The Morphogenesis of Multicellularity: 3 Boundary and Interior — Membranes as Relational Cuts

Every act of life begins with a cut. The membrane is not a wall but a relational threshold—the site where potential distinguishes itself without separation. In the emergence of multicellularity, these thresholds multiply and reconfigure, producing not division but nested interiors, a topology of coordinated difference.

From the relational perspective, a boundary does not exclude what lies beyond; it creates a relation by defining the conditions of mutual construal. The membrane is where the cell says I am this—yet only because it can sense, exchange, and align with what it is not. The act of bounding is already an act of meaning.

As multicellular organisation arises, these membranes begin to form structured alignments—tight junctions, plasmodesmata, signalling complexes—that transform local distinctions into systemic coherence. Each contact is a semiotic commitment: a promise that the inside will remain interpretable to the outside, and vice versa. The organismal interior thus emerges as a field of reciprocal readability.

In a purely mechanistic view, boundaries maintain gradients; in a relational ontology, gradients instantiate boundaries. The very sense of “inside” is constituted by the pattern of relational construal across the cut. A shift in potential—ionic, chemical, informational—does not merely reflect a difference; it actualises one.

Multicellularity therefore depends on the coordination of cuts. As membranes interlock, they create a topology of constrained possibilities—a grammar of interiorities. Organs, tissues, and cavities arise as differentiated clauses in this grammar: specialised construals of what it means to be inside in relation to the whole.

The paradox is that the organism’s unity depends entirely on its capacity to sustain heterogeneous boundaries. Without distinction, no dialogue; without dialogue, no coherence. The organism lives in the tension between closure and openness—each membrane a sentence in its ongoing negotiation of selfhood.

To be multicellular, then, is to live as a system of cuts in conversation: every cell a meaning of the whole, every boundary an act of relation. The organism is not contained by its skin—it is its skin, folded inward, multiplied, and continually re-drawn.