In chick, the discoidal or macroblastic cleavage remains restricted to the germinal disc, the yolk remains in uncleaved state and is encompassed eventually by the growing tissues of the embryo. The first cleave furrow consists of slight meridional incision near the center of the germinal disc. This furrow passes yolk ward but does not reach the lower portion of the disc where the cytoplasm is filled with coarse yolk granules.
The second cleavage plane is again meridional but at the right angles to the first. The third set of cleavage furrows is vertical, cutting across the second set of meridional furrows and consequently tends to parallel the first cleavage furrow.
The fourth set of cleavage furrows is also vertical and through not synchronous, but it proceeds gradually to form eight central blastomeres which are surrounded by twelve marginal blastomeres. Thus the meridional and vertical cleavages separate the daughter blastomeres from each other, but not from the yolk, so that the central blastomeres are continuous with the underlying yolk at their lower ends, and the blastomeres lying on the circumference are, in addition continuous with the uncleaved cytoplasm at their outer edge.
Surface view of germinal disc of hen’s egg showing early cleavage. A-Two-cell stage; B-Four-cell stage; C-twenty-cell stage; D-A late cleavage stage (after Balinsky, 1970)
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At the nuclei at the edge divide, more and more cells became cut off to join the arcs lying in the center, but the new blastomeres remain in continuity with the uncleaved yolk underneath.