@Incollection{, title = {Development of the concept of homotopy}, author = {Vanden Eynde, Ria}, booktitle = {History of topology}, year = {1999}, publisher = {North-Holland, Amsterdam}, pages = {65{\textendash}102}, url = {http://dx.doi.org/10.1016/B978-044482375-5/50004-3} }