The Nativity is flanked by the full-length figures of the two prophets who foretell the birth of Christ ^[fig1], ^[fig2] . Isaiah, to the left, as revealed by the text of his scroll and his leftward-turned gaze, is thematically linked to the previous scene of the front predella ^[1], representing the Annunciation ^[fig3], now in the National Gallery of London. The iconography ^[2] of the Nativity follows the figurative tradition of Byzantine art, combining the scene with the subsidiary episodes of the Glad Tidings to the Shepherds and the First Bath of the Child. Mary is shown semirecumbent on a mattress inside the cave setting, into which a simple wooden hut with sloping roof is inserted. At the center of the hut, in the background, we see the manger with the child and two animals. In the foreground the episode of the First Bath occupies a central position, with the two midwives portrayed in slightly smaller proportions than the Madonna. ^[3] To the left we see Saint Joseph seated on a rock, sunk in meditation, while to the right appear the two shepherds conversing with one of the fourteen angels that throng the upper part of the scene.

The painting was the second of seven scenes ( ^[fig4], ^[fig5], ^[fig6], ^[fig7], ^[fig-8] ) interspersed with standing figures of prophets that formed the predella of the front side of the two-sided altarpiece ^[4] placed over the high altar in Siena Cathedral ^[fig9] (see also Reconstruction). For a discussion of the multipart complex of which this work has always been recognized as an integral part, see the entry on The Calling of the Apostles.

This is one of the few early Italian panels in the collection that has not been cradled. The wooden support is a two-member poplar panel ^[14] of remarkable thickness (6 cm), with horizontal grain; engaged to this is a simple gilded molding that demarcates the three areas of the support to be painted. The panel and moldings were prepared with a fine fabric followed by gesso. A thin, orange bole was applied under the gilded areas. The ornamental border along the edges of the gold ground, the halos, and the contours of the figures of the prophets were incised in the preparation before painting. Mordant gilding is evident in the robes of the Virgin and of the angels. Infrared reflectography reveals a simple underdrawing. ^[15]

A photograph taken in or shortly before 1885 ^[16] suggests that the painting was subjected to a rather drastic restoration, of unspecified date but probably carried out before the acquisition for the Gemäldegalerie der Königliche Museen in Berlin, in order to integrate the abrasions and render the image more pleasing by extensive retouching. The inscriptions were also reinforced. Helmut Ruhemann treated the painting in 1929; ^[17] photographs made after this treatment show the worn areas of the painting. The figures of the prophets in particular are damaged by abrasion and by small flaking paint losses as well as by sharp craquelure. Dr. Max Friedlander “cleaned” the painting at some point between 1929 and 1937. ^[18] According to information in the William Suhr archives at the J. Paul Getty Museum, Los Angeles, William Suhr removed a varnish, inpainted, and revarnished the painting. ^[19] On the whole the painted surface, in spite of some abrasion, is fairly well preserved. Numerous small areas of inpainting affect the faces of the angels, the hair and beard of Isaiah, and the face of the Virgin.