The cleavage occurs more readily in the active cytoplasm than the yolk-laden cytoplasm or deutoplasm of the egg. The yolk which occurs in all animal eggs in little (e.g., microlecithal eggs) or larger (e.g., mesolecithal and macrolecithal eggs) amounts and remains distributed evenly (e.g., isolecithal eggs) or unevenly (e.g., telolecithal eggs) in the ooplasm of different kinds of eggs, has a profound determining influence on cleavage and mechanics of moving the germ layers into their final positions. The amount of yolk influences the course of cleavage by the following ways:
1) when the amount of stored yolk increases, the amount of active cytoplasm, gradually decreases, and the position of nucleus is variously affected. In mesolecithal eggs, such displacement of zygotic nucleus from the geometrical centre of the egg to the less yolky (active) cytoplasm is very common. The mitotic divisions of such a displaced nucleus result in unequal cytokinesis and unequal sized blastomeres.
2) Every mitosis of cleavage involves movements of the cell components, the chromosomes, parts of the cytoplasm constituting the achromatic figure, the mitochondria, and the surface layer of the cell, the activity of which along the equator of the maternal cell leads to the eventual separation of the daughter cells. During these movements, the yolk granules or yolk platelets behave entirely passively and are passively distributed between the daughter blastomeres. When the yolk granules or platelets become very abundant, they tend to retard and even inhibit the process of cleavage. As a result, the blastomeres which are rich in yolk tend to divide at a slower rate and consequently remain larger than those which have less yolk. The yolk in the uncleaved egg is more concentrated toward the vegetal pole of the egg. It is therefore, at the vegetal pole of the egg that the cleavage is most retarded by the presence of yolk, and where the blastomeres are of the larger size.
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