The development of the ovary of the sea lamprey (Petromyzon marinus L.)

This investigation is a histological study of the development of the ovary of the sea lamprey from south‐western Ontario, Canada. Development is described from the first appearance of primordial oögonia at the site of the ovary up to ovulation. After mitotic proliferation, oögonia enter meiotic prophase and become oöcytes which enlarge and acquire basophilic cytoplasm. During metamorphosis all oöcytes are at a uniform stage of development and no oögonia remain in the ovary. The follicular layer in the early adult produces nurse cells which are incorporated into the oöcyte and increase its mass of RNA‐rich cytoplasm. As the oöcyte enlarges, proteid yolk platelets are laid down in the cytoplasm. Vacuolation of the nucleolus, indicative of protein synthesis, is extensive during period of rapid growth and yolk formation. Immediately prior to spawning there is an accumulation of fluid under the follicular layer and the oöcyte emerges through a mound of follicular cells. Atresia of oöcytes occurs throughout the adult stages and following spawning. It is characterized by an enzymic dissolution of the yolk followed by phagocytic invasion. The phagocytes arise from the follicle in the early adult stages and from an unidentified source in the spawning‐phase adult.