The Segmentation of the Vitellus

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“The first change in the parent-cell is that by which it becomes broken up into a mass of cells, each of which is just like itself. This process is called segmentation of the vitellus; each one of the numerous resulting cells is called a cleavage-cell. The nucleus of the parent-cell divides into two; each attracts its half of the yelk; the halves furrow apart and there are now two cleavage cells in place of the one parent-cell a furrow at right angles to the first, and redivision of the nuclei; results in four cleavage-cells. Radiating furrows intermediate to the first two bisect the four cells, and would render eight cells, were not these simultaneously doubled by a circular furrow which cleaves each, with the result of sixteen cleavage-cells. So the subdivision goes on until the parent-cell becomes a mass of cells. This particular kind of cleavage, by radiating and concentric furrowing, is called discoidal, and the resulting heap of little cells assumes the figure of a thin, flat, circular disc. Segmentation of the vitellus, in whatever manner it may go on, results in a mulberry-like mass of cleavage-cells; and the original cytula has become what is called a morula. This process is shown closely here.” Elliot Coues, 1884


Bird Anatomy


Elliot Coues Key to North American Birds (Boston, MA: Estes and Lauriat, 1884)


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