Embryology Sacrococcygeal teratomas originate from an area of the primitive streak called the primitive knot or Hensen's node. As the mesoderm rapidly pro- liferates, the primitive streak comes to lie more and more caudally where the remnant of Hensen's node descends to the tip of the coccyx on its anterior surface. Normally it undergoes degenerative changes and disappears, but occasionally it persists and may give rise to a sacrococcygeal teratoma. The node contains pluripotential stem cells and so the tumour can contain tissues from the ectoderm mesoderm and endoderm