Few new ribosomes appear in the cytoplasm of embryos of Rana pipiens or Xenopus laevis (the South African" clawed toad") before the tail bud stage. 1 At this time the amount of cytoplasmic ribosomes begins to increase; this rise is cor-related with an increase of protein in the high speed supernatant fraction as well as with the first appearance or increase of many enzymes. Soon after these events, the embryos develop a requirement for magnesium ions in the medium. Mag-nesium-starved embryoscharacteristically stop growing in length anddie at early swimming stages (Shumway2 stages 21-23 for Rana pipiens, I or Nieuwkoop-Faber3 stage 40 for Xenopuslaevis4). The magnesium requirement coincides with the onset of intense ribosome synthesis and presumably is based on the important role of magnesium ions in maintaining the integrity of the functional ribosome particle. The study of ribosome synthesis during amphibian development has been extended utilizing the lethal anucleolate mutant of Xenopus laevis first described by Elsdale et al. 5 These workers discovered a heterozygote mutant with only one nucleolus (1-nu) in each cell, whereas wild-type Xenopus laevis have two nucleoli (2-nu) in the majority of their diploid cells. The progeny resulting from the mating of two heterozygotes (1-nu) fall into threegroups having two, one, or zero nucleoli per cell. The ratio of these genotypes is 1: 2: 1, respectively, 5'6 as expected of a typical Mendelian factor. The heterozygotes (1-nu) lack a secondary constriction (" nucleolar organizer") on one of two homologous chromosomesin diploid cells; 7 the two comparable chromosomes of the anucleolate homozygous mutants (0-nu) both lack this secondary constriction. The anucleolate mutant (0-nu) has numerous small nucleolar" blobs" instead of typicalnucleoli, and both nuclear and cytoplasmic RNA have been shown histochemically to be lower in 0-nu embryos after hatchingthan in controls (1-nu and 2-nu). 8 Development of 0-nu embryos is first retarded shortly after hatching. 5'9 The mutant embryos become microcephalic and oedematous and die as swimming tadpoles before feeding. It was apparent that magnesium-starved embryos were to some extent phenocopies of the homozygous mutants (0-nu) since retardation of embryogenesis and growth occurred in both groups of embryos at about the same developmental stage (Fig. 1). The above data, as well as recent studies relating nucleolar function to ribosome synthesis, suggested that the anucleolate mutant might be incapable of synthesizing ribosomes and ribosomal RNA.

Material and Methods.-Radioactivity was introduced into developing embryos by incubation with C1'02 at pH 6.0. 10 The methods for measuring ribosome and DNA contents have been described previously.'Total RNA was isolated from frozen embryos after homogenization in 0.1 M sodium acetate pH 5.0 containing 4, ug/ml polyvinyl sulfate (a ribonuclease inhibitor prepared synthetically by the method of Bernfeld et al.)" and 0.5% sodium lauryl sulfate (Mann Research Co.). The homogenate was shaken for 5-10 min at 0C with an equal vol of phenol. Nucleic acids were precipitated from the aqueous phase with 2 vol of ethanol and0. 1 vol of M