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
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