Electron microscopy of growing oocytes of Rana pipiens.

1. In the cytoplasm of oocytes of stage Y(0), prior to the appearance of yolk, one observes a few scattered profiles of endoplasmic reticulum and numerous filamentous mitochondria, usually distributed at random but sometimes clustered. As the nuclear membrane begins to bulge outward, small granules and short rods appear in the perinuclear cytoplasm and endoplasmic reticulum becomes more prominent throughout the cytoplasm. 2. Coincident with the appearance of the first yolk platelets, which are deposited in a narrow peripheral ring within the endoplasm at stage Y(1), protoplasmic processes, the microvilli, push out all over the surface of the oocyte. At the same time follicle cells pull away but remain attached to the oocyte at some points through finger-like processes which interdigitate with neighboring microvilli. It is estimated that the microvilli increase the absorptive area of the surface to about thirty-five times that of a simple sphere. Just beneath the microvillous layer is the basal protoplasm of the cortex, now containing tiny granules probably synthesized from newly absorbed raw materials. Cortical granules appear and become aligned below the basal layer on the external border of the endoplasm. Both the cortical granules and the yolk platelets measure up to 1 micro in diameter at this stage. 3. By stage Y(3) (yolk filling peripheral three-fourths of cytoplasm), the basal layer of the cortex is folded so that it appears in section as alternating ridges and valleys. The microvilli now extend from the summits of the cortical ridges. Small, ring-shaped granules are abundant in the cortex. Cortical granules have increased to 2 micro in diameter. 4. Yolk platelets continue to be synthesized around the cortical granules and in the subjacent endoplasm. The largest platelets measured in the interior cytoplasm at stage Y(4) (cytoplasm filled with yolk) were 3.7 micro wide by 5.8 micro long. Pigment granules increase in size from 0.15 micro in diameter at stage Y(3) to 0.30 micro in diameter at stage Y(4).