Primordium

A primordium ( /praɪˈmɔːrdiːəm/; plural: primordia) in embryology, is defined as an organ or tissue in its earliest recognizable stage of development. Cells of the primordium are called primordial cells. A primordium is the simplest set of cells capable of triggering growth of the would-to-be organ and the initial foundation from which an organ is able to grow.

Although it is frequently used term in plant biology, the terminology is used in biology of all multicellular organisms (for example: primordium of tooth in animals, primordium of leaf in plants or primordium of sporophore in mushroom.)