Sea Urchin staging series Invertebrate embryos develop at different rates depending on the temperature of incubation. Descriptive stages based on the number of cells and their arrangement in the embryo are used to enable researchers to compare results between experiments. Cleavage-stage sea urchin embryos are described based on the number of cells, followed by the blastula (central cavity surrounded by an epithelial layer) and gastrula (formation of the archenteron) stages. Feeding larvae are defined morphologically as first prism and then pluteus larva. Lytechinus embryos