The terminology of early development: history, concepts, and current usage.

The meaning of some terms used in describing the development of embryos is discussed in the light of their historical origin and current usage. Attention is focused on the terminology associated with the early development of animals, particularly that concerned with gastrulation and germ layer theory. It is suggested that terms linked with outdated concepts are not always appropriate for, and unless defined with care may limit, our appreciation of the significance of subsequent observation and experiment. Recommendations are made concerning the use of the terms mesoblast, mesoderm, and mesenchyme.