▁Vienna , ▁on ▁the ▁responses ▁of ▁but ter f lies ▁to ▁light ▁and ▁gravity . ▁ ▁After ▁comple ting ▁his ▁th esis ▁he ▁tra ve led ▁widely ▁in ▁Europe , ▁becoming ▁an ▁assistant ▁director ▁of ▁the ▁Bi ological ▁Research ▁Institute ▁of ▁the ▁Vienna ▁Academy ▁of ▁Sciences . ▁In ▁ 1 9 2 6 ▁he ▁married ▁Maria ▁Helen ▁Bl as ch ka . ▁ ▁His ▁studies ▁of ▁lim b ▁reg ener ation ▁in ▁new ts ▁showed ▁that ▁a ▁complete ▁lim b ▁could ▁reg ener ate ▁even ▁if ▁particular ▁t issue ▁forms ▁were ▁removed ▁from ▁the ▁st ump : ▁the ▁required ▁types ▁of ▁t issue ▁would ▁reform . ▁He ▁studied ▁cell ▁different iation ▁and ▁the ▁trans plant ing ▁and ▁reform ing ▁of ▁connections ▁in ▁the ▁n erves ▁of ▁lim bs , ▁using ▁new ts ▁and ▁fro gs ▁for ▁his ▁experiments . ▁He ▁went ▁on ▁to ▁consider ▁ne uro bi ology ▁and ▁morph ogen esis . ▁He ▁introduced ▁the ▁idea ▁of ▁the ▁" natural ▁experiment " ▁– ▁the ▁quest ▁for ▁suggest ive ▁examples ▁ ▁from ▁nature ▁– ▁and ▁this ▁became ▁a ▁favour ite ▁teaching ▁device . ▁ ▁In ▁ 1 9 3 0 ▁a ▁pros pective ▁post ▁at ▁the ▁University ▁of ▁Frankfurt ▁was ▁lost ▁due ▁to ▁the ▁de pression ▁and ▁We iss ▁moved ▁to ▁the ▁United ▁States . ▁In ▁ 1 9 3 1 , ▁after ▁studying ▁developing ▁cell ▁cult ures ▁for ▁some ▁time , ▁We iss ▁won ▁a ▁Ster ling ▁fellow ship ▁to ▁work ▁with ▁Ross ▁Gran ville ▁Harrison ▁at ▁Y ale . ▁He ▁took