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\b[<gastrulation> is coordinated at tissue scale but implemented through changes in individual <cells>: motility, adhesion, polarity and shape. “Individual” and “collective” describe different aspects of the mechanism, not mutually exclusive types of <embryo>.] A <cell> can move relative to its neighbours while transmitting force or receiving directional information through those same neighbours. Conversely, nearby <cells> can share a trajectory because they are passively carried by a moving tissue.
The principal outcomes are internalization and rearrangement of germ-layer progenitors, establishment of their relative positions and elongation or spreading of the embryonic axis. <Ingression> internalizes individual <cells>, whereas <invagination> bends a connected epithelial sheet; involution rolls tissue across a margin. <Convergent extension> changes tissue proportions, while <epiboly> spreads a tissue over a larger surface. These geometrical descriptions should be distinguished from the force-generating mechanisms. For example, two <embryos> can both extend but use different mixtures of oriented intercalation, migration, shape change and growth.
\b[Individual motility is especially apparent after <cells> leave an <epithelium>.] In mouse, <epiblast> <cells> approach the <primitive streak>, reorganize junctions, cross a locally permissive <basement membrane> and undergo an <epithelial-to-mesenchymal transition>. Once internalized, mesodermal <cells> migrate between <epiblast> and <endoderm>. Protrusions, cytoskeletal force production and transient attachment to neighbouring <cells> or <extracellular matrix> produce movement. This allows descendants to disperse to distinct destinations and respond to local signals. Downregulation of stable epithelial adhesion can facilitate departure, but productive migration still requires regulated interactions: complete loss of coupling can prevent traction or integration.
<FGFR1> illustrates the connection between regulatory state and <cell> behaviour. Its perturbation affects mesodermal <gene expression> as well as <Snail>/<E-cadherin> regulation and movement from the mouse streak.<FGF signaling regulates mesoderm cell fate specification and morphogenetic movement at the primitive streak by Brian Ciruna and Janet Rossant>[2001, pp. 37–49, separate differentiation and epithelial-to-mesenchymal-transition readouts]{ref} Therefore accumulation of <cells> at a streak cannot automatically be interpreted as a pure motility defect. I would determine whether they acquired the expected identity, whether they detached from the <epithelium>, and whether viable detached <cells> can migrate. A normal motility <protein> does not restore <gastrulation> if the progenitor population was never specified.
\b[Collective <invagination> coordinates local contractions through a mechanically continuous sheet.] In the <Drosophila> ventral furrow, mesodermal <cells> constrict their apical surfaces and become wedge-shaped, bending the <epithelium> inward. Apical <actomyosin> contraction is transmitted to neighbours through <adherens junctions>; surrounding tissue and basal <cell> properties constrain the resulting deformation. Live imaging showed pulses of medial apical <actin>–<myosin> contraction rather than a universally continuous circumferential purse string. Stabilization between pulses produces a ratchet-like reduction in apical area; <Snail> and <Twist> perturbations separate contraction initiation from stabilization.<Pulsed contractions of an actin-myosin network drive apical constriction by Adam C. Martin and others>[2009, Figures 1–4, live myosin dynamics, apical-area changes and transcription-factor perturbations]{ref} Asynchronous pulses in individual <cells> can thus contribute to a coherent tissue fold. Collective behaviour does not require every <cell> to contract simultaneously, and a <cell>'s change in shape need not equal its net translational displacement.
\b[Collective extension often requires <cells> to exchange neighbours rather than preserve one fixed group arrangement.] During Xenopus dorsal <mesoderm> convergence and extension, <cells> polarize, make biased protrusions and intercalate mediolaterally. The tissue narrows across its width and extends along the body axis without requiring a corresponding increase in <cell> number or total volume. Coordinated <planar cell polarity> signalling coordinates this orientation. <Dishevelled> perturbation disrupts polarized behaviour and extension even when the relevant <cell fates> remain, supporting a role in <morphogenesis> separable from specification.<Dishevelled controls cell polarity during Xenopus gastrulation by John B. Wallingford and others>[2000, pp. 81–85, time-lapse imaging, polarity-defective perturbations and fate controls]{ref} Individual <cells> are active force-generators, but their useful orientations and contact rearrangements depend on tissue organization. Merely increasing <cell> speed would not necessarily restore directional intercalation.
