Cleavage and Early Development
Cleavage and Early
Development
During cleavage the embryo divides repeatedly to convert the large, unwieldy cytoplasmic mass into a large cluster of small, maneuverable cells (called blastomeres). There is no growth during this period, only subdivision of mass, which continues until normal somatic cell size is attained. At the end of cleavage the zygote has been divided into many hundreds or thousands of cells (about 1000 in polychaete worms, 9000 in amphioxus, and 700,000 in frogs). Polarity is present in the egg in the form of a polar axis, which establishes the direction of cleavage and subsequent differentiation of the embryo.
During cleavage the embryo divides repeatedly to convert the large, unwieldy cytoplasmic mass into a large cluster of small, maneuverable cells (called blastomeres). There is no growth during this period, only subdivision of mass, which continues until normal somatic cell size is attained. At the end of cleavage the zygote has been divided into many hundreds or thousands of cells (about 1000 in polychaete worms, 9000 in amphioxus, and 700,000 in frogs). Polarity is present in the egg in the form of a polar axis, which establishes the direction of cleavage and subsequent differentiation of the embryo.
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