1. The endoderm was separated from the epiblast in chick and duck embryos of young primitive streak stages, and then the two layers of tissue were replaced so that their longitudinal axes were diametrically opposed: i. e. the anterior part of the endoderm lay under the posterior region of the epiblast, and the posterior part of the endoderm under the anterior region of the epiblast. The combination was then cultivatedin vitro by the watch-glass technique. 2. The displaced endoderm had in most cases an effect on the development of the epiblast. It either (1) hindered the normal growth in length of the primitive streak, or (2) caused the development of a new primitive streak. In some cases the secondary (induced) streak disappeared on further cultivation, in some cases both streaks persisted and gave rise to a double monster, and in still other cases the secondary streak persisted and the primary disappeared. In two cases primary and secondary united to form a single semicircular embryo. 3. It is argued that the endoderm does not induce the differentiation of a definite tissue, but that it induces the form-building movements which lead to the development of the primitive streak.