Figure 2.

Ctenophore development and body plan. (A) Early cleavage from egg to 60-cell stage, based on Martindale and Henry [20] and others. The top row shows the view from the aboral (or vegetal) pole and the bottom row shows the lateral view, with the oral pole at the bottom. The first two divisions are equal and meridional, and the third cleavage is unequal and oblique, giving rise to middle (M) and end (E) macromeres. Subsequent divisions are unequal, with micromeres given off at the aboral pole. (B) Basic ctenophore body plan, as shown during the cydippid stage. The oral pole is the location of the mouth, which opens to the pharynx. The pharynx leads internally to the gut and endodermal canals (yellow). Also shown are paired tentacle bulbs (from which the tentacles grow), the eight comb rows, and the apical sensory organ located at the aboral pole.