In most animal embryos early determinative steps appear to be controlled by uneven distributions of cytoplasmic components (Davidson, 1976). The segregation of localized cytoplasmic components into different blastomeres seems to determine certain cell lineages, or to set up polarities, which in turn become the basis for cell interactions later. Early investigators were often guided by the localization of visible markers such as pigments or basophilic granules in animal eggs. Removal or local inactivation of cytoplasm containing such components was usually followed by embryonic defects. However, studies of this kind do not prove that the visibly localized components determine cells, or that the defects were caused by inactivation of localized components.
Localized determinants are best defined by operational criteria. First, the components under consideration must be localized. The localization may be exclusive or merely quantitative. Second, it must be shown that the localized components are necessary and/or sufficient for cell determination.