The germinal vesicle of the mature Chaetopterus egg is invested by an envelope which can be seen in electron micrographs to contain "pores" in its bilaminar structure. While under continuous microscopic observation, individual germinal vesicles were isolated in various test solutions by an extremely gentle method. Repeated measurements of nuclear diameter and of optical path differences with an interference microscope provided data on changes in mass after isolation. It was found that bovine serum albumin can readily penetrate the nuclear envelope of the isolated nucleus and that there are soluble elements which rapidly diffuse out. A relatively non-diffusible mass is lost at a much slower rate, the proportion of soluble to non-diffusible mass being dependent on the ionic environment. Calcium and manganese increase the proportion of the non-diffusible mass at the expense of the soluble components, while potassium decreases it. The shape and size of the isolated nucleus is at least partially dependent on the non-diffusible mass of its interior. Digestion with trypsin causes a complete structural collapse and loss of the non-diffusible elements, along with disappearance of the nucleolus. The nucleus shrinks and becomes wrinkled. A small residual mass is left which is probably associated with the nuclear envelope. Digestion with RNase or DNase causes no detectable effect on the isolated nucleus. Micromanipulation of the isolated nucleus consistently indicates that there are strands emanating from the nucleus. They may be up to several hundred microns long, are structurally strong, and are not destroyed by trypsin, RNase, or DNase. Electron micrographs of thin sections of intact cells show that the germinal vesicle is highly irregular in outline with complex evaginations extending into the cytoplasm. With the light microscope the isolated nucleus looks spherical and smooth and no emanating strands can be seen. The nature of the strands is not known.