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Symmetry and asymmetry in the development of inner organs in parabiotic twins of amphibians (Urodela).

Newt embryos of different developmental stages were combined to parabiotic twins in different positions. The exterior appearance and the symmetry relations, particularly of the internal organs (intestinal tract, heart, nuclei habenulae, and vitelline vein) were studied. Experimentally caused organ inversions allowed conclusions with respect to organ asymmetry and unilateral dominance. There was no direct correlation between appearance and symmetry of the exterior and the internal organs. All internal organs showed a continuous transition between normal and ideally inverse situs. The concordance of the organ situs differs greatly. The "left-hand side" or "right-hand side" dominance is not uniform. It depends on the type of fusion, i.e. the relative position of the parabiotic twins, and is often specific for a given organ. In some cases a non-genetic "symmetrisation factor" appears to be strongly active, depending on the fusion type and resulting in a dominant transindividual organ mirror image symmetry in the parabiotic twins. The older twin generally dominates the processes of determination and induction. The "symmetrisation factor" also acts on members of different families, i.e. genetically completely heterogeneous parabiotic twins. The development of organ asymmetry appears to be a process with several phases.