Invertebrateembryosdevelopatdifferentratesdependingonthetemperatureofincubation.Descriptivestagesbasedonthenumberofcellsandtheirarrangementintheembryoareusedtoenableresearcherstocompareresultsbetweenexperiments.Cleavage-stageseaurchinembryosaredescribedbasedonthenumberofcells,followedbytheblastula(centralcavitysurroundedbyanepitheliallayer)andgastrula(formationofthearchenteron)stages.Feedinglarvaaredefinedmorphologicallyasfirstprismandthenpluteuslarva.