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Charles Darwin's proposed material account of heredity, in which contributions from parts of the organism could be represented in reproduction. This is distinct from his mechanism of natural selection, which does not depend on the truth of pangenesis.

Literal miniature-organism theories had lost their former standing, but the explanatory problem had been transformed rather than abolished. By 1900, arguments about inherited organization, cellular differentiation and the response of an embryo to intervention were central to embryology. These were recognizably related to the older contrast between preformationism and epigenesis, even though neither eighteenth-century label could be carried unchanged into the new work.

The basic distinction concerned how organization arose. In preformationism, development unfolded organization already present in the germ; an increase in visible size did not require the original construction of the organism's arrangement. Epigenesis treated organs and differentiated structures as arising progressively. These broad positions left many further questions open. A claim that new visible organs appeared did not itself explain the material conditions that generated them, while a claim about inherited organization did not necessarily require a fully formed microscopic adult. Failure to separate these levels makes an apparent victory over the crudest miniature-organism picture look like the solution of every developmental problem.

August Weismann's discussion supplies contemporary evidence that the old opposition remained analytically useful in altered form. In the 1889 Essays upon Heredity and Kindred Biological Problems by August Weismann, he distinguished older miniature-animal preformationism from more recent accounts involving organized hereditary material. He discussed Charles Darwin's pangenesis under the latter category. That classification is not a claim that Charles Darwin literally believed in a tiny complete adult: it identifies a theory in which material representatives of parts contribute to the germ. The same discussion contrasted this with an epigenetic process in which an initial material constitution generates successive developmental states.^[ref]

August Weismann's germ plasm also relocated the problem of continuity between generations. A continuity of hereditary material could distinguish reproduction of the lineage from changes acquired by the body. But identifying continuity did not by itself establish how different parts of an embryo received different developmental capacities. A hypothesized partition of hereditary determinants during successive divisions offered one possible explanation. That possibility could be investigated by intervening on an embryo, rather than only inspecting it with an optical microscope.

The contrast between Wilhelm Roux and Hans Driesch makes the continuing issue concrete. In the frog experiment published in 1888, Wilhelm Roux killed one of the first two blastomeres with a heated needle and observed development of a partial embryo from the survivor. The result suggested mosaic development: an early part might possess a restricted developmental contribution. It did not, without further assumptions, identify a particular material in the nucleus as the cause. Nor was killing a cell and leaving it in place equivalent to cleanly separating the two living cells.

In trials begun in 1891, Hans Driesch separated or isolated early sea-urchin blastomeres; some developed into smaller but whole larvae. This supported regulative development and contradicted the claim that the first division universally and irreversibly partitioned the capacities required for different halves of the animal. The different animal, manipulation and surviving material mattered. One could not infer from the contrasting outcomes alone that one investigator had simply observed correctly and the other incorrectly.^[ref]

These experiments did not prove an absence of structure in the egg, and they did not establish that every embryonic cell retained every developmental possibility at every stage. Regulative development can depend on existing organization and interactions. Equally, Hans Driesch's eventual advocacy of vitalism was an interpretation of the findings, not an observation directly forced by a whole larva. The source cited here is his 1908 retrospective of pre-1900 work; its later philosophical formulation must not be silently treated as the precise position held at the original trials.

The debate had therefore become irrelevant only in a narrow sense: the old binary, if taken to mean a tiny complete organism versus the production of organization from wholly unstructured matter, no longer adequately described the best questions. In a broader sense it remained active as a dispute over what was inherited, what became progressively determined and how parts depended on the whole. The emergence of experimental embryology changed the evidence by which those claims could be tested. It did not make the relation between heredity and biological development disappear, and it should not be redescribed using modern epigenetics as though that were simply a new name for historical epigenesis.