1. The shape of the embryonic blood island, the appearance of the vitelline vein under normal and experimental conditions as well as the relationship between the vitelline vein and the position of the asymmetrical viscera (heart, intestine) were investigated in larvae ofAmbystoma mexicanum andTriturus alpestris. 2. The approximately symmetrical, Y-shaped blood island shows conspicous species and individual form differences. Larvae often show alterations of shape which indicate reorganization activities in the blood island. Remnants of the blood island can persist in the presence of a fully developed and functioning vitelline vein. 3. Normally, the vitelline vein runs asymmetrically to the left (normal position), but even in controls and especially in experimental animals it shows great variation, ranging from normal to completely inverse position. The vitelline vein can be missing in developmentally damaged larvae. It is often originally paired and roughly symmetrical; furthermore, it can show abnormal local ramifications. The occasional position alterations of the vitelline vein, which in extreme cases can lead to inversion of the position, are especially remarkable and are without parallel in the evolution of the heart and the intestine. The caliber of the vein and the duration of its functional period are highly variable. 4. In the greater number of cases, the course of the vitelline vein and the position of the intestine and the heart agree; only a small percentage of the larvae show a distinct discrepancy in this respect. According to this evidence, the intestinal vein is positionally and functionally not strictly dependant on the heart and the intestine. 5. Defects on the left side of postneurulae have, on the average, a more pronounced effect on the course of the vitelline vein than defects on the right side. During the functional period of the vein alterations of its course occur more frequently toward the normal position (toward the left) than in the opposite direction. 6. The phenomenon of the leftward tendency or "left-dominance" of the vitelline vein is discussed in connection with similar observations in the viscera. Apart from a general lateral determination of the asymmetrical organs in the gastrula and neurula stages, to which the vitelline vein is probably also subject, the effect of an unknown non-determinative factor on the vitelline vein must be assumed when one considers its occasional positional changes. The variability of the vitelline vein, considerable in every respect, becomes more readily understandable when one regards it as a component of a more comprehensive, dynamic functional system.

No abstract

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.

1. 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.

Neurulae (HARRISON 15-17) ofTriturus alpestris were kept within an electrical set up for 1 hour. At a later stage the larvae showed irregularities and inversions of their viscera in a much higher frequency than the untreated controls.