The trilaminar embryonic disc

At S6, the formation of the primitive streak causes pluripotent epiblast cells to develop into endodermal and mesodermal cells. These pluripotent cells leave the epiblast in the region of the primitive streak and primitive node, and spread to form layers under the epiblast. As endodermal cells they replace the hypoblast. As mesodermal cells they form the mesoderm, which is a middle layer between epiblast and endoderm. Epiblast cells, which remain in the epiblast layer, become the ectoderm. The embryonic disc now consists of three layers: ectoderm, mesoderm, and endoderm. Due to this triple-layer configuration, it is known as the trilaminar embryonic disc. The mixed population of cells settling between the epiblast and embryonic endoderm is known as the mesoblast until their specific determination is known. The terms mesoderm and mesenchyme often are used as synonyms.
All tissues and organs of the body develop from the germ layers. However, most organs arise from more than one layer. Thus, it is worth distinguishing between germ layers and tissues. A germ layer is a sheet-like and coherent layer of cells of the same kind which arise from the first differentiation of the embryoblast. By contrast, tissues are associations of cells that have differentiated in the same manner, including specific non-cellular structures, e.g. the intercellular substance.
There are two main groups of tissue: border tissues (epithelium, diathelium) and intermediate tissue (mesenchyme, embryonic connective tissue). Border tissues delimit internal or external surfaces, and separate phases of different compositions. Intermediate tissues fill the space between border tissues. In addition to these two main types, four groups of tissues can be distinguished: 1. epithelial tissue, 2. connective tissue (including blood, cartilages and bones), 3. muscle tissue, and 4. nervous tissue.