The development of anucleolate embryos of Xenopus laevis.

A MUTATION of Xenopus has been found which reduces the maximum number of nucleoli per diploid nucleus from 2 in the wild-type to 1 in heterozygotes. Homozygous mutants possess no true nucleoli, hence being termed anucleolate. They do, however, possess pyronine-staining intranuclear organelles that are smaller and more numerous than typical nucleoli. The mutation can be considered as a recessive larval lethal, unlinked to sex: heterozygotes of both sexes are fully viable, but anucleolate larvae die at about the time their sibs begin to feed. These points have been recorded in two preliminary reports (Elsdale, Fischberg & Smith, 1958; Fischberg & Wallace, 1960). The development of anucleolate embryos is described here as a basis for the design of an experimental analysis of the ways in which the mutation acts. The description rests mainly on the progeny of a single mating of heterozygous toads, conforming with notes made on other such matings, from which additional data are drawn when required. As no developmental differences have been detected between the wildtype and heterozygotes, they are treated together as nucleolate (+«) controls, to which the anucleolate (On) embryos are compared. Nieuwkoop & Faber's (1956) stages of normal development are used here to indicate age, made independent of temperature. After stage 40, the retarded On larvae are assigned to the stage of their controls.