Of 10,695 larvae ofTriturus alpestris, 419 exhibitedcomplete orincomplete situs inversus in at least one of their asymmetric organs, i.e.alimentary canal, heart, andnuclei habenulae. The abnormal (nonregular) animals appeared in the following groups: a) 33 (0.9% of 3,801) among the control animals (spontaneous reversal), b) 50 (2.6% of 1,947) in a series in which the eggs were not stripped from the grass leaves to which they had been attached ("blade series"). c) 83 (3% of 2,748) afterx-irradiation between cleavage and late neurulation. d) 253 (of 2,199) after exposure tolithium sulfate hydrate between the morula and early postneurula stages. Thus x-irradiation and particularly lithium treatment proved capable of provoking situs inversus. These two nonsurgical methods were most effective when applied at the gastrula stage. 2. The asymmetric organs were completely or incompletely reversed. Thevarious degrees of reversal formed graded series of reactions ranging from almost normal situs to ideal situs inversus. In 65% of the nonregular larvae, all three organs were nonregular. The three did not differ in the faculty of assuming the reversed state. In each of them, incomplete reversal occurred more frequently than complete reversal, and incompletely reversed larvae outnumbered the completely reversed by, for example, 73∶27 in the control group or 88∶12 in the lithium series. The several grades of reversal of the three organs were combined in almost any conceivable way; the anlagen are obviously capable of responding independently of one another. Nonetheless, in most cases gut, heart, and habenulae reacted jointly and similarly. 3. InTriturus, no convincing evidence for monohybrid inheritance of spontaneous reversal is available. A polygenic basis to account for the lability of sidedness is discussed. 4. There was no direct relationship between the larval malformations (e.g. flexures of the body) caused by x-irradiation or lithium treatment and the occurrence of reversal. 5. Presumably, a microstructure that is asymmetric in itself and capable of determining the orientation of the asymmetric organs is present in the egg and transmitted to all rudiments so that each of them possesses an intrinsic orienting tendency. Reversal is probably an incidental effect of the disturbance of normal development. Surgical methods, such as replantation, constriction, or extirpation, and nonsurgical methods, such as x-irradiation or lithium treatment, provoke regulatory movements, in the course of which the anlagen are rearranged. The rudiments may become rotated with respect to their original orientations, and their intrinsic microstructures may therefore assume abnormal relative positions to the geometrical axes of the embryo. This change in the relative positions is the direct cause of situs inversus, the degree of reversal depending on the angle of rotation. In the small egg, the three organs are often affected similarly, though the looseness of the connections between the rearranging cell complexes permits local differences in the amount of rotation and hence in the degree of reversal. This interpretation is termed theregulation hypothesis of situs inversus.
