@article{CMH_1951__25_139026,
     author = {Shiing-Shen Chern and E. Spanier},
     title = {A {Theorem} on {Orientable} {Surfaces} in {Four-Dimensional} {Space.}},
     journal = {Commentarii mathematici Helvetici},
     pages = {205--209},
     publisher = {mathdoc},
     volume = {25},
     year = {1951},
     zbl = {0043.38403},
     url = {http://geodesic.mathdoc.fr/item/CMH_1951__25_139026/}
}