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Chicken or the Egg?

The age-old question persists: which came first, the chicken or the egg? Approximately 24 hr after fertilization occurs in a chicken, the egg is laid. The newly formed single cell begins to divide; tissue begins to form on the yolk by the end of day 1. By the end of day 2, the heart begins to beat. On day 14, claws begin to form and the embryo is moving into position to hatch. On the 21st day, the chick finishes its escape from the shell.

Image: This image depicts a 13-day-old chicken embryo. The networks of chorioallantoic membrane blood vessels used for gas exchange and the yolk sack blood vessels used for nutrient uptake can easily be seen. By day 8, the beak and feathers are also visible. The allantois (sac involved in gas exchange and excretion) shrinks to become the chorioallantoic membrane (vascular membrane). And so begins the appearance of claws and leg scales. Inset: Photo of two fully developed chicks with their mother.

 This image shows the development of a chicken embryo from day 0 to day 21. Approximately 24 hr after fertilization occurs, the egg is laid. The newly formed single cell begins to divide; tissue begins to form on the yolk by the end of day 1. By the end of day 2, the heart begins beating. On day 14, claws begin to form and the embryo is moving into position to hatch. On the 21st day, the chick finishes its escape from the shell.
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