After the answers of Lars and Freddy I looked on familysearch for the birth record of Cornelis Collens. As starting point I use http://genver.nl/fs/fsindex.htm
This is an index in dutch.
Rotterdam is in Zuid-Holland So click on ZH and search for Rotterdam. Next click on 988 Reeksen. This brings you to http://genver.nl/fs/zh/0599.htm
There I searched for Geboorten 1821 and found
6582198 Geboorten 1820-1821 v/a 10 jul t/m 23 jun; index Geboorten 1821 [498 img] and clicked on the link The birtt record is on https://www.familysearch.org/search/image/show#uri=https%3A/...
bottom left. The father and the 2 witnesses are not able to sign. For easy find a tiny urlhttp://goo.gl/OLqAH