Fertilization in chickens occurs in the infundibulum of the reproductive tract within 15 min after ovulation of the yolk. The first cleavage division occurs 5 h later as the egg enters the shell gland. As the shell is calcified during the next 20-22 h, the embryo develops into a radially symmetric dome of cells (the blastoderm) containing approx 60,000 cells. Development within the oviduct and in the first few hours after the egg is laid has been described by Eyal-Giladi and colleagues (1 ,12 ); these stages of development are given in roman numerals followed by E-G&K. Later stages of development, which occur after embryos are placed into an incubator at 37.5 C and 50% relative humidity, are described by Hamburger and Hamilton (3 ); these stages of development are given in arabic numbers followed by H&H.
