Staging Systems

Abstract

Differences between the brains of species belonging to different vertebrate classes are evident from the onset of neurulation (Richardson et al. 1997). Nevertheless, embryos of different vertebrate classes share a common organization and at, for instance, the tailbud stage, have many features in common such as the presence of somites, the neural tube, optic anlagen, the notochord and pharyngeal arches. But, evolution has produced a number of changes in the embryonic stages of vertebrates including differences in body plan, changes in the number of units in repeating series such as somites and pharyngeal arches, and changes in the pattern of growth and the timing of development of different fields (see Richardson et al. 1997). To be able to compare data obtained from studies on embryos, systems for staging prenatal development are advocated.