The study of intrauterine development and growth while consistently respecting the time factor enabled the authors to determine the time course of organogenesis of all organ systems. The resulting comparison with other mammals detected the time lag in ossification, retardation of odontogeny, the time of the origin and development of the fluke, dorsal fin and flipper, etc. The authors believe the Staging and Ageing Method serves for much more efficient evaluation of existing embryological collections which very often contain embryos of unknown age.