A review of the 4 outstanding cases of polyembryony in animals: (1) embryonic fission in the Cyclostomatous Bryozoa; (2) twinning in the earthworm; (3) polyembryony in the parasitic Hymenoptera; (4) polyembryony in the Armadillos. The various theories of polyembryony are discussed, and the point is made that polyembryonic development is an hereditary phenomenon.