The development of the chick begins in the single cell formed by the union of two parental cells, eggs and sperm, in the process known as fertilization. In birds, fertilization occurs about 24 hours before the egg is laid.
Many elaborate physiological processes take place during the transformation of the embryo from egg to chick. These processes are respiration, excretion, nutrition, and protection.
Several changes take place during the 18th and 21st days. The abdominal wall surrounds the residual yolk sac on the 19th and 20th days of incubation.