原坑是原結中央的凹陷處。與未來脊索相連接，屬於原線的一部份（原線最末端）。
外部連結
- http://www.embryology.ch/anglais/hdisqueembry/triderm01.html
- https://web.archive.org/web/20080809142409/http://isc.temple.edu/marino/embryology/EMBII97/img020.GIF
- http://cancerweb.ncl.ac.uk/cgi-bin/omd?primitive+pit
- https://web.archive.org/web/20070908045428/http://embryology.med.unsw.edu.au///Medicine/BGDlab3_11.htm
- https://web.archive.org/web/20060925080426/http://www.ana.ed.ac.uk/database/humat/notes/embryo/ectoderm/primstr.htm
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