much Catholic imagery, but not to imagery itself, and Lucas Cranach the Elder, a close friend of Luther, had painted a number of "Lutheran altarpieces", mostly showing the Last Supper, some with portraits of the leading Protestant divines as the Twelve Apostles. This phase of Lutheran art was over before 1550, probably under the more fiercely aniconic influence of Calvinism, and religious works for public display virtually ceased to be produced in Protestant areas. Presumably largely because of this, the development of German art had virtually ceased by about 1550, but in the preceding decades German artists had been very fertile in developing alternative subjects to replace the gap in their order books. Cranach, apart from portraits, developed a format of thin vertical portraits of provocative nudes, given classical or Biblical titles. Lying somewhat outside these developments is Matthias Grünewald, who left very few works, but whose masterpiece, his Isenheim Altarpiece (completed 1515), has been widely regarded as the greatest German Renaissance painting since it was restored to critical attention in the 19th century. It is an intensely emotional work that continues the German Gothic tradition of unrestrained gesture and expression, using Renaissance compositional principles, but all in that most Gothic of forms, the multi-winged triptych. The Danube School is the name of a circle of artists of the first third of the 16th century in Bavaria and Austria, including Albrecht Altdorfer, Wolf Huber and Augustin Hirschvogel. With Altdorfer in the lead, the school produced the first examples of independent landscape art in the West (nearly 1,000 years after China), in both paintings and prints. Their religious paintings had an expressionist style somewhat similar to Grünewald's. Dürer's pupils Hans Burgkmair and Hans Baldung Grien worked largely in prints, with Baldung developing the topical subject matter of witches in a number of enigmatic prints. Hans Holbein the Elder and his brother Sigismund Holbein painted religious works in the late Gothic style. Hans the Elder was a pioneer and leader in the transformation of German art from the Gothic to the Renaissance style. His son, Hans Holbein the Younger was an important painter of portraits and a few religious works, working mainly in England and Switzerland. Holbein's well known series of small woodcuts on the Dance of Death relate to the works of the Little Masters, a group of printmakers who specialized in very small and highly detailed engravings for bourgeois collectors,