Figure 13

Stage 17, Dorsal closure. All scale bars 100 μm. Sytox staining, a-a''', c; SEMs, b, d, e. a: Lateral view. The white line indicates the increased distance from the precheliceral lobes (PcL) to the opisthosomal tail compared to previous stages (compare with Figure 12a). a': Frontal view. The white arrows indicate the direction of the epidermal and cuticular overgrowth of the brain region. a'': Posterior view. a''': Dorsal view. The white arrows indicate the dorsad growth of tissue that eventually forms the heart (H). b: Dorso-lateral view showing the crumbled appearance of the dorsal tissue directly posterior to the head lobes after dorsal closure. The prosomal tergites continue to extend dorsally (white arrows) (compare with Figure 12a). c: Detail of anterior prosoma. As a result of the forward positioning of the cheliceres and/or posterior positioning of the stomodeum, the labrum (Lb) is now between the bases of the cheliceres (Ch). Cuticular formations (white arrows) are visible in the sternal regions (white asterisks) of the prosomal segments. d: The right limb bud on opisthosomal segment two (O2) shows four pulmonary furrows (black arrows) and a lateral opening of the pulmonary sac (PuS). At the latero-posterior insertion of the limb bud at opisthosomal segment three (O3), the opening of the tubular trachea (TrO) is visible. e: Posterior opisthosomal region. The anlagen of the left and right tergite plates (Ter) meet dorso-medially (compare with a later stage, Figure 14c). en, endite; L1-L4, walking legs one to four; P, pedipalp; VS, ventral sulcus.