Fig. 3

Illustration of the developmental hourglass model for chordates, specifically vertebrates. The upper portion represents early development, the lower portion late development, and the middle constricted portion the phylotypic stage. The greater width of the upper and lower portions represents the greater morphological diversity that is representative of vertebrates during these developmental periods. For example, the figures in the upper half of the hourglass depict stylized vertebrate embryos and highlight the differences between different species at the beginning of gastrulation. Representative embryos include: i frog, ii bird, iii human, and iv fish. The lower half depicts the developmental differences that occur late in development that lead to the vast differences we see among adult vertebrate species. The middle constricted portion illustrates the relative lack of morphological variation between vertebrate species during the phylotypic stage