in 1888, roux published the results of a series of defect experiments in which he took 2 and 4 cell frog embryos and killed half of the cells of each embryo with a hot needle. he reported that they grew into half-embryos and surmised that the separate function of the two cells had already been determined. this led him to propose his "mosaic" theory: after a few cell divisions the embryo would be like a mosaic, each cell playing its own unique part in the entire design.