A kinematic check illustrates the distinction from growth. If a roughly rectangular tissue of constant thickness and area changes from width $W$ and length $L$ to $W/2$ and $2L$, it has undergone strong convergence and extension while preserving $WL$. Actual gastrulating tissues can also change thickness, proliferate and exchange <cells>, so this is an illustrative conservation argument rather than an assertion of strict area conservation. Quantifying all three dimensions and <cell> number is required before attributing axis elongation entirely to intercalation.
The <zebrafish> <prechordal plate> provides a particularly clear experiment distinguishing individual movement from collective guidance. Isolated prechordal <cells> remain polarized and motile but lose their normal directional migration toward the <animal> pole. Directionality returns when they contact the endogenous plate, with <E-cadherin>, Wnt/planar-<cell>-polarity components and <Rac1> implicated in the response. Even an isolated group is not automatically oriented correctly.<Collective mesendoderm migration relies on an intrinsic directionality signal transmitted through cell contacts by Julien G. Dumortier and others>[2012, pp. 16945–16950, isolated-cell/group transplantation and contact-dependent orientation]{ref} Thus collective movement is more than the sum of many <cells> independently sensing the same external attractant. Nor does the experiment prove that the frontmost <cell> alone is a permanently designated leader. The informative observation is retention of motility but loss and restoration of directional information.
\b[<epiboly> demonstrates how an adjacent tissue can supply a substantial driving force.] In <zebrafish>, the superficial <enveloping layer> spreads over the yolk while deeper populations rearrange. Here, <radial intercalation> helps thin and spread deep tissue. Contractile <actomyosin> in the <yolk syncytial layer> contributes pulling at the epithelial margin. Force and flow analysis showed that retrograde <actomyosin> flow can generate pulling through resistance, in addition to circumferential contraction; spreading can proceed when circumferential constriction alone would be unproductive.<Forces driving epithelial spreading in zebrafish gastrulation by Martin Behrndt and others>[2012, pp. 257–260, actomyosin flow, mechanical analysis and epithelial spreading]{ref} Observing a <cell> move vegetally therefore does not establish that its own crawling supplies the principal force. Collective <morphogenesis> can couple embryonic and extraembryonic compartments.
The movement classes are related developmentally. A coherent <epithelium> can first bend, then release <cells> that migrate individually, which can later establish new collective arrangements. Partial retention of contacts can permit a mesenchymal population to remain coordinated. Such <cell> rearrangement changes exposure to inducing signals and the mechanical environment, so movement can influence fate as well as execute a prior fate decision. The comparison should therefore ask where force originates, how it is transmitted, what gives direction and which contacts persist, rather than assign one universal movement mode to <gastrulation>.
I would test these alternatives with simultaneous nuclear, membrane, junctional and <actomyosin> imaging. Trajectories should be measured relative to neighbouring <cells> and matrix landmarks to subtract <tissue advection>. Speed, directional persistence, neighbour exchange, <cell> shape and spatial velocity correlations answer different questions; a common external pull can correlate velocities without contact-mediated guidance. Sparse mosaics distinguish a <cell>-intrinsic motility defect from a surrounding-tissue defect. Isolated-<cell> versus group transplants test whether direction requires neighbours. Acute, local perturbation after fate specification, followed by rescue, reduces the ambiguity of early patterning <mutants>. Local <laser> severing and recoil can reveal prestress, but recoil also depends on dissipation and is not an absolute force measurement without a mechanical model. Matching viability, proliferation, adhesion and differentiation controls is essential. \b[The strongest explanation connects an individual behaviour to a measurable tissue deformation and shows, by perturbation, which interactions make that behaviour productive.]
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