The frame also contains carvings of crescent moons, stars, vegetation, and lions’ heads. These symbols are, perhaps, references to the Doni and Strozzi families, taken from each one's coat of arms. As depicted on the frame, “the moons are bound together with ribbons that interlock with the lions,” possibly referring to the marriage of the two families. There is a horizontal band, possibly a wall, separating the foreground and background. The background figures are five nudes, whose meaning and function are subject to much speculation and debate. Because they are much closer to us, the viewers, the Holy Family is much larger than the nudes in the background, a device to aid the illusion of deep space in a two-dimensional image. Behind Saint John the Baptist is a semi-circular ridge, against which the 'ignudi' are leaning, or upon which they are sitting. This semi-circle reflects or mirrors the circular shape of the painting itself and acts as a foil to the vertical nature of the principal group (the Holy family). Mary and Joseph gaze at Christ, but none of the background nudes looks directly at him. The far background contains a mountainous landscape rendered in atmospheric perspective. The "Doni Tondo" is believed to be the only existing panel picture Michelangelo painted without the aid of assistants; and, unlike his "Manchester Madonna" and "Entombment" (both National Gallery, London), the attribution to him has never been questioned. The juxtaposition of bright colors foreshadows the same use of color in Michelangelo's later Sistine Ceiling frescoes. The folds of the drapery are sharply modelled, and the modelling of the figures is distinctly sculptural, suggesting they are carved in medium marble. The nude figures in the background have softer modelling and look to be precursors to the "ignudi", the male nude figures in the Sistine Ceiling frescoes. Michelangelo's technique includes shading from the most intense colors first to the lighter shades on top, using the darker colors as shadows. By applying the pigment in a certain way, Michelangelo created an ""unfocused"" effect in the background and focused detail in the foreground. The most vibrant color is located within the Virgin's garments, signifying her importance within the image. The masculinity of Mary could be explained by Michelangelo's use of male models for female figures, as was done for the Sistine Chapel. Michelangelo used a limited palette of pigments comprising Lead White, Azurite, Verdigris and a few others.