Figure 3.

Early neurogenesis switch in N. vectensis. During very early development (blastula to ea rly gastrula; (a)), individual neurons start appearing in the aboral half, yet excluding future apical organ at the aboral pole. The location and morphology of the cells speak in favour of neuronal determination in situ, either guided by stochastic processes, or by the action of a yet-unknown gradient. Neurogenesis during later stages of embryonic development (b) is still poorly understood, but seems to have a more localized character. In the planula, many neurons are now born at the oral side and in the endoderm. It is still unclear whether and to what extent neuronal precursors migrate towards the aboral side of the embryo.