→ Ernst Haeckel gave the embryological basis of development.
→ Based on his observation some traits are common in all lineages during the embryonic stage, but absent in the adult organism.
→ For example, in the embryos of all vertebrates including humans, there is a row of vestigial folds behind the head, but it is only functional in fish, not in other adult vertebrates.
→ However, the proposed embryological basis of this development was rejected in a careful study by Karl Ernst von Baer.
→ He noted that embryos sometimes do not pass through the adult stages of other animals.