AbstractAxolotl females homozygous foroproduce eggs which cleave normally, but never develop beyond gastrulation. This cessation of development is due to a cytoplasmic deficiency which can be corrected by injecting eggs ofo/ofemales with cytoplasm from normal eggs. When so injected, the recipient eggs develop beyond gastrulation and may attain larval stages. The corrective component of normal cytoplasm has the following characteristics: 1. It remains in the supernatant of normal egg homogenates after two hours of centrifugation at 105g, but is gradually sedimented thereafter. 2. The corrective activity persists for at least eight days when preparations are stored at O°C. It is abolished on heating for one hour at 50° to 55°, and on incubation with crystalline trypsin. 3. The active material is customarily extracted from egg homogenates in 0.1 M or 0.2 M KCl, with 0.01 M Tris, pH. 7.6. It can be precipitated by 2M to 3M ammonium sulfate and retains its corrective activity. It is also precipitated by distilled water, but is then irreversibly denatured. 4. The corrective component is found mainly within the germinal vesicle of large oocytes; then in the egg cytoplasm or extracts thereof following germinal vesicle breakdown. Comparable extracts of blastulae show a reduced corrective activity. Attempts to extract the active material from older embryos and from organs of adults have so far failed. These results suggest that the corrective material, presumably a product of the normal allele ofo, depends upon a protein or proteins for its activity. It is produced during oogenesis and later plays an indispensible role in early organogenes