Evidence is presented that oocytes representing species from widely diverse phyla, including mammals, have several characteristic features in common. These include relatively large nuclear and cytoplasmic volumes with proximity to and interrelationships with "nurse" or follicular cells, microvilli and pinocytosis channels on their plasma membranes, stored or "masked" messenger RNAs in their cytoplasms, facility for sequestering macromolecules from the circulation (which may play important nutritional and informational roles in immediate and later development), and tendencies toward a predominantly glycolytic metabolism as the egg advances in maturation. Major emphasis in the present paper has been placed on macromolecules in the oocyte of the hen whose large size and exaggerated processes of maturation permit a relatively detailed and accurate examination of many of the above-mentioned features.