Light and electron microscopic studies have been made on young oocytes of the trout; i.e., during previtelline and early stages of vitellogenesis. Balbiani's basophilic body has been followed from its origin in the youngest oocyte to its disappearance at the beginning of vitellogenesis. It assumes the form of a duplex body composed of a non-basophilic component (idiosome) and a highly basophilic portion, the pallial substance. Electron micrographs reveal the Balbiani body to be a dense, finely granular substance apparently unrelated to the Golgi apparatus or mitochondria. Its role in oogenesis is not clear, but probably constitutes an essential precursor substance (perhaps RNA) necessary for oocyte growth and vitellogenesis.
At given periods in early oocyte growth, an extensively developed system of endoplasmic reticulum is formed and the cisternae contain numerous granules. These intracisternal granules probably represent precursor elements for a specialized type of yolk substance. The granules collect in dilated regions of the smooth-surfaced cisternae where the latter undergo anastomosis. Here the granules undergo transformation into a finely granular particle. Older and more vitellogenetic oocytes have not been included in this study.