[Larval form and visceral situation in the newt (Triturus alpestris) after one-sided UV-irradiation of neurulae and postneurulae].

Neurulae and postneurulae ofTriturus alpestris (up toHARRISON stage 29) were irradiated with mixed ultraviolet light from the left, right, dorsal or ventral side, respectively. The duration of treatment ranged from 1-600 seconds. Visible effects occurred already after a 5 sec irradiation. Embryos and larvae showed defects of histological and external features. In this paper "inversions" are distinguished from "partial inversions". In "inversions" the curvature of the gut and the liver sinus (or, in some cases, only the latter) are inverted. "Partial inversions" include all cases of lesser expression. The frequency of total and partial inversions of the digestive system depends on the duration, the stage and the side of irradiation. The frequency of total and partial inversions after dorsal, ventral and right side irradiation is not markedly higher than it is in normal controls (an exception are partial inversions of the gut after treatment within a certain dose range). Irradiation of the left side leads to a marked increase of partial inversions in all age classes. A significant increase (p<0.001) of total inversions occurs only after treatment in early stages (HARRISON 15-17) with high doses. A maximum of inversions of the digestive tract at low doses is still to be proven. The frequency of the experimentally elicited situs inversus viscerum decreases during neurulation. Inversions of the heart are less frequent than are inversions of the gut. The results are not in agreement with the hypothesis of a mechanical curving of the gut discussed bySPEMANN; rather, they indicate an influence from irradiation disturbance to the morphogenetic determination processes.