The preparation of a series of normal stages of the cliick embryo does not need justification at a time when chick embryos are not only widely used in descriptive and experimental embryology but are proving to be increasingly valuable in medical research, as in work on viruses and cancer. The present series was planned in connection with the preparation of a new edition of Lillie’s Deuelopiiaent of the Chick by the junior author. It is being published separately to make it accessible immediately to a large group of workers. Ever since Aristotle “discovered” the chick embryo as the ideal. object for embryological studies, the embryos have been described in terms of the length of time of incubation, and this arbitrary method is still in general use, except for the first three days of incubation during which more detailed characteristics such as the numbers of somites are applied. The shortcomings of a classification based on chronological age are obvious to every worker in this field, for enormous variations may occur in embryos even though all eggs in a setting are placed in the incubator at the same time. Many factors are responsible€ or the lack of correlation between chronological and structural age.-4mong these are: genetic differences in the rate of development of different breccls (eg, the embryo of the White Leghorn breed develops more