The structure and distribution of organelles within oocytes of developing antral follicles were studied qualitatively and quantitatively. In the smallest category of follicle (0\ m=. \ 2\ p=n-\ 0\ m=. \ 4mm diam.) the Golgi was present in a peripheral position, the endoplasmic reticulum was distended and mitochondria were intimately associated with it. Processes from surrounding cumulus cells were in contact with the oolemma which was thrown up into slender villi, increasing the oocyte surface

more »

... rea 5-fold. In these and all subsequent follicles the oocyte cytoplasm contained numerous 30 nm particles or vesicles. As the follicles grew to about 2\m=.\0mm the mitochondria became located in a peripheral band, interior to which were numerous large vesicles, and the villi became shorter and thicker. In follicles >2\m=.\0mm the mitochondrial band was largely dispersed and cortical granules rested close to the oolemma. Little structural change was then observed until late oestrus. In oocytes from preovulatory follicles at this stage an internal dense zone was formed in the zona pellucida. Most of the cumulus cell processes degenerated and most mitochondria assumed a hooded appearance.