Birds and bats have independently evolved wings from their forelimbs (an example of convergent evolution). However, while their wings look superficially quite different, examination of the underlying bones reveals them to be homologous. The forelimb of the embryonic bird begins its development with much the same structure as that of a mammalian embryo. As the bird develops the forelimb becomes ever more wing-like and less leg-like. Many of the hand bones fuse with each other and some are lost. In contrast, the bat retains all the bones of its hand, but these are greatly elongated. Another group of flying vertebrates, the pterosaurs, had similar modifications of the basic tetrapod forelimb. All three organisms, reptile, mammal, and bird, have the same bones of the upper and lower arm, although their proportions differ. However, the hand bones supporting the wing surface are quite different. In a pterosaur the wing membrane is supported by the 5th finger only. A bird's primary flight feathers are mounted on the 2nd finger, while in a bat the 2nd, 3rd, 4th and 5th fingers all support the wing membrane.