Ovary from cat (100X)

Here you get a closer look at the follicles. The primordial follicles are quite small. They consist of the oocyte and it's nucleus, the cytoplasm and a surrounding layer of flattened follicular cells.

The primary follicles, which you have one example of here, are a bit larger than the primordial follicles. The oocyte is even larger, and the surrounding follicular cells (also called granulosa cells) have a cubical shape. Inbetween the follicular cells and the oocyte, you can see a thick pinkish membrane. This is the zona pellucida. The cells in the stroma surrounding the primary follicle have organized themselves into concentric bands and become the theca folliculi. In the stroma you also see lumps of pale, epithelial-like cells forming the so called interstitial glands of the ovary.

In the secondary follicles (not seen in this picture), the follicular cells (granulosa cells) have become much more numerous compared with primary follicles, giving the follicle a granular appearance at low-power.