Morphogenesis and form · Article

Polarity

Polarity is an asymmetric distribution of a property along an axis, such as head versus tail or apical versus basal.⁶ Vochting's plant experiments, reported by Morgan, showed that cut pieces keep their orientation.¹² Later work asked what fixes it: a gradient, nerves or a bioelectric pattern.³⁸⁹

Earliest held: 1880, Darwin, C.
Most discussed in: Physiological Foundations of Behavior, 1924
In the library: 1,252 passages in 100 works
Rewritten: 2026-10-04

Early plant observations

Morgan (1901) reviewed Vochting's grafting work on plants. Pieces grafted with their original orientation united perfectly. Pieces grafted in other ways sometimes united incompletely or died later.¹ Vochting concluded that every part has a polar orientation, and that radially symmetrical bodies also have a radial polarity. He placed the basis in the cells, each with its own apical and root pole.¹ Short internodal pieces of stem put out leaf-buds at the apex even when the apical end pointed down.² Loeb (1916) found that leaves on a stem piece favoured roots toward the base and shoots toward the apex.⁴

Gradients and their limits

Child (1915) discussed double apical regions and heads in pieces of animals. Other investigators had called these axial heteromorphoses, because the apical structure at the basal end seemed out of place. Child argued they were not abnormal. They follow the same laws as a normal single individual, in pieces where the original gradient is almost absent and activity at the basal end can set up a new gradient in reverse.³ Child (1924) reported that in very short pieces polarity could be set by contact with the substratum and by differential exposure, independent of the original polarity.⁵ The original polarity could therefore be overridden.

Planarian fragments and nerves

Lobo, Beane and Levin (2012) describe planarian polarity along anterior-posterior and dorsal-ventral axes. A trunk fragment with head and tail removed regrows its head in the original orientation, never toward the original tail.⁶ Pietak and colleagues (2019) set two hypotheses against each other: pre-existing morphogen gradients and the direction of net innervation. In small fragments lacking the VNC, the regenerated axis rotated by 90 degrees. They concluded that average neural axon polarity is the dominant driver, through directional transport of morphogens.⁸ Fragments that contain the VNC keep the original axis because its large nerves dominate transport.⁸

Cells and bioelectric memory

In epithelial cells, apicobasal polarity appears in the cytoskeleton. Chifflet and Hernandez (2012) describe microtubules running along the cell with their minus ends toward the apical surface. They call this arrangement crucial for the targeted traffic of vesicles that sustains distinct apical and basolateral domains.⁷ Levin (2025) reviews a bioelectric prepattern that sets planarian head-tail polarity. Altering it for a few hours can convert regenerating planaria into a two-headed form.⁹ Levin calls the circuit a true memory, because the two-headed state persists without further manipulation and is not reflected by genetic changes.⁹

SourcesEach quotation was checked word for word against the passage it opens.

every part and every portion of a part has a polar orientation in one direction, and furthermore, in a body having a radially symmetrical form, there is also a radial polarizationMorgan, T. H., 1901 · Regeneration · open at passage 455
leaf-buds appear at the apex of the internodal piece (Fig. 33, A), even when the apical end is downward (Fig. 33, B).Morgan, T. H., 1901 · Regeneration · open at passage 185
the development of these double or biaxial structures is, as I have shown, subject to exactly the same laws as the development of the usual single individualChild, C. M., 1915 · Individuality in Organisms · open at passage 169
the leaves accelerate the growth of roots towards the basal end and inhibit it towards the apical end; and they favour the growth of shoots towards the apical endLoeb, J., 1916 · The Organism as a Whole, from a Physicochemical Viewpoint · open at passage 266
In very short pieces this determination of polarity in relation to the substratum and the differential exposure of the pieces may be quite independent of the original polarityChild, C. M., 1924 · Physiological Foundations of Behavior · open at passage 318
a worm trunk fragment generated by removing both the head and tail will always re-grow its head in the same orientation as the original wormLobo D, Beane WS, Levin M, 2012 · Modeling planarian regeneration: a primer for reverse-engineering the worm · open at passage 23
in epithelia microtubules organize in long apicobasal-oriented parallel fibers that span the whole length of the cell, with their minus end pointing towards the apical surfaceChifflet S, Hernández JA, 2012 · The plasma membrane potential and the organization of the actin cytoskeleton of… · open at passage 4
average neural axon polarity in a fragment is the dominant driver of anatomical polarity via the directional transport of morphogens with respect to nerve polarityPietak A, Bischof J, LaPalme J, Morokuma J, Levin M, 2019 · Neural control of body-plan axis in regenerating planaria · open at passage 61
Altering the bioelectric pattern for just a few hours can convert regenerating planaria into aLevin M, 2025 · The Multiscale Wisdom of the Body: Collective Intelligence as a Tractable… · open at passage 44

Linked ideas

Axial and metabolic gradientsrelated to

Child explains polarity and double-headed pieces through an axial gradient; Pietak et al. test gradients against nerve direction as the source of polarity.

