The egg (ovum) like the embryo during the first embryonic divisions, do not possess any sign of polarity. This development mechanism can be called regulative development(in contrast to the mosaic model) since the fate of the cells that originate is not fixed and can be modified during development. In regulative embryos, part of the embryo can be removed and the remaining cells can compensate for the loss and give a complete individual as the final product.The strongest evidence that continues to support this regulatory model is based on the plasticity or potential that mammalian cells possess before implantation. It is known that the blastomeres produced by the first divisions can be replaced with each other or even can be eliminated without apparently altering the embryonic development and therefore they are equivalent (without polarity).
