Source
Nicholson (2026), LIT-1821, §5 and nn. 30–31, reporting Weiss 1969a, 1970a, 1973 and 1974; read in NOTE-1340.
What was actually shown
The claim. “there are innumerable ways to arrive to each developmental stage, and no two embryos ever undergo cellular differentiation in exactly the same way” (§5). Fig. 5, redrawn from Weiss 1969a, shows embryonic regions A–E whose topological relations persist from stage S1 to S2, while “even two very closely related embryos (top and bottom) differ drastically in the trajectories taken by the individual cells” (caption). Weiss’s summary is “determinacy in the gross despite demonstrable indeterminacy in the small” (Weiss 1973, quoted in §5). The river delta is the image: “one can only tell that the water will end up in the ocean, but never what precisely the distribution and configuration of the branches will be” (Weiss 1970a, quoted in §5).
The evidence offered. Weiss’s own embryological experience, as the schematic; and four citations on the stochasticity of gene expression and cell-fate decisions (Elowitz et al. 2002; Raj and van Oudenaarden 2008; Losick and Desplan 2008; Balázsi et al. 2011), said to call “[t]his deterministic understanding” into question (§5). No data are reproduced, and no measurement compares cell-level and tissue-level variability.
The exception conceded. “for very simple multicellular organisms such as the tiny nematode worm Caenorhabditis elegans we can actually track the individual lineages of all 671 of its embryonic cells (Sulston et al. 1983). But Weiss’s general point stands, as such perfectly predictable behaviour is the exception rather than the rule” (n. 30). How rare the exception is, the paper does not show.
What could have failed. Cell trajectories could have proved as reproducible as organ outcomes across embryos, as they are in C. elegans; or the outcome could have varied with the trajectories. Neither test is reported in the paper.
What this does not say
- That genes or molecular mechanisms do not matter. Nicholson says so: “taking systems dynamics seriously does not entail denying the importance of genes in development”, which supply “‘boundary conditions’” (§5).
- What imposes the regularity. “imposed from the top down by the developmental system as a whole” (§5) names a level, not a mechanism. Cell–cell signalling, mechanical feedback and selection among cells would each produce reproducible outcomes from variable cells, and the paper does not choose among them.
- That developmental programs are a myth. A gene regulatory network can specify outcomes without specifying each cell’s path; the conclusion is against microdeterminacy, not against regulation by genes.
- That Weiss’s variance inequality has been measured here. The link §5 draws to it is by analogy (THEORY-1122 on what that inequality can show).