition independent of metabolism.7 He added that penetration rate runs parallel to oxidation rate and that differential acclimation results oppose a differential permeability reading.7

04

Later use of the term

The term axial gradient reappears in recent modelling work by Levin and colleagues. Manicka and Levin (2022) built a cellular network that develops an axial gradient pattern from homogeneous conditions, with activity falling from high positive to low negative values along the anterior-posterior axis.8 Shreesha and Levin (2023) used virtual embryos consisting of a single axis of positional information values to test how cellular competency alters evolutionary dynamics.9 The passages held here do not tie these models to metabolic rate or to susceptibility to depressants.

SourcesEach quotation was checked word for word against the passage it opens.
  1. posterior regions, which possess a lower metabolic rate than anterior regions, do not acclimate to the conditions as readily as the latter and are therefore retarded or inhibitedChild, C. M., 1915 · Individuality in Organisms · open at passage 105
  2. Where acclimation does not play a part the anterior regions of the embryo may be most, the posterior least, affected and another type of monsters results.Child, C. M., 1915 · Individuality in Organisms · open at passage 105
  3. regions of high metabolic rate are least susceptible in the long run because they become acclimated more readily, while regions of lower metabolic rate undergo less acclimationChild, C. M., 1915 · Individuality in Organisms · open at passage 94
  4. The susceptibility gradients observed with these two modifications of the method are themselves opposite in direction, but are different expressions of the same metabolic gradient.Child, C. M., 1915 · Individuality in Organisms · open at passage 94
  5. Another suggestion made by Child,2 is that there exists an axial gradient in the stem whereby the cellsLoeb, J., 1916 · The Organism as a Whole, from a Physicochemical Viewpoint · open at passage 273
  6. modifications of the same general type with respect to a particular axial gradient can be produced in widely different organisms, e.g., flatworms, echinoderms, fishes, frogs.Child, C. M., 1921 · The Origin and Development of the Nervous System · open at passage 71
  7. the susceptibility gradients cannot be interpreted in terms of permeability of cell surfaces as a physical condition independent of metabolismChild, C. M., 1924 · Physiological Foundations of Behavior · open at passage 230
  8. the network develops an axial gradient pattern where the activity states drop mostly smoothly from high positive values to low negative values along the anterior-posterior axisManicka S, Levin M, 2022 · Minimal Developmental Computation: A Causal Network Approach to Understand… · open at passage 22
  9. Virtual embryos consisted of a single axis of positional information values provided by cellsShreesha L, Levin M, 2023 · Cellular Competency during Development Alters Evolutionary Dynamics in an… · open at passage 0
Linked ideas
The general concept; this article covers the specific experiments with depressing agents and acclimation that Child used to read the gradient.
Treats the susceptibility method itself; acclimation is the indirect form that reverses the apparent order of regions.
The high-rate anterior region that resists depressants in the long run is the region treated as dominant.
Child rejected a purely permeability account of susceptibility gradients, while permeability measures are used elsewhere as indicators of physiological state.
Regenerationrelated to
Loeb relayed Child's axial gradient suggestion in discussing where a new polyp forms on a cut stem.
Both tie regional metabolic rate to what a region becomes, here shown by graded developmental defects.
Polarityrelated to
Child's 1921 and 1924 passages treat axial (apico-basal, antero-posterior) gradients as the physiological basis of polarity; B's susceptibility gradients are the experimental evidence. Depressants aren't mentioned in the passages.
Child's passage reports experimentally determined, lawlike dependence of depressant susceptibility and acclimation on metabolic rate along the axis, an instance of conditions-determine-response thinking, though passages never name determinism.
Both concern planarians. Child's passage describes susceptibility and inhibition gradients in Planaria along body axes; the passages say nothing about memory, so the link is the shared organism and axial form.
Where it is discussedPassages matching axial gradient, acclimation, anaesthetics and developmental modification, susceptibility to depressing agents
1924Physiological Foundations of Behavior · Child, C. M.63
1921The Origin and Development of the Nervous System · Child, C. M.62
1915Individuality in Organisms · Child, C. M.37
2022Minimal Developmental Computation: A Causal Network Approach to Understand… · Manicka S, Levin M2
2026Aspects of plant intelligence (ii) · Trewavas A1
2023Cellular Competency during Development Alters Evolutionary Dynamics in an… · Shreesha L, Levin M1
1925Elements of Physical Biology · Lotka, A. J.1
1916The Organism as a Whole, from a Physicochemical Viewpoint · Loeb, J.1
2018RNA from Trained Aplysia Can Induce an Epigenetic Engram for Long-Term… · Bédécarrats A, Chen S, Pearce K, Cai D…1