Embryos ofTriturus alpestris in the tail-bud or post-tail-bud stage were severely damaged by removal of a large quantity of germinal tissue from either the left or the right side (Fig. 1a). The resulting extreme curvature of the torso and the tail toward the region of the defect sometimes remains more or less pronounced but is usually corrected in the course of regulative events (Figs. 1b-d, 2). These appear as intense right-left interactions of the embryonic and larval body which are aimed toward compensation of the primary defect. 2. The lateral defects have a very strong influence on the course of the vitelline vein and there is sometimes a difference between the effects of left and right defects. Changes in the course of the vitelline vein, to the extent of complete situs inversus, occur relatively often; the transposition always takes place toward the defect or toward the restitutive tissue (Fig. 3). 3. Neither left nor right lateral defects lead to fundamental alterations of the (already determinate) visceral position. Only the intestine often has abnormalities of form and position which can be considered as disturbance of the normal position (Fig. 4). These results directly contradict Spemann's hypothesis that lateral transposition of the viscera is due to the body curvature mechanically caused by loss of material on the left side.
