The unequal distribution of certain materials—cytoplasmic determinants—in the cytoplasm of a fertilized egg cell (zygote), early embryo cell, or stem cell. Following cell division, the daughter cells receive varying amounts of those determinants, which influence their subsequent development. The determinants include mRNAs and regulatory proteins, which activate certain genes and so influence the developmental fate of the recipient cell(s). Cytoplasmic segregation specifies polarity of the zygote, i.e. which end is ‘top’ and which is ‘bottom’, and this fundamentally influences future embryonic development, as the cells divide repeatedly to form tissues and structures appropriate for the ‘head’ or ‘tail’. See also animal pole.