ORGANIZATION

Modified from: Kimmel et al., 1995. Developmental Dynamics 203:253-310. Copyright © 1995 Wiley-Liss, Inc. Reprinted only by permission of Wiley-Liss, a subsidiary of John Wiley & Sons, Inc.

We have named the stages rather than numbering them as in most other series, because named stages are more flexible and easier to remember or recognize than numbered ones. A stage defines more than an instant in time, it is merely a device for approximately locating a part of the continuum of development. With named stages one can easily add detail as it is learned, and intercalate new stages into the series as necessary for a particular study without resorting to cumbersome devices like decimal numbers, minuses, and pluses. For example, here we describe a 5-somite stage, and next a 14-somite stage, but a particular study might use eight stages in between (6-somite, 7-somite, and so on), and one will immediately understand their meaning. We emphasize that we do not mean to exclude these extra stages, wherever they are in the series, by ignoring them.

As an aid to communicating a broader perspective towards development, we group the stages into larger time-blocks called periods (Table 1), and summarize in the text the principal events during these periods. Specialized terms are defined in the glossary, and bold-face type in the text highlights terms important for staging. Table 2 collects brief descriptions of all the stages, and Fig. 1 shows corresponding sketches. One can use these two sources to locate approximately a stage of interest, and then find additional detail as necessary in the text and other figures.