on the first day of recording. Beatles biographer Philip Norman comments that the two shared a disregard for the other's new compositions; Lennon found McCartney's songs "cloyingly sweet and bland", while McCartney viewed Lennon's as "harsh, unmelodious and deliberately provocative". Harrison and Starr chose to distance themselves partway through the project, flying to California on 7 June so that Harrison could film his scenes for the Ravi Shankar documentary Raga. Lennon's, McCartney's and Harrison's individual projects outside the band in 1968 were further evidence of the group's fragmentation. In Lennon's case, the album cover of his experimental collaboration with Ono Two Virgins featured the couple completely naked, a gesture his bandmates found bewildering and unnecessary. On 20 August, Lennon and Starr were working on overdubs for "Yer Blues" in Studio 3, and visited McCartney in Studio 2 where he was working on "Mother Nature's Son". The positive spirit of the session disappeared immediately, and engineer Ken Scott later claimed that "you could cut the atmosphere with a knife". Starr abruptly left the studio on 22 August during the session for "Back in the U.S.S.R.", feeling that his role in the group was peripheral compared to the other members, and upset at McCartney's constant criticism of his drumming on the track. Abbey Road staff later commented that Starr was usually the first to arrive at the studio, waiting in the reception area for the others to arrive. In his absence, McCartney played the drums on "Dear Prudence". For "Back in the U.S.S.R.", the three remaining Beatles each made contributions on bass and drums, and the drum part is a composite of Lennon, McCartney, and Harrison's playing. Lennon, McCartney, and Harrison pleaded with Starr to reconsider. He returned on 5 September to find his drum kit decorated with flowers, a welcome-back gesture from Harrison. Mono version The Beatles was the last Beatles album to be mixed separately for stereo and mono. All but two tracks exist in official mono mixes; the exceptions are "Revolution 1" and "Revolution 9", both direct reductions of the stereo master. The Beatles had not been particularly interested in stereo until this album, but after receiving mail from fans stating they bought both stereo and mono mixes of earlier albums, they decided to make the two different. Several mixes have different track lengths; the mono mix/edit of "Helter Skelter" eliminates the fade-in at the end of the song