Ninety-eight duck double monsters (siamese twins) were studied between 48 and 72 h of incubation in whole-mount or serial-sectioned preparations. Each case was classified into one of four categories depending on the orientations of the two embryonic axes. In all classes, defects in primary neural induction of various parts of the embryonic nervous system were observed which depended to a large extent on the distance separating the anterior tips of the two notochords or on the relative positions of the notochords. Our observations suggest that: when the same inducing field of one embryo overlaps to a critical extent with that of the twin, the two inducers act as one; induction of forebrain and optic vesicles is purely prechordal; the notochord exerts an inhibition on induction of forebrain by prechordal mesoderm; and the presence of rhombencephalic structures depends on both prechordal and chordamesodermal interactions.