Fig. 166. Nuclear equality in the sea-urchin egg. (.AD) Normal cleavage. While nuclei and black nuclei theoretically so designed to show nuclei in animal and vegetal pole cells respectively. (E) Four-cell stage flattened by pressure, showing position of spindles for the third cleavage parallel to pressure surface. (F) Fight-cell stage under pressure. Compare with (C), normal. (G) Horizontal cleavage resulting from release of pressure after eight-cell stage. Note mixed distribution of nuclei. Later development normal, with cytoplasmic, organ-forming substances determining development as in fig. 163. Thus it appears that the nuclei are equal within the blastomeres, whereas the cytoplasm is unequally (i.e.. qualitatively) distributed to the respective blastomeres, the particular type of development of the blastomeres being dependent upon the cytoplasmic substance present.