Andasinthemulticellularplantssointhemulticellularanimals，alike
primarycontrastisforthwithrepeatedintheinitialclustersofcells。
Producedbytherepeatedfissionsoftheprimitivegerm—cell，eachsuchcluster
presentlyformsitselfintoahollowsphere:the\"cleavagecavity\"
beingmanifestlyhomologouswiththecavityoftheVilvox—sphere。*InsimpletypesofMetazoa，asthehydroidpolyps，theblastula，