Nitrogen Excretion in Xenopus Tadpoles

fact that Munro's experiments were carried out under less satisfactory conditions; he himself states that more than half the tadpoles died during the first week, and that from the original 250 eggs only 10 survived to metamorphosis. Our own animals were well fed on a high-protein diet and had a high survival rate. Munro, on the other hand, does not state that his animals were fed but only that they were kept in water 'to which had been added an infusion of raffia, mud and weed obtained from a pond supporting a large frog population'. According to our findings the changes during metamorphosis are similar to those found in other amphibians for which the ratio of ammonia to urea during development has been investigated. These are Rana temporaria, Bufo bufo bufo, Tritu?8 vulgari8, cri8tatu8 and Axolotl (Munro, 1939, 1953). The only difference between Xenopu8 and these other species is that whereas Xenopu8 returns to a predominant excretion of ammonia after displaying a tendency to ureotelism during its larval life, the other species become and remain ureotelic during adult life. It appears in fact that a progressive tendency towards ureotelism is a feature of the larval life of amphibia in general, but, whereas the completion of metamorphosis is commonly attended by the development of total ureotelism, Xenopu8 shows only a transient move towards ureotelism and reverts towards ammoniotelism again even before the forelimbs begin to emerge. Taken together with the observations of Cragg (1953) and Munro (1939, 1953), our results suggest that if Xenopu8 were to live a terrestrial or semiterrestrial instead of an aquatic life, it would be capable of meeting the problems of nitrogen disposal in the manner usual among amphibia, i.e. by ureotelism. Indeed, it seems likely that its survival during the dry season depends upon precisely this device. Certainly the dependence of Xenopu8 on an aquatic habitat is not to be attributed to failure to develop the machinery necessary for ureogenesis.