Name: 16-cell stage
ECAO ID: 0000190
Definition: "An embryo that is composed of 16 cells and is surrounded by a fertilization envelope. Note for euechinoid indirect developers: although the fourth cleavage occurred simultaneously in all cells, the pattern of cell division differed in the animal and vegetal blastomeres. In the animal hemisphere the four cells divided meridionally, producing eight blastomeres of equal volume arrayed in a single tier. These cells are called the mesomeres. In the vegetal hemisphere, the four cells underwent an unequal, equatorial cleavage, producing a tier of four large cells, the macromeres, and a tier of four small cells, the micromeres, located at the vegetal pole. At the 16-cell stage, the embryo is hence composed of three groups of cells with distinct sizes. In most sea urchin species,this is the first morphological indication of the animal-vegetal axis."
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