Fig. 2
From: A review of the reproductive system in anuran amphibians
Ovarian morphology and follicular development in frogs. The fat bodies are attached to the ovaries, and the ovaries connect with the oviducts. The ovary is formed by a simple epithelium and ovarian cortex. Follicular development occurs in the ovarian cortex. Stage I. The oocytes are formed by cytoplasm, Balbiani bodies, and the yolk. The follicle is surrounded by the follicular cells. Stage II. Then, the oocytes accumulate yolk and form the acellular envelope. Stage III. Theca and granulosa cells begin to form in the follicle. The height of the cubical follicular cells increases and numerous blood vessels develop. The oocyte accumulates yolk for the later nutrition of eggs. Stage IV. Multiple layers of the granulosa surround oocytes and form a follicular cavity. The blood vessels surround theca cells. Stage V. The zona pellucida and microvilli formation begins on the oocyte’s surface. The oogonia form cellular nests near the follicle. Stage VI. Finally, numerous microvilli, a zona pellucida, and a double vitelline layer surround the oocyte. The follicle finishes its development, and oocytes are ready to be released