Name: 28-cell stage
ECAO ID: 0000200
Definition: "An embryo that is composed of 28 cells and is surrounded by a fertilization envelope. Note for euechinoid indirect developers: In most sea urchin species, the fifth cleavage signed the end of cell division synchrony, with the mesomeres and macromeres dividing prior to the micromeres with a significant time difference. In the animal hemisphere, the mesomeres underwent an equal and equatorial cleavage, giving rise to two animal cell tiers, each composed of eight blastomeres of equal size: the an1 tier located at the animal pole and the an2 tier located below an1. In parallel, the macromeres divided meridionally, forming a ring of eight cells of equal volume located below the an2 tier. At that stage, the micromeres have not yet divided and are still at a number of four cells."
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