Final answer:
The theory of preformation, put forth by Hartsoeker in 1864, stated that the embryo is already preformed within the sperm or egg. According to this theory, the embryo unfolds or enlarges its preexisting structures during development.
Step-by-step explanation:
The theory of preformation, as put forth by Hartsoeker in 1864, stated that the embryo is already preformed in a miniature form within the sperm or egg. This theory suggested that the embryo does not undergo significant changes during development, but rather unfolds or enlarges its preexisting structures. Hartsoeker's theory was based on the observation of homunculus-like figures in human semen under a microscope.
With advancements in science, we know this theory is inaccurate, and modern embryology has disproven it with the understanding of genetic material and epigenetics, which show that complex processes and interactions at the cellular and molecular levels are involved in development.