mosaic egg

At one time a distinction was drawn between those organisms in which the egg seemed to have a firmly committed fate map built in and "regulating" embryos. In the former, after the first cleavage one blastomere was committed to produce one set of tissues, the other blastomere a different set, and removal of one blastomere led to the production of an incomplete embryo. This was particularly obvious in mollusc development where one blastomere had the polar lobe material. This early differentiation (or determination) of blastomeres for particular fates was in distinction to "regulating" embryos in which the removal of one blastomere did not matter, the other blastomere(s) compensating and producing a full set of tissues. The distinction is, however, only based upon the timing of differentiative events, and within a few divisions the regulating embryo also becomes a mosaic of determined cells.