Cell division begins soon after fertilization, even while the rest of the egg is being formed. Cell division will continue if the egg is kept warmer than 67oF. The first cell division is completed about the time the egg enters the isthmus. Additional cell divisions take place about every 20 minutes; so, by the time of lay, several thousand cells form two layers of cells called a "gastrula."
At this time the egg is laid, it cools, and embryonic development usually stops until proper environmental conditions are established for incubation. After incubation begins, the cellular growth resumes. At first, all the cells are alike, but as the embryo develops, cell differences are observed. Some cells may become vital organs; others become a wing or leg.