Final answer:
The Graafian follicle is the sac within the ovaries that contains the developing egg prior to ovulation and transforms into the corpus luteum afterwards, playing a critical role in human reproduction.
Step-by-step explanation:
The sac containing the egg prior to ovulation is known as the Graafian follicle. This structure is found within the ovaries and is part of the process by which eggs, or oocytes, develop and are eventually released during the menstrual cycle. Inside the Graafian follicle, the egg matures and when ready, the follicle ruptures during ovulation, releasing the egg into the fallopian tube where it may be fertilized by sperm. Post-ovulation, the remnants of the follicle transform into the corpus luteum, which secretes hormones to help maintain the uterine lining in preparation for potential pregnancy.
If fertilization does not occur, the corpus luteum degenerates into the corpus albicans. Fertilization usually takes place in the fallopian tube, and following this, the zygote travels to the uterus to implant and begin the development of the embryo. The Graafian follicle is, therefore, crucial in the reproductive process as the site of egg development and the predecessor to the hormone-secreting corpus luteum.