Cleft lip results from failure of fusion between the medial nasal and maxillary processes, not the median and lateral nasal processes. These two processes normally merge to form the upper lip. The medial nasal process contributes to the central part of the upper lip and the philtrum, while the maxillary process forms the lateral parts. Failure of fusion here leads to a cleft lip, which may be unilateral or bilateral. Understanding this fusion is crucial for surgical repair, as realigning these segments restores both function and aesthetics. Precise knowledge of these embryological origins guides the design of flaps and suturing techniques in cleft lip surgery.

Reference: Gray's Anatomy, Richard L. Drake, 42nd Edition.