[The symmetrical relation of intestines, the habenular nuclei and the intestinal venous system in dorsal-doral and ventral-ventral grown parabiontic larva of mountain salamanders (Triturus alpestris)].

Embryos of the newt Triturus alpestris were fused in their dorsal regions (DD-parabionts) of their ventral regions (VV-parabionts), while the longitudinal axes were in equal direction. In these parabionts situs and symmetry of the gut, the heart, the habenular nuclei and the vitelline vein were studied. The fusion of the embryos of the DD-parabionts was performed in three developmental stages (after Harrison): phase of neurulation (14-18, N-parabionts), late phase of neurulation (19-22, E-parabionts) and tail bud stage (23-27, S-parabionts). VV-parabionts were only fused in phase of neurulation (14-18). The external development of the parabionts in most cases was normally. DD-parabionts showed a more or less pronounced deformation of their longitudinal axes. In DD-parabionts all organs developed separately in nearly all cases. In VV-parabionts a partial fusion of the heart was sometimes observed, while fusion of large part of gut occurred regularly. In DD-parabionts complete and incomplete inversions of all organs and the vitelline vein are extremely abundant in the N-parabionts, much lower in E-parabionts and nearly absent in S-parabionts. The differences are statistically of high significance. The "pair situs" symmetries of N-parabionts (DD-parabionts) show always a strong dominance of a transindividual organ- and vitellin vein-symmetry. In VV-parabionts (N-parabionts) the tendency for inversion of the asymmetric organs is statistically significant lower as in DD-parabionts. In regard to this observation there is a preference for an asymmetric "pair situs" in the heart, habenular nuclei and vitelline vein system. In "polarisied" DD-parabionts ("left" as well as "right" parabionts) the asymmetric organs as well as the vitelline vein system inversions were more often observed in the "left" parabiont compared with the "right" parabiont in N- and E-parabionts. These right-left differences partial are statistically significant (P < 0.05). The organ situs-correlation as well as the situs-correlation between vitelline vein and gut or heart, respectively, are mostly positive (concordant). The frequency of dominance of "left" organ inversions argues against a morphogenetic "left dominance" of the amphibian embryo. The determination of organ-asymmetries is irreversible only after the end of neurulation in tail-bud-stage. The results suggest the efficiency of a "symmetry-factor", which determines the organ symmetry during neurulaphase. The differences between DD- and VV-parabionts are statistically significant and point to a dorso-ventral polarity of the embryo, with a maximal morphogenetic potency in the dorsal region.