AbstractIn vitro systems afford a unique opportunity for investigating the direct interaction of substances with developing morphogenetic systems (MGSs), since maternal influences are excluded. As a carrier of a complete set of MGSs, the chick embryo in ovo manifests an advantage over those in vitro systems that employ isolated embryos or embryonic tissues that have only limited survival. Under controlled experimental conditions including standardization of subjects, administration technique and mode of evaluation according to the basic principles of teratology, the chick embryo test was demonstrated to be reliable and to afford quantifiable endpoints for evaluation (e.g., specific embryotoxicity effect level, positive dose response, and a stage response and effect). Individual compounds, mixtures of compounds, and even poorly identified extracts can easily be tested. The chick embryo possesses its own basic enzyme‐catalyzed drug‐transformation capacity and, moreover, it can be used for screening specific human metabolites. Comparative studies have indicated a high predictive value of the chick embryo system relative to other in vitro systems as well as to whole‐animal te