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File:Echinoidea nuclear equality in the cleaving egg.jpg

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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.

Black cytoplasm = micromeres which form primary mesenchyme. Coarse dotting = entoderm, secondary mesenchyme and coelomic material. White, light stipple, and vertical lines = ectodermal cells.
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Source https://archive.org/details/comparativeembry00nels/page/335/mode/1up?view=theater&q=BLASTULATION+ Comparative embryology of the vertebrates; with 2057 drawings and photos. grouped as 380 illustrations.
Author Nelsen, Olin E.

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current20:19, 11 March 2024Thumbnail for version as of 20:19, 11 March 20241,067 × 649 (806 KB)Rasbak (talk | contribs){{Information |description=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 mix...

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