Bulletin 633 Embryonic Development Fertilization occurs about 24 hours before the egg is laid. A sperm cell, actually a number of sperm cells, penetrates the blastodisc on the yolk, and a new life begins. When the egg is laid, the blastoderm contains several hundred cells. When the egg cools, embryonic development stops.
Embryonic development starts again when the temperature is increased.
If the temperature is then decreased to room temperature a second time, the embryo will die. Prior to incubation, all cells in the embryo look the same and have the same function. That changes, however, during the first 4 days of incubation. As cell division continues, the cells receive messages that cause them to become different types of tissues and organs. Some cells will develop into the circulatory system, others into the brain, and still others into muscles.