Notochordal rest

In all bilaterian animals, the mesoderm is one of the three primary germ layers in the very early embryo. The other two layers are the ectoderm (outside layer) and endoderm (inside layer), with the mesoderm as the middle layer between them.The mesoderm forms mesenchyme, mesothelium, non-epithelial blood cells and coelomocytes. [Source: Wikipedia ]

May Cause
Sacral neoplasm