Embryos of California Grunion, Leuresthes tenuis (Atherinopsidae), must be able to tolerate incubation in an unpredictable environment, and then respond rapidly when the environment becomes favorable for hatching. Since embryos of L. tenuis do not hatch until triggered by an environmental cue, we were able to experimentally generate and examine hatchlings of specific known ages for a series of embryos from the same fertilization date, by hatching them at intervals throughout the extended incubation period. During this time, most morphological features did not develop as they would have if the embryo had hatched into larval life. This study examined the development of embryos of L. tenuis extending incubation from the time of hatching competence, at approximately ten days post fertilization (dpf), to the time of embryonic death at 35 dpf. The embryos during extended incubation arrest most aspects of development, including organogenesis and fin development. Melanophores, sagittal otoliths, and marginal teeth continue to develop in embryos until hatching or embryonic death. Thus individual L. tenuis of the same chronological age may be either embryonic or larval, at very different stages of development, depending on which day hatching takes place. This example of heterokairy may be a useful new model for studies in developmental biology.