Figure 3

A segment can be composed of one or more tissue layers. Bracket marks one cell or segment, except in (A), where it marks one putative cell or segment. (A) Yellow circles represent cells (putative segments) in one layer of tissue. (B) Each cell in a column of cells can be called a segment if there is a-p cell polarity in each cell. Cells (circles) now have a-p cell polarity, represented by difference in coloration from yellow (anterior) to red (posterior). (C, D) Segments are often defined as having reiterated units (segments) composed of derivatives of both mesoderm (yellow-red circles) and ectoderm (yellow-blue (C) and blue (D) circles). Each dorsal-ventral row of cells forms one segment. (C) Both the mesoderm and the ectoderm have a-p cell polarity, represented by difference in coloration. (D) Only the mesoderm (yellow-red) has intrinsic a-p cell polarity. The ectoderm (blue), does not have reiterated pattern on its own, but does contribute to the segmental pattern of each segment as a whole, since it is associated with the anterior of each segment. a, anterior; p, posterior.