in mammals, the existence of primitive endoderm had been observed as early as the end of the 19th century as first recognized by duval and sobotta. however, it took long time before people realized that the primitive endoderm will be replaced by definitive endoderm which will further develop into the gut tube. the first convincing experiment was conducted by bellairs in chick embryo with the careful observation under electron and light microscopy. in his experiment, bellairs demonstrated that there is a transitory endoderm cell layer in the chick embryo at its ventral surface before the formation of primitive streak. this layer of cell was replaced by definitive endoderm migration from the primitive streak through ingression and de-epithelialization. later on, more insights on primitive endoderm and definitive endoderm origin and formation have been provided in different species including rat and mouse, rhesus monkey, baboon et al.