primordium

[prahy-mawr-dee-uh m] /praɪˈmɔr di əm/
noun, plural primordia
[prahy-mawr-dee-uh] /praɪˈmɔr di ə/ (Show IPA). Embryology
1.
the first recognizable, histologically differentiated stage in the development of an organ.
Origin
1665-75; < Latin prīmōrdium, in plural: beginnings, elementary stage, equivalent to prīm(us) first (see prime) + ōrd(īrī) to begin + -ium -ium; cf. exordium
British Dictionary definitions for primordium

primordium

/praɪˈmɔːdɪəm/
noun (pl) -dia (-dɪə)
1.
(biology) an organ or part in the earliest stage of development
primordium in Medicine

primordium pri·mor·di·um (prī-môr'dē-əm)
n. pl. pri·mor·di·a (-dē-ə)
An aggregation of cells in the embryo indicating the first trace of an organ or structure.