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.