Regenerationpart of / contains

Polarity decides which end regrows a head or tail in fragments, as in Vochting's plant pieces and planarian trunks.

Bioelectric prepatternrelated to

Emmons-Bell et al. say voltage gradients regulate positional information and axial polarity.

Resting membrane potentialrelated to

Resting-potential gradients across cell types are named as instructive cues for axial polarity.

Morphogenetic fieldrelated to

Pietak et al. model morphogen gradients whose transport direction sets the regenerated anterior-posterior axis.

Left-right asymmetryrelated to

Lobo et al. link left-right to dorso-ventral axes in vertebrates and describe medial-lateral polarity in planarians; Carneiro et al. note the left-right axis is oriented relative to other axes.

Memory in planariarelated to

Child holds that cells' environment, not heredity alone, sets which body region they form; Durant et al. link neurotransmitters to polarity.

Tropismrelated to

Driesch's negative geotropism of the main axis, taken over by a side branch, ties a directed response to the organism's axis.

Neurotransmitters before neuronsrelated to

In planarians, praziquantel-induced bipolar phenotypes are rescued by exogenous serotonin, tying neurotransmitter signalling to polarity.

Determinism in physiologyrelated to

Child argues something must determine why cells with identical nuclei form a head at one axial end; Bose seeks the conditions that predispose or reverse polar effects. Polarity is treated as condition-determined.

Galvanotaxisrelated to

Child ties galvanotactic orientation to the axial gradient: the high end faces the negative pole. Funk links cell polarity (front-rear pH gradient, NHE3) to electrotaxis. Lillie sees current direction determining polar action.

Depolarization and hyperpolarizationrelated to

Vmem depolarization patterns are tied to body axes: a depolarized A/P boundary stripe in Drosophila wing discs, ivermectin depolarization promoting head regeneration in planarian and Macrostomum fragments. Electrical 'polarization' otherwise is a separate sens

Gap junctionsrelated to

Blocking gap junctions in planaria gives secondary heads, so junction communication is needed for head-tail polarity.

Heliotropism, geotropism and related tropic responses in plantsrelated to

The thinner ectoplasmic wall on the lower side of a slime mould tube was described as morphological geopolarity.

Ion channels and pumpsrelated to

Passages link them causally: asymmetric ion channel and pump distribution, from the egg onward, creates asymmetric ion gradients and Vmem patterns that drive left-right and A/P axis patterning; epithelial apical/basolateral domains differ.

Electrical response of living tissuerelated to

Bose treats 'polar effects' of electric current as a feature of excitation shared by plant and nerve; the organoid passage uses polarity as cell asymmetry in budding, a different sense. Link is partial.

Turing patterns, reaction-diffusion and autocatalysisrelated to

Planarian axis polarity is the case where reaction-diffusion alone is argued to fall short without added nerve-based transport.

Inherency of form, mesoscopic materials and the physical origin of body plansin tension with

Child denied that polarity is inherent in protoplasm and traced it to external factors, the opposite emphasis from inherency of form.

Forced movements and compensatory orientationrelated to

Jennings found no permanent anterior or posterior in Amoeba. Child described axes as physiological conditions, not fixed regions.

Gap gene network and positional informationrelated to

Axis polarity is set by first-division asymmetry in some embryos and by nerve direction in planarian fragments.

Mitotic spindle, asters and the centrosomerelated to

The two poles of the spindle, each with a centrosome and aster, give division an axis.

Cephalization and physiological dominance of the anterior regionrelated to

Anterior-posterior polarity sets which end can form a head and which end dominates.

Differential susceptibility as a measure of metabolic raterelated to

Susceptibility ranking reveals a metabolic gradient along the hydroid axis (apical to basal), and differential inhibition can abolish original polarity, so the method probes polarity's gradient basis.

Membrane potential patterns in Drosophila oogenesisrelated to

Apicobasal, anteroposterior and transversal voltage gradients in the follicle and oocyte bear on cell and tissue polarity.

Electrical polarization and axon outgrowthrelated to

Thompson-style visible polarity of the cell and Child's dynamic polarization of the neuron both concern an axis set by electrical or metabolic difference.

Gradient modification by depressing agents and developmental acclimationrelated 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.

Where it is discussedPassages matching polarity, anterior-posterior, head-tail, apical

1924	Physiological Foundations of Behavior · Child, C. M.	184
1915	Individuality in Organisms · Child, C. M.	120
1913	Researches on Irritability of Plants · Bose, J. C.	92
1901	Regeneration · Morgan, T. H.	72
1906	Plant Response as a Means of Physiological Investigation · Bose, J. C.	70
1992	On Growth and Form · Thompson, D. A. W.	65
1923	Protoplasmic Action and Nervous Action · Lillie, R. S.	62
1907	Comparative Electro-Physiology: A Physico-Physiological Study · Bose, J. C.	59
1894	Materials for the Study of Variation Treated with Especial Regard to… · Bateson, W.	41
1928	The Motor Mechanism of Plants · Bose, J. C.	34