The behaviour of the egg pigment in wild-type and ‘rusty’ tadpoles of Xenopus laevis

The behaviour of the egg pigment was studied by histological analysis of wild-type and ‘rusty’ embryos and tadpoles of Xenopus laevis as well as by experimental procedures. The histological analysis of the wild-type animals showed that the various tissues, notably the skin, neural tube, alimentary system and cement gland go through progressive stages of egg pigment migration and concentration at the apical ends of the cells. In the ‘rusty’ mutants the migration and concentration of pigment occur to a slight extent only, the majority of the pigment granules remaining dispersed. The experiments (tail cultures, squashes of cement gland mucus and of meconium) showed that in wild-type animals the pigment, after migration and concentration, is eliminated from the cells by expulsion. In ‘rusty’ animals, this expulsion does not take place. Parabiotic tadpoles of a ‘rusty’ wild-type combination possess a coloration corresponding to their genotype. Ectodermal grafts performed at the neurula stage between ‘rusty’ and wild-type embryos develop according to their origin. The amount of egg pigment found in wild-type and ‘rusty’ tadpoles, and the exceptional case of the cement gland are discussed. It is concluded that the behaviour of the egg pigment is an active cell-specific process, and that the pigment is eliminated by expulsion. The non-elimination of the egg pigment in the ‘rusty’ mutant, accounting for its characteristic colour, appears to be due to a failure of the expulsion mechanism.