The Marlborough Gallery in London is showing two perfectly complementary exhibitions together. Both are prime examples of the interplay between artist and model. The show of Picasso etchings and aquatints is, in fact, simply called "Pablo Picasso: artist and model". The sculptures of Aristide Maillol are displayed under the title "Maillol and Dina", and chart the compelling relationship in bronze between the 73-year-old sculptor and the 15-year-old girl, Dina Vierny, who became his most important muse from their meeting in 1934 until his death ten years later.
Vierny was the daughter of a Ukrainian Menshevik and pianist who, in 1926, fled to Paris, where she grew up in a typical emigre artistic circle. An architect recommended her to Maillol, who wrote to her: "Mademoiselle, they tell me you resemble a Maillol and a Renoir. I shall be satisfied if it's a Renoir." She spent her school holidays with Maillol and his dour, disapproving wife at the sculptor's birthplace of Banyuls-sur-Mer, at the foot of the Pyrenees. With the advent of war and the fall of France, she moved permanently to Banyuls, where she was joined by her father in 1940. When he remonstrated with Maillol about his daughter's tomboyish tree-climbing and the sculptor's inadequate supervision, he responded: "You may have made her, but I created her." (It is rather difficult, today, to accommodate Maillol's attitudes to the women who gave him his art, but it was still the 19th century when he said of his wife: "I lifted her chemise and found marble.")
Vierny, when not posing for Maillol, guided refugees from Vichy France through the mountains and across the border into Spain. After the war, she opened her eponymous art gallery in Rue Jacob in Paris, an event photographed by BrassaI. Eventually, in 1995, she cemented her relationship with her mentor by persuading the then president, Francois Mitterrand, to open the Fondation Dina Vierny-Musee Maillol. Rarely can a modelling career have ended so auspiciously.
Sadly, the show at the Marlborough Gallery has none of Maillol's paintings of Vierny. Missing is a fine nude that gives a far better representation of her catlike face than the bronzes, and two or three affectionate versions of her in the red dress she wore to identify herself to the refugees who arrived at Banyuls railway station in order to follow her through the mountains.
Maillol made great sculptures long before Vierny came into his life, and several are on show here, including the striking and unusual Monument to Paul Cezanne. This reclining nude is unusual because the torso is much more elongated than Maillol's regularly chunky figures. It is as if he wanted to avoid any comparison with the often coarse, heavy-set nudes done by Cezanne himself, most notably in the late Bathers sequence. It is certainly one of the few sculptures that does not immediately announce Maillol's authorship.
For 40 years, his nude figures set a template of the eternal feminine, with a rugged, but never coarse, sensual beauty - as if they were Renoir's paintings wrought in three-dimensional form, but without that sugary sentimentality from which Renoir, even in his own bronzes, never wholly escaped.
It is only Maillol's late work that was inspired by Vierny, so his early work, particularly La Mediterranee, was based on his wife Clotilde - although, as he shrewdly explained, it was not enough "to have a model and copy it. No doubt nature is the foundation of an artist's labours. But art does not lie in the copying of nature." This is especially true of the many bronze versions of Vierny who, in her prime, for all her voluptuousness, was diminutive in stature.
Large or small, Maillol's work always channels every bit of energy into the women. With Picasso, even in the often delicate engravings on show at the Marlborough, the energy is channelled into his overweening egomania. The highly charged eroticism is intensely self-regarding. In these studies of the artist and his model, there is no ambiguity about identity. The artist is always himself, the model always his prey.
In one superb aquatint, Picasso is at his easel as the model stands at the side of the canvas, like a Playmate of the Month, gazing at the painting, which is of several similar models cavorting. The old goat - this is a 1965 work - is busy having it all, but the viewer forgives everything because the self-referential joke, with its harking back to his neoclassical nudes of the Thirties, is so funny (and the quality of the line is incomparable). Two great 20th-century masters at the height of their powers in a few small Mayfair rooms. Unmissable.
Maillol and Picasso can be seen at the Marlborough Gallery, London W1 (020 7629 5161), until 22 June