| Http://: http://en.wikipedia.org/wiki/Regional_specification#Dorsal.2Fventral_axis_.26_or |
| Http://: http://en.wikipedia.org/wiki/Primitive_knot |
| Http://: http://en.wikipedia.org/wiki/Regional_specification#Dorsal.2Fventral_axis_.26_or |
| Http://: http://en.wikipedia.org/wiki/Primitive_knot |