results of Edmond B. Wilson, Edwin G. Concklin, and Theodor Bovari, which led to his continuing interest in dynamic causation in biology. He chose physics as his minor subject, and this, with his training in mathematics and engineering, had a strong effect on his later work in biology. Weiss's work was carried out under Hans Prizbram, director of the Biological Research Institute of the Academy of Sciences in Vienna. In his thesis, published in 1922, he studied responses of butterflies to light and gravity, arguing that the nervous system cannot be reduced to a rigid tropistic machine, but that the elementary steps in behavior are subordinated to the state of the whole, a view he extended later in studies of the vertebrate nervous system.

Weiss's nonscientific activities centered on music (he played the violin), sports, and travel. In 1926 he married Maria Helen Blaschka. After receiving his degree he worked in a number of European laboratories and traveled and lectured widely. Weiss later moved to the United States where career opportunities were greater. He became an American citizen in 1939. He taught first at the University of Chicago and later moved to Rockefeller University. In 1947 he was elected to the National Academy of Sciences.

During the period in which Weiss worked in Europe he initiated research in several areas that he was to return to later, most notably in cellular differentiation during regeneration, neural coordination of movement, and patterns of cell growth in tissue culture. In his studies of regeneration in newts he demonstrated that regeneration of the limb involved not merely the reformation of each tissue from the corresponding tissue of the stump, but differentiation of skeletal elements from non-skelatinous tissue, since a normal limb could form after removal of all skeletal components from the stump.

In subsequent work with the newt limb, Weiss was able to