In a homeotic transformation, one body part is replaced by another

A late-nineteenth-century drawing from one of the first studies of homeotic transformations in nature. (a) Homeosis in a sawfly, with the left antenna transformed into a leg. (b) Homeosis in a frog. The middle specimen is normal. The specimen on the left has extra structures growing out of the top of the vertebral column. The specimen on the right has an extra set of vertebrae.

[From W. Bateson, Material for the Study of Variation. Macmillan, 1894.]