Electron microscopic studies on the development of interstitial (steroidogenic) cells in embryonic chick gonads were carried out in a chronological sequence from the time of their appearance to the end of incubation, in a comparative aspect in both female and male sexes, and through a comparison with light microscopic findings. An asynchrony in the development of these cells in both sexes is established. In the ovaries (left and right) individual steroidogenic cells are detected for the first time towards the 7th embryonic day, grouping themselves towards the 9th...10th embryonic day in nests, interstitial organoids. In the testis this takes place towards the 10th and 14th...15th days, respectively, the interstitial organoids forming as incomplete muffs around the seminiferous cords and later on (towards the 16th embryonic day) also as nests in stroma. On the basis of observations that the precursers of the steroidogenic cells differentiate within the sex cords (first proliferation) and separate from them, falling into the stroma, the authors adopt the concept of the mesothelial (epithelial) origin of those cells. Particular protrusions (multivesicular structures) of the cytoplasm of the interstitial cell are described whose role is believed to be connected with the mechanism of the steroid secretion of the gonads. The results are discussed in relation to some biochemical and experimental data on the morphogenetic function of the embryonic sex hormones.