![]() Note that the cleavage furrows have not continued down through the yolk of the egg. Kessel, shows the zebrafish (Danio) embryo at the 32-cell stage. This type of meroblastic cleavage is also found in the eggs of fish, reptiles, and 4 species of mammals - the monotremes. Therefore, each of the cells produced in the earliest stages is bound on the top and on the sides by a plasma membrane, but the bottom of the cell is in direct contact with yolk. ![]() ![]() When the first cleavages occur in the hen's egg, the cleavage furrows do not continue down through the mass of yolk. The hen's egg consists of just a tiny patch of cytoplasm resting on the surface of a large ball of yolk (the "white" of the egg is noncellular accessory protein). In eggs that contain a large amount of yolk, cytokinesis does not divide the egg completely. \)Ĭleavage refers to the early cell divisions that occur as a fertilized egg begins to develop into an embryo.
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