Trilaminar blastocyst

A trilaminar embryo (or trilaminary blastoderm, or trilaminar germ disk) is an early stage in the development of triploblastic organisms, which include humans and many other animals.

It is an embryo which exists as three different germ layers - the ectoderm, the mesoderm and the endoderm. These layers are arranged on top of each other like a stack of paper, giving rise to the name trilaminar, or "three-layered". The mesoderm is segmented further into the paraxial, intermediate and the lateral plate mesoderm.

These three layers arise early in the third week (after gastrulation) from the epiblast (a portion of the mammalian inner cell mass).