The mutation "distended lungs" (dl) was found in the offspring of a clone composed by three isogenetic males resulting from the transplantation of a mesodermal nucleus into an enucleated egg. This mutation is recessive and its precise origin is unknown .

From stage 49/50 onwards, the phenotype of the mutants is characterized by the dumpy aspect of the body, the bent and wavy tail, the distension of the lungs, the abnormal red colour of the heart and blood vessels. The growth is slower than that of the normal tadpoles and the majority of the tadpoles die at different stages up to the end of the metamorphosis (semilethal mutation). Thin section of the lung walls have revealed a state of disorganization and even of degeneration in the "dl" tadpoles . The measures of the number of erythrocytes and the microhematocrit values showed an abnormal increase of these values at stage 52/53. In the case of lung anlagen grafts between wild-type and mutant embryos the grafted organs develop according to the host genotype. The analysis of this mutation suggests that the Xenopus lungs probably have a respiratory function during early larval life already.