Day 5: What Came First: the Chicken or the Egg?

Development of the Ovary in the Female Embryo

Embryonic cells destined to someday become eggs or sperm are called primordial germ cells. Primordial germ cells first become recognizable in the yolk sac of the embryo. If the chick embryo is genetically female, these germ cells will migrate from the yolk sac through the mesentery to the body cavity of the embryo and develop into an ovary. The ovaries can be seen around day eight in the female chick embryo. The embryonic ovary is made up of germ cells which have multiplied to form clusters. Only one cell within a cluster will actually become an egg, destined to undergo meiosis and carry on the gene pool. This cell is called an oocyte. The remaining cells will become specialized to protect and nuture the developing oocyte. The oocyte and its surrounding cluster of specialized cells are called an ovarian follicle.

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