We examined the ontogeny of hatching-related traits in A. callidryas to assess how they vary with the ability to hatch in response to different threats at different developmental stages. We measured the diameter of the perivitelline chamber throughout development, as an indicator of breakdown of the vitelline membrane, to infer whether HE is released in advance of hatching (for justification, see Cohen et al., 2018). We used scanning electron microscopy (SEM) to examine HGC morphology, distribution and abundance throughout embryonic development, and the timing of HGC disappearance in embryos and hatchlings. We also used SEM to observe lateral lineneuromasts, as candidate mechanosensors whose development might limit the onset of vibration-cuedhatching. In examining HGC ontogeny, we found two morphologically distinct HGC types. Given that no