(Chapter 29) Footnote 57:

Vesalius cites Book 2 of De anatomicis administrationibus in the margin; see 2.299.16 ff. Galen does not say this muscle goes to the tibia, but the semimembranosus accessorius he describes there exists only in the ape. See Singer 1956, p. 40. This reference to Galen was deleted from the 1555 edition.