A teacher, experimenter and theorist, he made a lasting contribution to science in his lengthy career, throughout which he sought to encourage specialists in different fields to meet and share insights.

He absorbed the studies of Edmond B. Wilson, Edwin G. Concklin, and Theodor Boveri and completed his doctoral thesis in 1922 under Hans Leo Przibram, then director of the Biological Research Institute of the Academy of Sciences in Vienna, on the responses of butterflies to light and gravity.

After completing his thesis he traveled widely in Europe, becoming an assistant director of the Biological Research Institute of the Vienna Academy of Sciences.

In 1931, after studying developing cell cultures for some time, Weiss won a Sterling fellowship to work with Ross Granville Harrison at Yale.

In his work on tissue cultures Weiss outlined several features of cell proliferation: he showed how cell-patterns are affected by their substrate and, through grafts, proved that basic neural patterns of coordination were self-differentiating rather than learned, though higher vertebrates can "retrain" reflexes.