Schematic representation of the octopus arm anatomy and embryonic development. (A) Schematic of a cross section through an adult O. vulgaris arm. (B) Sketch drawing of an octopus hatchling’s arm, demonstrating the arm axes and terminology used in this study. Inset on the top left illustrates the embryonic body axes with respect to the arm’s axes. (C) Illustration of selected developmental stages highlighting the major events of the embryonic development from fertilization to hatching. Embryos are shown with anterior to the left and dorsal to the top, except for the hatchling, which is shown in a frontal view. Dark gray shading in stage 11 represents the embryo proper during gastrulation; light gray shading highlights a single cell layer of the not yet gastrulated area. Embryos stages 22 to 26 are displayed without yolk sac. Abbreviations: ar, artery; ch, chromatophore; e, eye; f, funnel; gza, growth zone of the arm; m, mantel; s, sucker; tr, trabeculae; in, intramuscular nerve; y, yolk; yp, yolk plug; I-IV, arm numbers I to IV.