Which observation contradicts Haeckel's idea of "ontogeny recapitulates phylogeny"?

The observation that pharyngeal structures of embryos do not match adult forms of fish is significant in evaluating Haeckel's idea that "ontogeny recapitulates phylogeny," which suggests that the development of an organism (ontogeny) follows the same progression as the evolutionary history of that organism's species (phylogeny). Haeckel proposed that during embryonic development, organisms exhibit features reflective of their evolutionary ancestors.

The contradiction arises because if ontogeny were to truly recapitulate phylogeny, we would expect the embryonic forms of organisms to closely resemble the adult forms of their evolutionary ancestors at corresponding stages. However, the pharyngeal structures in the embryos of various species do not correspond directly to the adult forms of fish. Instead, mammalian embryos may develop structures, such as pharyngeal arches, that are present during embryonic development but regress or transform considerably by the time of adulthood, demonstrating a divergence from their adult ancestors.

This highlights that there are complex mechanisms and evolutionary changes involved in development that Haeckel's hypothesis does not adequately account for, leading to the conclusion that embryonic stages may reflect a more generalized evolutionary history rather than a direct or simple lineage reflection as H