Chickens, unlike mammals, do not develop with an umbilical cord. The concept of an umbilical cord, which connects a fetus to a placenta for nutrient and waste exchange, is specific to placental mammals. Avian embryos, developing within an eggshell, rely on specialized structures to support their growth. This system allows the chick to develop independently within its self-contained environment.
Inside the Egg: Nourishing the Embryo
Avian embryos are nourished and manage waste through several extraembryonic membranes. These membranes are the yolk sac, the allantois, and the chorion, which collectively facilitate the chick’s development.
The yolk sac is a membrane that envelops the yolk, the primary food source for the developing chick. It produces enzymes that convert the yolk’s fats and proteins into a form the embryo can absorb, acting somewhat like a digestive system. The yolk sac is highly vascularized, transporting absorbed nutrients to the growing embryo.
The allantois is another membrane that plays a dual role in avian embryonic development. It serves as a reservoir for metabolic waste products, particularly uric acid. Additionally, the allantois develops an extensive network of blood vessels that connect to the embryo’s circulatory system, facilitating gas exchange through the porous eggshell. This membrane also contributes to the absorption of calcium from the eggshell, used for bone formation in the embryo.
The chorion, often fusing with the allantois to form the chorioallantoic membrane, is positioned just beneath the eggshell. This combined membrane is crucial for efficient gas exchange between the embryo and the external environment. It also aids in the absorption of albumen (egg white) and calcium from the shell. These specialized membranes collectively perform the functions that, in mammals, would be handled by the placenta and umbilical cord, ensuring the chick’s development within the egg.
Hatching and Beyond
As the chick nears hatching, significant changes occur in these extraembryonic structures, preparing the chick for life outside the egg. A key event is the absorption of the remaining yolk sac into the chick’s abdominal cavity. This internalized yolk provides nourishment for the newly hatched chick, sustaining it for the first few days until it learns to find food independently.
The allantois, having completed its functions of waste storage and gas exchange, dries up and detaches from the chick. It remains adhered to the inner surface of the eggshell and is left behind when the chick hatches. The chorioallantoic membrane, which facilitated respiration and calcium absorption, also detaches and is discarded with the shell.
The chick then uses a specialized “egg tooth” located on its beak, along with its leg muscles, to break through the shell in a process known as pipping and hatching. This allows the chick to emerge from the egg, ready to begin foraging. The complete utilization or discarding of the extraembryonic membranes highlights the self-sufficient nature of avian embryonic development.