In 1890, he was born into a Jewish family in Mogilno, County of Mogilno, Province of Posen, Prussia (modern Poland). He was one of four children born into a middle-class family. His father owned a small general store and a farm.[3] The family moved to Berlin in 1905. In 1909, he entered the University of Freiburg to study medicine, but transferred to University of Munich to study biology. He became involved with the socialist movement and women's rights around this time.[3] He served in the German army when World War I began. Due to a war wound, he returned to the University of Berlin to complete his Ph.D., with Carl Stumpf (1848–1936) the supervisor of his doctoral thesis.