Various stages of Nematostella vectensis embryonic development. Development stages from egg to polyp are shown, with the oral pole to the left in panels H-L (indicated with an asterisk). A) Unfertilized egg, with the female pronucleus visible near the cell membrane at the oral pole (arrowhead). B) The first cleavage (1–2 hpf) occurs at the oral pole of the fertilized egg. C-F) Cleavage stages, with the 4-cell stage often arising after the first two cleavages finished simultaneously (C). G) Cleavages result in a hollow sphere of cells called a coeloblastula (10–20 hpf). H) Invaginating cells at the oral pole mark the beginning of gastrulation (24 hpf). I) The early larva (72 hpf) displays a doubly layered body wall and swims with the aboral end forward (arrowhead indicates the apical organ). J) The late larva starts to elongate and to form two primary septa (arrows). K) Early polyp, with four tentacle buds around the mouth at the oral pole. L) Juvenile polyp; the primary and secondary axes are indicated with a green and a blue line, respectively. Scale bars are 60 μm in panels A-H and 90 μm in panels I-L. Development times in hpf = hours past fertilization (original image from , with development times estimated from [13, 29]).