Some of these possible animal embryos apparently record the early stage of cellular division. (fossilmuseum.net)
An embryo that has undergone multiple cellular divisions with the formation of a cavity within it. (pacificfertilitycenter.com)
They remain alive for a time after they stop dividing, but sometime after cellular division ends, cells do a particularly disturbing thing: Essentially, they commit suicide. (science.howstuffworks.com)