What theory speculates that embryos go through stages resembling adult forms of ancestral organisms?

The theory that speculates embryos go through stages resembling the adult forms of ancestral organisms is known as the theory of recapitulation. This theory, originally proposed by the German zoologist Ernst Haeckel in the 19th century, is often summarized by the phrase "ontogeny recapitulates phylogeny." This means that the development of an individual organism (ontogeny) follows the same progression as the evolutionary history (phylogeny) of that organism's ancestors.

According to this theory, during certain stages of embryonic development, embryos of various species exhibit characteristics or structures that resemble the adult forms of their evolutionary predecessors. For example, human embryos have stages where they possess features similar to those of fish, such as pharyngeal arches, which are noticeable during early development. This observation is significant as it suggests a connection between development and evolutionary history, illustrating how evolution can shape developmental processes.

The other choices represent concepts related to biology and development but do not specifically encapsulate the idea of embryonic stages reflecting ancestral forms as effectively as the theory of recapitulation.