paintings, many of which are influenced by his warm relationship with her, in addition to his increased exposure to French painting. The generally upbeat and optimistic mood of paintings in this period is reminiscent of the 1899–1901 period (i.e., just prior to the Blue Period), and 1904 can be considered a transition year between the two periods. By 1905, Picasso became a favourite of American art collectors Leo and Gertrude Stein. Their older brother Michael Stein and his wife Sarah also became collectors of his work. Picasso painted a portrait of Gertrude Stein and one of her nephew Allan Stein. Gertrude Stein became Picasso's principal patron, acquiring his drawings and paintings and exhibiting them in her informal Salon at her home in Paris. At one of her gatherings in 1905, he met Henri Matisse, who was to become a lifelong friend and rival. The Steins introduced him to Claribel Cone and her sister Etta, who were American art collectors; they also began to acquire Picasso's and Matisse's paintings. Eventually Leo Stein moved to Italy. Michael and Sarah Stein became patrons of Matisse, while Gertrude Stein continued to collect Picassos. In 1907, Picasso joined an art gallery that had recently been opened in Paris by Daniel-Henry Kahnweiler, a German art historian and art collector who became one of the premier French art dealers of the 20th century. He was among the first champions of Pablo Picasso, Georges Braque and the Cubism that they jointly developed. Kahnweiler promoted burgeoning artists such as André Derain, Kees van Dongen, Fernand Léger, Juan Gris, Maurice de Vlaminck and several others who had come from all over the globe to live and work in Montparnasse at the time. African art and primitivism: 1907–1909 Picasso's African-influenced Period (1907–1909) begins with his painting Les Demoiselles d'Avignon. The three figures on the left were inspired by Iberian sculpture, but he repainted the faces of the two figures on the right after being powerfully impressed by African artefacts he saw in June 1907 in the ethnographic museum at Palais du Trocadéro. When he displayed the painting to acquaintances in his studio later that year, the nearly universal reaction was shock and revulsion; Matisse angrily dismissed the work as a hoax. Picasso did not exhibit Les Demoiselles publicly until 1916. Other works from this period include Nude with Raised Arms (1907) and Three Women (1908). Formal ideas developed during this period